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User Manual/Handbook: Microphone Handbook

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1. 10 100 1k 10k Frequency Hz 100k 950910e Fig 2 15 Real and imaginary part of Equivalent Volume calculated with the values of Table 2 3 Notice that the imaginary part is negative 2 30 Microphone Handbook Br el amp Kj r Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier In practice the equivalent volume of measurement microphone diaphragms de pends on the type of microphone The total range varies from about a tenth of a cubic millimetre to some hundred cubic millimetres The diameter of the micro phone diaphragm has a great influence on the volume fourth power Pressure microphones in a 1 size have typical volumes of 130 to 160 mm while correspond ing 4 2 microphones have a much smaller volume of approximately 10 mm The diaphragm volumes of 1 4 and g microphones are correspondingly smaller The equivalent volume is also a function of the diaphragm tension therefore the high sensitivity 4 2 microphones which have lower diaphragm tension have greater equivalent volume typically 40 mm 2 4 Combination of Microphone and Preamplifier The condenser microphone generates electrical signals These signals need to be transferred to associated equipment such as an analyzer or recorder which may be placed in some distance from the microphone itself Because the microphone has a very high electrical impedance it cannot withstand the load made up by the instru ment and the cable wh
2. Characteristics of Microphones Frequency Response 3 3 5 Free field Response For background information on the free field response of a microphone refer to Section 2 5 4 To determine the microphones performance in a free sound field a correction is added to the actuator response This correction is denoted as the Free field Correc tion This correction must not be confused with corrections used with sound cali brators 3 3 6 Random Incidence Response The random incidence response is the response of a microphone in a diffuse sound field A diffuse sound field exists at a given location if the field is created by sound waves arriving more or less simultaneously from all directions with equal probability and level A diffuse sound field may be created within a room with hard sound reflecting walls and which essentially contains no sound absorbing materials Sound fields with a dose resemblance to a diffuse field may be found in environ ments such as factories where many simultaneous sound or noise sources exist or in buildings with hard walls for example in halls or churches The diffuse field or random incidence sensitivity of a microphone refers to this type of field even if in most cases the diffuse field sensitivity is calculated from meas urements performed under free field conditions The calculation method is described in the international standard Random incidence and Diffuse field calibration of Sound
3. Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Techniques How to Monitor Using CIC The procedure is as follows l Install the system and perform an acoustical calibration 2 Immediately afterwards measure the ratio between the output and input volt ages of each measurement channel 3 Store the measurement results as reference ratios and the system is ready for use 4 Verification of the system can now be performed as often as necessary by com paring the present measured ratios with the stored reference ratios 5 For normal temperature and pressure variations within 0 2dB could be expect ed Under controlled conditions for example in special test cells a repeatability of better than O 1dB can and should be obtained For results which vary more than expected the system should be checked with an acoustical calibrator Frequent initial measurements will create a database valid for the actual set up on which the threshold for acceptance can be based As experience and confidence is built up the interval between acoustical calibrations can be ex tended Note that some types of microphone have a deliberate variation of capacitance as a function of temperature which will be reflected in the measured ratios 4 8 3 CIC Input Signal Requirements Test L evel A test signal close to the allowed maximum limit is recommended in order to obtain a
4. Characteristics of Preamplifiers Introduction to Characteristics of Preamplifiers 4 1 Introduction to Characteristics of Preamplifiers 4 1 1 Definition of a Microphone Preamplifier A microphone preamplifier is an impedance converter between a high impedance microphone and its following cable A low output impedance is necessary to drive long signal cables 4 1 2 Selection of a Microphone Preamplifier In principle microphone preamplifiers have more or less identical electrical charac teristics i e a gain of unity and a frequency range of a few hertz to more than 200kHz The most obvious difference between preamplifiers is the diameter with the most common diameter being 1 2 For acoustical reasons it is usual to select a preamplifier with the same diameter as the microphone to be used Bruel amp Kj r produce 1 2 and 14 preamplifiers Adaptors are available to connect these to l g or 1 microphones Apart from diameter other important selection parameters indude the transmission principle for example current voltage or digital signals system verification facili ties phase characteristics inherent noise and current supply requirements 4 1 38 Contents of this Chapter 4 2 4 2 1 Certain characteristics are commonly referred to in discussion of the performance and design attributes of preamplifiers This chapter describes these characteristics in some detail The information is intended to promote both
5. Cy is between 1Hz and 2Hz but other types with longer time constants are availa ble for measurements at lower frequencies Microphones with higher cut off frequencies are also available They may reduce possible disturbance from infra sound while doing low level sound measurements in other parts of the frequency range The magnitude and phase response curves which are shown on the above graphs are calculated using a model shown in Section 2 3 10 BE 1447 11 Microphone Handbook 2 19 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Fig 2 9 SER ENN AU O Vent exposed JOOYNN poe 00 60 PENNE CNN Eoo T O 0 1 Frequency Hz The phase of the low frequency microphone response is influenced by the ambient pressure The calculated responses shown are valid for a microphone which has 109 air stiffness at nominal ambient pressure 101 3 kPa a 1 bar b 2 bar c 10 bar d 0 5 bar 2 3 8 High Frequency Response Sound pressure measurements are made at both very low and very high levels of sound pressure as well as at both very low and very high frequencies Measure ments are also made in different types of sound field preferably without disturbing the fields However it is not possible to design one single microphone which can fulfil all needs Several types of microphone must be designed to cover the many different applications Some main design parameters are the stiffness and
6. In the comparison method both the measurement and the reference objects are present at the same time and are exposed to the same sound pressure As a result a simultaneous measurement can be performed In principle something unknown is compared with something known The method is often confused with the substitu tion method described above The method reduces the number of error sources and also reduces the stability requirements in situations where external sound sources are used It also covers the compressor loop principle used in a number of sound level calibrators where a reference microphone inside the calibrator constantly monitors the sound pressure In this situation the reference microphone must be very stable as it is the known object This method is also used for calibration and checking of sound intensity measure ment equipment which ideally requires two identical measurement channels both BE 1447 11 Microphone Handbook 6 11 Vol 1 Chapter 6 Calibration Calibration Methods phase and magnitude In a calibration situation two microphones are subjected to the same sound pressure field inside a coupler Calibration is performed by applying a well known sound pressure at a single frequency and then adjusting both chan nels to show the correct sound pressure simultaneously The checking of the pres sure residual intensity index is performed by applying a broad band noise spectrum Both microphones are subjec
7. Temperature C 950896e Fig 2 33 Stability Factor for nicka diaphragms non Falcon mi crophones To obtain the stability at any temperature multiply the reference stability of 150 C see micro phone specification by the stability factor Note that the above calculation method leads to a rather rough estimates of dia phragm and electret stability and that the stability of most microphone units are actually several times higher than estimated BE 1447 11 Microphone Handbook 2 53 Vol 1 Chapter 2 Microphone Theory Electrostatic Actuator Calibration Stability Factor 150 200 250 300 Temperature C 950897e Fig 2 34 Stability Factors as functions of temperature valid for the dectrets used by Prepolarized Microphones Multiply the stability value stated for the reference temperature with the stability factor to estimate the stability at other temperatures The curve marked a is valid at 90 rda tive humidity while b is valid for dry air 2 Electrostatic Actuator Calibration 2 7 1 ntroduction The calibration of microphones using an electrostatic actuator is a widely applied laboratory method for determination of frequency response characteristics of meas urement microphones The actuator produces an electrostatic force which simulates a sound pressure acting on the microphone diaphragm In comparison with sound based methods the actuator method has a great advantage It provides a simpler means
8. This divides the possible choices into roughly two groups of microphones free field or pressure field measurements Other factors such as the measurement environment the international standards which may need to be adhered to and the type of polarisation charge may also need to be considered These and other main considerations are discussed in more detail in this chapter But first as a prelude to any specific selection guidelines it should be stressed that selection considerations cannot be taken in isolation Many of the parameters are inter dependent To illustrate this point examples of two separate but inter linked considerations are given The first being frequency response the second being that of dynamic range Three diagrams illustrate this point 40 80 160 Lower Limiting Frequency Upper Limiting Frequency 950745e Fig 5 1 Upper limiting frequency response of 4 typical measure ment microphones of different sizes 1 g 1 4 15 and 1 5 2 Microphone Handbook Br el amp Kjeer Vol 1 Chapter 5 Selecting a Microphone Guidelines on Selecting Microphones 160 dB 396 Total Harmonic Distortion 950746e Fig 5 2 Dynamic range of the same 4 measurement microphones The lower limit is given in dB A The upper limit is given in dB at the levd at which 3 total harmonic distortion occurs 60 50 40 30 Inherent Noise dB 20 10 0 0 20 40 60 80 100 120 140 Upper limiting frequency kHz 950747
9. r preamplifiers is typically a little less than unity for example 0 997 0 025 dB Typical capacitance values for 1 2 micro phones and preamplifiers are 20pF and 0 2pF respectively These values yield an overall gain G of 0 1dB 0 025 0 125 dB 3 2 3 Correction factors K and Kg 3 3 3 3 1 Some Bruel amp Kj r measuring amplifiers and analysers display their input voltage in terms of dB re 1uV This feature can be used for determining the sound level by adding a correction to the displayed value The correction is zero dB for a microphone with a sensitivity of luV per 20uPa because LuV and 20uPa are the reference values for the levels of the displayed voltage and the sound pressure respectively This microphone sensitivity equals 50 mV per Pa which corresponds to 26 dB re 1V per Pascal microphone sensitivi ties are specified in dB relative to 1V Pa For microphones with this sensitivity the instrument will display the sound level directly in dB For microphones with other sensitivities the correction factor K defined below should be added to the display reading The correction factor K is defined as K 26 Sc o K Kg G dB where K correction factor Sc loaded sensitivity Kg open circuit correction factor G gain of the microphone and preamplifier combination The Kg factor can be found on most Br el amp Kjaer microphone calibration charts Frequency Response Introduction and Optimised respon
10. 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier measure lower signal levels depending on the type of acoustic signal By using bandwidths of 1Hz levels of 35 to 25 dB may be measured in the audible part of the frequency range According to international sound measurement standards measurements must be performed with a signal to noise ratio of at least 5dB The graph in Fig 2 19 shows how non correlated system noise adds to the sound pres sure level and increases the instrument reading 0 2 4 6 8 10 12 14 16 18 20 S N reading dB N reading dB 960238e Fig 2 19 Addition of Noise The noise of a measurement system is composed by microphone noise preamplifier noise and noise from the succeeding measurement amplifier The graph see Fig 2 20 illustrates the contribution of each of these three sources for a typical 1 microphone system The noise voltages of the preamplifier and the measurement amplifier were converted to equivalent SPL by dividing the noise voltage by the microphone sensitivity The noise from the measurement amplifier contributes to the system noise at higher frequencies where it is of same order of magnitude as the preamplifier noise At low frequencies the preamplifier noise dominates while the microphone noise is dominating from about 200Hz to 10kHz Microphone Noise The noise produced by the microphone is basicly due to thermal or Brownian move ments of the diaphragm
11. 33 Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier m C G dB 20 to Fo g dB m The magnitude and phase of a typical electrical frequency response which corre sponds to the values given in the table are shown in Fig 2 17 and Fig 2 18 respec tively Magnitude dB Frequency Hz 950893e Fig 2 17 Magnitude of the dectrical frequency response calculated with the values of Table 2 4 The response is flat within a wide range induding the frequency range where acoustical meas urements are most commonly performed 1M Frequency Hz 9508946 Fig 2 18 Phase of the eectrical frequency response calculated with the values of Table 2 4 The re sponse is flat within a wide range and meets the requirements for most acoustical measure ments The constant magnitude ratio in the mid frequency range is determined by the capacitance ratio first factor and by the amplifier gain g Their product is named G in the Bruel amp Kj r calibration literature and on calibration charts The low frequency roll off is determined by the input circuit and the high frequency roll off by the output circuit 2 34 Microphone Handbook Br el amp Kj r Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier 2 4 3 Even if the preamplifier output resistance may be sufficiently low to withstand a certain cable load the signal may in extreme cases be distort
12. 5 110 52 6 3 16 S41 zin de 3 17 3 12 raes sj E OE 3 17 COnN a e E VIN OR aL nU 3 17 E a TONE AN nantes 3 17 SNOET SOD E e I VR EEE ER A TR 3 17 0 2 Microphone Handbook Br el amp Kj r Vol 1 Contents 3 43 Reversible Changes TENNIS TREE 3 18 Ened o Temperature sina annan ansa annann Again nn 3 18 Effect of Ambient Pressure nennen nenne nnn nnn nnn nnn nnne nnn nna nna 3 19 EN TETTE TETTE 3 21 capped qm digo MET m 3 21 ened o Magnete F elg 3 21 Electromagnetic Compatibility ST IR 3 21 4 Characteristics of Preampliflers s 4 1 4 1 Introduction to Characteristics of Preamplifiers eeeeeeeeeneeenennn 4 2 Definition of a Microphone Preamplifier eese 4 2 Selection of a Microphone Preamplifier esses 4 2 Contents of this Chapter kk REKA anna ann 4 2 4 2 Frequency ico 605 a eee ee ee ee eer 4 2 Low erases sts E LL LOU 4 2 High Frequency 0 os tc 4 3 4 3 ROBLEDO 4 5 Upper limit DYR 4 5 iic ggg ib y OOE asrorini nnne EEEE E RAEE 4 6 MUN ET E e E E 4 6 Maximum Slew Rate sss ssssnesnrssnrssrrnsrrrrrnrrrnrssnnrrnnrrnsrrnnnnrnnrrnerrnnnnnnnnrnnrrnennnnnnnn EE 4 7 Lower Limit of Dynamic RA 4 8 4 4 FD BOUM ronen REO 4 11 4 5 Effec
13. Microphone and Preamplifier 8 16 31 5 63 125 250 500 1k 2k 4k 8k 16k 31 5k Frequency Hz 951155 1e Fig 2 20 Equivalent leves of inherent noise 1 3 octave bandwidth produced by the denents of a system equipped with a 1 freefidd microphone 50 mV Pa The microphone the preampli fier and the measurement amplifier are all significant noise sources in certain parts of the frequency range Microphone System Noise Noise spectra valid for microphone systems equipped with free field microphones of different sizes are shown in Fig 2 22 For smaller and less sensitive microphones in sizes such as 1 4 and wg only the preamplifier noise needs to be taken into ac count as this dominates over the microphone noise System With Extremely Low Inherent Noise A specific method of lowering microphone system noise can be applied for the de sign of a system with extremely low inherent noise The method is based on the idea of reducing the diaphragm damping resistance as this makes up the main noise source of the microphone This will lower the noise but it will also create a peak on the frequency response at the diaphragm resonance frequency H owever this peak may be equalized by an electrical network which can be combined with Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier Sound Pressure Level re 20 Pa dB Microphone and Preamplifier
14. Very often artificial ears are used up to such high frequencies that standing waves occur in the coupler Depending on the coupler cavity shape it may be quite complicated to determine the influence of the microphone It is simpler in cases where the coupler makes a cylindrical cavity with the sound source and a microphone is mounted at each end In this configuration the coupler acts as an acoustic transmission line which connects the source to the microphone see Fig 2 25 The transfer function and the microphone influence may then be determined by the general transmission line theory by taking its character istic impedance its complex propagation coefficient and its length into account see Fig 2 26 a At lower frequencies where standing waves do not ocaur the circuit may be further simplified to take only the compliances into account They represent the reciprocal BE 1447 11 Microphone Handbook 2 43 Vol 1 Chapter 2 Microphone Theory Microphone Types Dedicated to Different Sound Fields Coupler Cavity Coupler Telephone Microphone Diaphragm Diaphragm 950939e Fig 2 25 Schematic diagram of coupler used for headphone testi ng Z source source TT C coupler Psource Fig 2 26 Moda of headphone coupler using acoustic transmission line upper circuit and simplified model lower circuit Only the first mode should be used at high frequencies The microphone is represented by its complex impeda
15. a general understand ing of preamplifiers and to support the information given in Volume 2 of this hand book Frequency Response The frequency response of a microphone preamplifier typically covers a range from a few hertz and up to approximately 200 kHz which is far greater than the audible range of humans Within most of this range the amplifier acts as a buffer with a near perfect flat frequency response The responses at the extremities of the fre quency range are more complex and are described in Section 2 4 and here in more detail under separate headings Low Frequency Response Usually the low frequency response of a microphone and preamplifier combination is determined by the static pressure equalisation system of the microphone There is however another important factor to be considered The electrical low frequency Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Frequency Response response of the microphone capacitance in combination with the input impedance of the preamplifier This electrical response at low frequencies is determined mainly by the highpass filter R C circuit created by the capacitance of the connected microphone Cm and the input impedance of the preamplifier Ri giving the 3dB cut off frequency A 1 The typical electrical low frequency responses are normally shown with capacitanc es equivalent to common 15 and 1 microphones typi
16. acoustic and electric elements properly under the condition that all elements are given in equivalent units See the table below Acoustic Parameter Unit Equivalent Electric Parameter Volume Displacement Table 2 1 LES Acoustic and Equivalent Electric Parameers used for modelling of condenser mi crophones Microphone designers may use very complex models which take many design de tails into account and allow calculation of their influence on the response For users or designers of acoustic systems that indude microphones simpler models are gen erally sufficient Models which describe the acoustic diaphragm impedance as a function of frequency are frequently applied for determining the influence of the microphone on the sound pressure of narrow channels or closed cavities such as couplers used for calibration of microphones or earphones BE 1447 11 Microphone Handbook 2 25 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design The equivalent electric circuit shown in Fig 2 13 and the indicated component val ues explained in Table 2 2 form a model of a microphone The example shown corresponds to a microphone with a diaphragm system resonance of 10 kHz which is critically damped Quality factor Q 1 like the diaphragm of a pressure pressure field microphone 1 Ca La Ls Rs Cc Diaphragm o I 9 aaa f 41 gt 3 Ry C Acoustic Pressure C3 Electric Terminals Equalization Termi
17. all be obtained in practice and they are utilized in microphones designed for different purposes BE 1447 11 Microphone Handbook 2 21 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design The critical damping b quality factor 1 is used for pressure pressure field microphones while the high damping c quality factor 0 316 is used for free field microphones see 2 5 3 and 2 5 4 Low damping corresponding to the upper curve on Fig 2 10 a quality factor 10 is used by Bruel amp Kjae for a microphone with extremely low inherent noise Microphones dedicated to the various types of sound field are discussed later in this chapter The influence of the diaphragm diameter on the sensitivity and on the frequency response is illustrated by Fig 2 11 For the calculation of these curves the dia phragm tension thickness and quality factor Q 1 were assumed to be constant The applied parameters correspond to those of typical one 1 1 2 and 4 micro phones The results show that the flat frequency range is extended upwards when the diaphragm diameter becomes smaller The upper operation frequency is inverse ly proportional to the diameter while the sensitivity is proportional to the square of the diameter Real microphone specifications confirm this Microphone types of equal diameter may have a different sensitivity and frequency range In practice this is especially the case for 4 2 microphones The
18. also be encountered n such a case it may be necessary to use a probe microphone However this type of microphone does not have the highest upper limit of dynamic range Alternatively where it is possible to use more conventional externally polar ised microphones the upper limit of dynamic range of these microphones can be extended slightly by reducing the polarisation voltage to 28 volts 5 2 4 Microphone Venting As the microphone is only able to withstand a fairly small static pressure difference between the front and the rear side of the diaphragm it is important to consider the choice of side or rear vented microphone in relation to measurement situation There are basically two different types of static pressure equalisation channels rear vented where the microphone is vented through the rear and via the preamplifier and side vented where the pressure equalisation channel is located on the side of the microphone housing just in front of the thread for the protection grid Rear vented microphones have the advantage that they can be used with a dehu midifier However for certain applications it is necessary to use a side vented type of microphone with its pressure equalisation close to the diaphragm This is impor tant when the microphone is flush mounted in an air duct where there is typically a large static pressure difference between the inside and the outside of the duct 5 2 5 Phase Response Phase response should be consider
19. and as an electret Bruel amp Kj r found that this solution would not work for a high quality measurement microphone and decided to separate the two func tions of the foil in the new Bruel amp Kj r design Thus the electret was placed on the top of the back plate In this way the electret material could then be selected for optimal charge stability while the highly stable metal diaphragm used for the existing microphones could be maintained Information about the stability characteristics of Bruel amp Kj r Prepolarized Micro phones may be found in Section 3 11 of this handbook 2 3 5 Diaphragm and Air Stiffness The microphone sensitivity is inversely proportional to the stiffness of the dia phragm system which therefore must be carefully controlled The dominating part of the stiffness is due to the mechanical tension of the diaphragm which is stretched like the skin of a drum The higher the tension the higher the stiffness and the lower the microphone sensitivity A pressure sensing microphone has an internal air filled cavity which is generally formed by the housing the insulator and by the diaphragm Ideally there is no sound pressure in this cavity therefore an external pressure will displace the dia phragm and produce an electric output signal However due to the diaphragm dis placement a minor sound pressure is in practice produced in the cavity This pressure reacts against the external pressure and reduces the d
20. and free field reciprocity calibration mechanical engineering for example when controlling small mechanical toler ances electrical engineering such as frequency analysis and capacitance measure ments environmental testing such as measurement of resistance to humidity and tem perature tests Development skills and knowledge are also applied in research into the optimum choice of materials and to devise effective forms of testing microphones before they go into full production These tests include resistance to shock vibration tempera ture humidity and in the case of preamplifiers resistance to electromagnetic fields is also tested Bump tests in which the microphone is subjected to repeated knocks BE 1447 11 Microphone Handbook 1 5 Vol 1 Chapter 1 Introduction Development of Microphone Products Fig 1 4 Anechoic chamber used for measurement of freefidd re sponse simulate everyday use while shock tests reproduce the possible effect of impacts received in transport typically up to the equivalent of 1000 m s Additionally mi crophones are tested for severe impacts by a drop test from a height of one metre onto a wooden block The tested microphone must show less than 0 1dB variation in sensitivity after the fall As temperature and humidity are both factors which pose the greatest threat to the performance of condenser microphones pre production microphones are thoroughly tested for resist
21. and is thus a function of the absolute temperature In connection with microphone design and selection for specific purposes noise estima tion or analysis may be made in a very practical way by using equivalent circuit models Acoustic resistances which produce noise like electrical resistances make the sources of the microphone while the reactive circuit elements do not produce any noise There are two sources resistances in a microphone see the equivalent circuit model shown in Fig 2 13 The diaphragm damping resistance and the static pressure equalisation resistance The noise pressure produced by an acoustic resist ance is defined by the formula below p JAk T R Af Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier Time average of noise pressure Boltzmanns Constant Nm K 1 38 x 10 23 A Acoustic Resistance Pas m or Ns m Frequency Bandwidth 1 Table2 5 Parameters used for calculation of noise pres sure produced by an acoustic resistance The pressure of each of the acoustic resistances may be calculated by using the above formula but their contribution to the microphone noise depends also on the transfer function from the source location inside the microphone to the output ter minals In practice only the diaphragm damping noise is significant as the noise produced by the pressure equalisation vent is lower than that of the preamplifie
22. applied for testing of earphones or calibration of microphones They also occur in most types of sound level calibrator and in pistonphones The pressure field conditions of small cylindrical couplers have been carefully ana lyzed because such couplers are specificly used for primary calibration of micro phones by the reciprocity technique This technique which can be used to achieve very high calibration accuracy is applied by most national calibration laboratories Microphone calibrations performed in small couplers are pressure field calibrations but under certain circumstances they may be applied for microphones which are to be used in other types of field Pressure field calibrations are widely applied as well defined pressure fields are rather easily produced 2 1 6 Free field A free sound field or just a free field may be created where sound waves can propagate freely i e in a continuous medium without any disturbing objects In this document it is considered that a free field is made up by a plane wave which propa gates in one defined direction Microphone Free field Sensitivity refers to this type of field Ideal free fields are difficult if not impossible to realise However in practice free fields which are applicable for instrument verification and calibration may be creat ed either in anechoic rooms or outdoors away from reflecting surfaces A small sound source point source may create a satisfactory plane wave at t
23. correct reading of the measurement display or out put voltage If the frequency of the calibrator is so high that the sensitivity in the pressure field and free field environments are not the same it should be noticed that sound calibrators always establish a pressure field As a result a free field calibration can only can be performed by applying a suitable correction 6 6 5 Actuator method The electrostatic actuator is well suited for calibration of a relative frequency re sponse of microphones with a metallic or metalised diaphragm It consists of a metallic grid positioned dose to the diaphragm approx 0 5 mm By applying 800 VDC and 100VAC to the actuator electrostatic forces equivalent to a sound pressure of approximately 100dB re 20 micropascal are established The actuator is not suited for absolute calibration due to the extreme dependency of the equivalent sound pressure level on the distance between actuator and diaphragm Microphone Handbook Br el amp Kjaer Vol 1 Chapter 6 Calibration Calibration Methods 6 6 6 The equivalent sound pressure is only applied to the diaphragm and not to the ambient pressure equalisation vent so that the response measured at low frequen cies only applies when the vent is not exposed to the sound field The actuator method is a reliable method for determination of the microphones relative frequency response under laboratory conditions The response obtained with the actuator
24. does not however correspond to any of the acoustical sound fields Although there is no difference between the actuator response and the pressure field response at low frequencies and only minor difference at higher frequencies for microphones with high impedance diaphragms It should however be noted that different types of actuators will give slightly different frequency responses even for the same microphone The frequency responses of the microphone in pressure random and free sound fields are determined by adding actuator and microphone type specific corrections to the individual actuator response The disadvantages of the method are that care is required to position the actuator that the system uses high voltages and that the grid must be removable Insert Voltage Calibration Method Insert voltage calibration is a technique which can be used for two purposes l In calibration laboratories it is used to assess the open circuit sensitivity of microphone cartridges 2 t can provide a convenient means for checking in the field the electrical sensi tivity of a complete sound measuring system including preamplifiers and cables However the method does not account for the mechanical parameters which determine the acoustical properties of the microphone cartridge itself The method requires a special preamplifier that can isolate the microphone housing from the preamplifier housing This makes it possible to apply an electrical sign
25. due mainly to their stability and accurately documented performance pa rameters Fig 1 2 Microphone Types 4180 1 2 and 4160 1 are used by DPLA as highly stable reference microphones for labora tory calibrations Further innovations ensued notably in improvements to electret processes during the 1970s resulting in the production of stable prepolarised microphones which became standard for use with sound level meters The 1980s brought further devel opments in particular in the field of sound intensity measurement The mid 1980s also saw the development of specialised types of probe microphone This microphone made use of a revolutionary and now patented tube system which gives a flat BE 1447 11 Microphone Handbook 1 3 Vol 1 Chapter 1 Introduction Historical Background to Microphone Development at Bruel amp Kj r frequency response and allows for measurements in places where access for stand ard measurement microphones is difficult Improvements have continued into the 1990s with the introduction of highly accu rate yet robust microphones the Falcon Range and a microphone unit specially designed for permanent outdoor use These microphones have proven their ability to perform effectively in harsh measurement environments Throughout the history of Bruel amp Kj r a collaborative approach with customers has led to improvements in the design performance and reliability of measurement microphones In addition B
26. el amp Kj r microphone products are supported by a wealth of information and services In addition to this handbook information about microphones is provided in technical journals product data sheets and application notes Training in different aspects of acoustics can be provided Bruel amp Kj r also offers a range of accredited calibration services for calibration of microphones and associated equipment BE 1447 11 Microphone Handbook 1 11 Vol 1 Microphone Handbook Vol 1 Br el amp Kjaer Chapter 2 Microphone Theory BE 1447 11 Microphone Handbook Vol 1 2 1 Chapter 2 Microphone Theory Sound Levels and Sound Fields 2 1 Sound Levels and Sound Fields This chapter provides the background required to understand the properties speci fications and use of condenser measurement microphones The main subjects are the principle of operation the main design parameters how the design parameters influence the properties of microphones the principle of how microphones interact with different types of sound field the need for different microphones that are designed for free field pressure field and diffuse field measurements related subjects including microphone modelling diaphragm impedance and electrostatic actuator calibration 2 1 1 Sound Field Parameters Sound pressure has always been the sound field parameter of the greatest interest because sound pressure i
27. good signal to noise ratio between the attenuated calibration output signal and the signal produced by the acoustical background noise If available use filters for the measurement of the test signal to improve the repeatability of the test meas urement This will lead to more stable results For a t4 microphone 15 pF the attenuation ratio will be in the range of 35dB to 40 dB This means that for a test signal of 10V an output of 180 mV to 100 mV is obtained To estimate whether background noise has an influence on the results it is worth noting that a test signal of 10V using a microphone with 50mV Pa sensi tivity corresponds to a sound pressure level of more than 100 dB There are no spedal requirements for the long term stability of the test level provided that the ratio between the output and input voltage is determined BE 1447 11 Microphone Handbook 4 19 Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Techniques Test Frequencies Even if the system has the possibility to measure over the entire frequency range it is recommended to limit the amount of data by only using two test frequencies Use one in the mid frequency range for example 1000 Hz and one at a low fre quency for example 20 Hz The measurement at low frequency has a higher sensitivity to possible problems caused by humidity leakage whilst the mid frequency measurement monitors the stability of the mic
28. in the distortion level Contrary to this the microphone exhibits an almost linear relation ship between sound pressure level and distortion see Chapter 3 for details There fore the contribution of the preamplifier to the total distortion can practically be ignored until one of the mentioned limits occur BE 1447 11 Microphone Handbook 4 7 Vol 1 Chapter 4 Characteristics of Preamplifiers Dynamic Range 50 Total Supply Voltage 120 V 40 Vnus 10 Total Supply Voltage 28 V 200 k Frequency Hz 950309 1e Fig 4 3 Maximum output voltage vs frequency showing the influ ence of preamplifier limitations caused by a maximum supply voltage b slew rate The curves for different ca ble lengths and loads are all caused by current limita tion 4 3 5 Lower Limit of Dynamic Range The lower limit of the dynamic range of the preamplifier is set by the inherent noise The noise is primarily generated by two independent sources resistor noise Np and transistor noise NET hese two noise sources can be considered as being located in the passive and active part of the preamplifiers input circuit respectively as shown in Fig 4 4 950393 2e Fig 4 4 Schematic diagram of preamplifier showing noise sourc es The Np noise is the thermal noise created by the high value resistors in the input circuit R Thermal resistor noi
29. main physical difference between the existing high sensitivity 50mV Pa and low sensitivity 12 5 mV Pa types is the tension in their diaphragms which the designer may select within certain limits 100 1k 10k Frequency Hz 100 k 950794 Fig 2 11 Magnitude of frequency responses pressure The curves are valid for models of microphone with critical damp ing and different diaphragm diameters relative scale 1 a 0 5 b 0 25 c The numbers chosen for the calcula tion approximate the parameters of existing 1 5 and 1j 4 microphones The two upper curves of Fig 2 11 also illustrate typical sensitivities and frequency responses which are obtainable for 1 2 microphones by using different diaphragm The peak on the frequency response of the low noise microphone is equalized by an electrical network Microphone Handbook Bruel amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design tensions The phase characteristics which correspond to the above magnitude char acteristics are shown in Fig 2 12 30 degrees 950905e Fig 2 12 Phase of frequency responses pressure The curves are valid for modes of microphone with critical damping and different diaphragm diameters rdative scale 1 a 0 5 b 0 25 9 The numbers chosen for the calculation approximate the parameters of existing 1 1 5 and t mi crophones The above curves were worked out by using a simple mathemat
30. mass of the diaphragm system These two parameters determine the diaphragm resonance frequency which sets the upper limit of the microphones frequency range The fact that the microphone sensitivity is also a function of the stiffness makes the stiffness an especially im portant design parameter The stiffness is mainly due to mechanical tension in the diaphragm which is perma nently stretched like the skin on a drum The mass is partly composed of the diaphragm mass itself and partly by the mass of the air in the narrow slit behind the diaphragm Even if the physical air mass is low in comparison with the dia phragm mass it is important as the air moves with a much higher velocity than the diaphragm The energy required to accelerate the air mass is therefore of the same order of magnitude as that required by the diaphragm mass The effective mass of Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design the diaphragm system is thus significantly greater than that of the diaphragm itself Typically the air mass makes between 10 and 50 of the total system mass As the air mass varies with ambient pressure it changes the frequency response at high frequencies A microphone with a large fraction of diaphragm mass should be selected for applications where large pressure variations occur for example in div ing tanks as the frequency response of such microphones changes less with a
31. on Actuator Calibration eeeeeeeeeee ener ener nnn nnn 2 58 2 8 dva qe 2 59 3 Characteristics of Microphones ae CN Introduction to Characteristics as i sa 3 2 The Calibration Chart and Diskette sseesceeeeeeeee enne nennen nnn nnns 3 2 3 2 a a 3 3 0 iriiri eee ee ee 3 3 Esze cape aena rn 3 4 Bo genase pei sre K ANO denn 3 5 3 3 di s lqedii s SR eee ee ee ee 3 5 Introduction and Optimised response eeesseesseeeeeeeee nennen nennen nenne nnns 3 5 Es 12 fe RS IN 3 6 ulace ie dii t nanna a aaa ETT 3 7 Electrostatic Actuator RESPONSE a a nn 3 7 Free field ReSponSe OTT 3 9 Random Incidence Response oseeesseeesseeee seen nenne nnn nnn nnns annia nnn 3 9 FI a eris etii tipos iras a n dE Enea D v prvR S xd Rd dMa i ines 3 9 3 4 Direcional TR 3 10 a Bi chalet TETTE PD DUE 3 11 inherent Pa T RN I SS A LA a arai idadi ii 3 11 Maximum Sound Pressure LENDI uscito eite a Rhe E Pea pio eR ker demas 3 12 3 6 Equivalent Diaphragm ds IE IM LEOTE ETT 3 13 3 7 assu ic V AUTE senen a a E E A E EEEE 3 13 3 8 dla NEM MK ee 3 13 3 9 Gar a a a E na 3 14 Externally Polarized Microphones aussdreesctusasuscunepSexva ve eveepedudastx ses E sur Eee Y resina des 3 14 g tstgritpglegosqpei T SR I e 3 15 3 10 NI
32. other plate the back plate is stationary Movements lead to distance and capacitance changes and to a corresponding AC voltage across the plates The AC voltage produced is sepa rated from the polarization voltage by a capacitor contained in the preamplifier The instantaneous value of the output voltage may be derived from the formula below BE 1447 11 Microphone Handbook 2 11 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Microphone Preamplifier Diaphragm Back plate bye d Polarization Pressure Voltage Polarization J Supply Resistor Capacitor b Amplifier 950604e Fig 2 3 Capacitive Transduction Principle The constant dectri cal charge used for polarization is supplied from an ex ternal source e P II Area of capacitor plate Instantaneous capacitance between plates o Distance between plates at rest position Displacement of moveable plate diaphragm from rest position Instantaneous voltage between plates o Polarization voltage omma g o Il Voltage change caused by plate displacement O o II Constant charge on plate capacitor Dielectric constant of air Note that the output voltage of the system is proportional to the displacement of the moveable plate This is also the case for large displacements In other words there is a linear relationship between output voltage and displacement even if the corresponding capacitance changes a
33. signal reaches a maximum level set by the supply voltage minus a voltage drop determined by the preamplifier construc tion See the specifications for the relevant preamplifier for details Clipping is responsible for the maximum output at lower frequencies To make the most of the dynamic operation range of the condenser microphones the supply voltage has to be quite high i e 100 120 V If the input signal exceeds the limits set by the supply voltage an abrupt genera tion of harmonic distortion will result The maximum sound pressure level that can be handled by the microphone pream plifier can be calculated by e k SPL peak IX 944 olog BE ja B where SPL peak peak acoustical sound pressure level in dB re 20uPa 1Pa 94dB amp eak 4 peak output voltage of the preamplifier in V S loaded sensitivity of the microphone in V Pa Do pressure level for stated microphone sensitivities 1Pa Note The peak signal is typically 3 to 10dB higher than the RMS value for a pure tone 3dB for a noise signal typically 10dB 4 3 3 Maximum Current A second limitation is the maximum output current This current is normally deter mined by the design of the output stage in the amplifier however the current capacity of the power supply could also be a limiting factor The current limitation should be considered when high frequencies long cables and relatively high signal levels are combined The relation between the maxim
34. the diaphragm This is specified either as inherent noise or as cartridge thermal noise see Section 2 4 3 for a discussion of inherent noise Because the microphone cartridge is always used with a preamplifier the inherent noise of the combination is composed of both microphone and preamplifier noise The combined noise level may be calculated by the following formulae L Mic p Mic Pre 10 x0 e PA p4CPA S p2 Mic p2 PA L Combined 10 1og 7 Pref where Ln Mic cartridge thermal noise level dB SPL pn Mic cartridge thermal noise Pa pn PA equivalent preamplifier noise pressure Ln Combined combined noise level dB SPL en PA preamplifier noise voltage V 6 Pref Prop 20 10 Pa BE 1447 11 Microphone Handbook 3 11 Vol 1 Chapter 3 Characteristics of Microphones Dynamic Range 3 5 2 The noise data L Mic and e PA are given in the respective type specifications It is important to note that the noise levels must be for the same frequency band widths Maximum Sound Pressure L evel The dynamic range of condenser microphones is so great that for most practical situations the user will not encounter any limitation due to the maximum permissi ble sound pressure level The maximum output of the microphone is limited by the displacement of the dia phragm To ensure correct operation the microphone should not be exposed to sound pr
35. three ratios are then used to solve three equations with three unknowns the sensitivities of the microphones To obtain reliable results dean and stable environments are required Practice at performing the calibration is also important The method is described in detail in EC 1094 1 Pressure Calibration and IEC 1094 3 Free field Calibration Substitution method The substitution method involves the measurement of a device in a given measure ment set up The device is replaced substituted by a reference device preferably of the same type and the measurement is repeated Thus the ratio of the sensitivities of the devices are obtained directly This method is well suited for both microphones and sound pressure calibrators If the measurement and reference objects are of the same type the measurement uncertainty is reduced due to identical measurement conditions The measurement capability requirements are also reduced as only a small part of the dynamic range iS used i e there is no need to know the absolute level Any sound source used for the calibration of microphones must obviously be very stable during the measurement and must not be affected by differences in micro phone configurations In this case the method is well suited for calibration of free field microphones provided that a suitable reference microphone is available This method is often confused with the comparison method described below 6 6 3 Comparison Method
36. 10k Frequency Hz 100k 950605e Fig 3 3 The frequency response curve is composed of an individual high frequency response and a typical low frequency response The curve is normalised to OdB at the reference frequency For a full discussion about types of microphone designed for different sound fields see Section 2 5 3 3 2 Low Frequency Response In general the data related to the low frequency response given in the calibration data is typical for the type of microphone cartridge at the reference ambient pres sure 101 3 kPa only The 3dB point on the low frequency response is proportional to the ambient pres sure in practice the influence is insignificant because the lower limiting frequency is below the usual frequency response of interest Note that the stated low frequen Cy response is valid when the diaphragm and static pressure equalisation vent are exposed to the sound field Microphone Handbook Br el amp Kjaer Vol 1 Chapter 3 Characteristics of Microphones Frequency Response For rear venting microphones the venting takes place through the preamplifier housing In practice the low frequency response of a measurement system is often deter mined by other factors such as the electrical lower limiting frequency of the micro phone preamplifier combination or by a high pass filter in the conditioning amplifier All Bruel amp Kj r microphones are tested acoustically to ensure that the lower limit ing
37. 1447 11 Microphone Handbook 2 23 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design The above explanation shows that the microphone size the sensitivity and the fre quency range are tied together and cannot be selected separately Very often the user must make a compromise when selecting a microphone Generally small mi crophones work to higher frequencies and create less disturbance in the sound field but they also have lower sensitivity and higher inherent noise and thus may not be usable at the lowest sound levels of interest 2 3 9 Microphone Sensitivity The sensitivity of a measurement microphone may either be expressed in Volts per Pascal V Pa or in decibels referring to one Volt per Pascal dB re 1V Pa The term sensitivity generally means the sensitivity at the Reference Frequency which is most often 250 Hz but in some cases this may be 1000Hz The magnitude of the frequency response characteristic represents the ratio between the sensitivity at a given frequency and that of the Reference Frequency This ratio is generally ex pressed in dB The sensitivity of a microphone depends on the type of sound field Sensitivity is therefore generally referred to in terms of Free field Diffuse field or Pressure field sensitivity see 2 1 5 to 2 1 7 and 2 5 However at the reference frequency the sensi tivity is essentially equal for all types of sound field It should be noted that pressurefidd sensitiv
38. 37 BE 1447 11 Microphone Handbook Index 3 Index Nose cone 5 8 5 9 Nyquist and J ohnson Noise 2 26 Output impedance 4 4 Phase Response 4 11 Preamplifier and microphone combination 2 O 31 Open Circuit Sensitivity 3 3 ha a a ORE Open circuit sensitivity 3 3 6 7 MU au M e Optimised response 3 5 e Output resistance 2 32 Effect of magnetic fields 4 13 Prepolarised microphones 2 14 Pre polarization principle 2 11 P Prepolarized microphone 2 13 Parallel capacitors 2 43 Prepolarized microphones 3 15 5 6 Particle velocity 2 2 Pressure coefficient 3 19 Passive capacitance 2 40 Pressure equalisation 5 5 Phase match 6 7 Pressure Field Microphone 2 45 Phase response 5 5 Pressure sensing condenser microphones 2 5 Polar plot 3 10 Pressure sensitivity 2 4 POlarisation Pressure Fiel 2 43 resistor 2 11 Pressure field 2 4 Polarisation 5 5 Pressure field calibration 2 4 field strength 2 14 Pressure field correction 3 10 Polarisation resistor 3 14 Pressure field microphone 2 48 2 50 2 Polarisation Voltage 3 14 59 Electret 3 14 Pressure field response 3 9 Polarisation voltage 2 10 2 14 Primary calibration laboratories 6 6 Resistor 3 14 Properties of microphones 2 7 Polarization voltage 2 10 2 14 2 35 Protection grid 2 46 2 58 Polarzation 2 11 R phe ana IA Rain cover 5 8 Built in preamplifier 5 8 Raincover Ba Calibration Technique 4 15 Rendons cs mcd a Random Incidence response 3 9 pane Mi e Rear vented micro bones 2 16 E
39. 5 6 Types 0 to3 5 6 Static Pressure Equalization 2 16 Stray capacitance 2 8 2 41 3 13 Substitution method 6 11 System Noise Spectra 2 40 5 6 5 6 T Temperature 2 51 2 52 5 7 Effect of 3 18 effect of 1 6 Testing environmental 1 5 shock temperature and humidity 1 5 Thermal resistor 4 8 Transduction principle 2 11 Transduction principles 2 11 Turbulence screen 5 8 Turbulent pressure 5 10 Type of Sound Field 5 4 BE 1447 11 Microphone Handbook Index 5 Vol 1 Index Index 6 Microphone Handbook Br el amp Kjaer Vol 1
40. 7 Equivalent Volume of the Diaphragm System eeeeseeeeneeee nnne 2 28 2 4 Combination of Microphone and Preamplifier eeeeeeeeeeeeeee 2 31 Villas spa SL ts a 5 6 rf c 2 31 Electrical Frequency Response of Microphone and Preamplifier 2 32 Inherent Noise of Microphone Systems 2 35 Witusgio OTT T senemnenend 2 39 2 5 Microphone Types Dedicated to Different Sound Fields 2 43 Influence of the Microphone on the Pressure F ield 2 43 BE 1447 11 Microphone Handbook 0 1 Vol 1 Contents Pressure Field Microphone and Sensitivity ccecccccscceeseceeseeeeesseeeseeeeesseeeessaass 2 45 Influence of Microphone on the M easured Sound Pressure in a Free F ield 2 45 Free Field Microphone and Sensitivity eeseeeeeeeeeeeenennnene nnn 2 48 Influence of the Microphone on the M easured Sound Pressure in a Buc E M 2 49 Diffuse Field Microphone and Sensitivity sessa 2 50 2 6 Long Term ia sce 2 51 d Electrostatic Actuator Calibration cm 2 54 Aigat anie p TS RT RT HH 2 54 SA RS e ad ML 2 55 Electrostatic GANA c 2 55 Electrostatic Actuator Operation sta anna ia be vods eva Ue oos eraimc des 2 5 7 Actuator and Pressure field Responses sera 2 58 General Note
41. Combination ll Q Preampli o rophone O Frequency Hz 940718e Fig 2 21 Third Octave Noise Spectra of a system consisting of a free fidd microphone 1 2 50mV Pa and a high quality preamplifier The eectrical preamplifier noise dominates at low fre quencies while Brownian movements of the microphone diaphragm create the most signifi cant noise at high frequencies The columns to the right show Linear 20Hz 20 kHz L and A weighted A noise leas for the microphone M preamplifier P and combination C the preamplifier This technique which requires careful control with the micro phone resonance frequency and Q factor has lead to the development of a system with an equivalent inherent noise level as low as 2 dB A see Fig 2 23 2 4 4 Distortion Both the microphone and the preamplifier may distort the output signal The dis tortion which is produced by the microphone is the dominating component within the major part of the dynamic range while preamplifier distortion may only be detected dose to the dipping limit at the highest operational levels BE 1447 11 Microphone Handbook 2 39 Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier 40 dB re 20 uPa 1 4 4 mV Pa 8 16 31 5 63 125 250 500 1k 2k 4k 8k 16k 31 5k 63k Frequency Hz 950941e Fig 2 22 Third Octave Noise Spectra of free field
42. IVC technique The system is based on a relative measurement of the capacitance of the microphone cartridge which is a reliable indicator of the microphones condition 4 8 1 Insert Voltage Calibration Insert Voltage Calibration is a standardised method of determining the open circuit voltage and open circuit sensitivity of a transducer Insert Voltage Calibration is a substitution method where the inserted voltage substitutes the open circuit voltage produced by the transducer see chapter 6 for more details Insert Voltage Calibra 4 16 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Techniques tion can be used to monitor a complete acoustic system but it only checks the function of the preamplifier cable and the conditioning amplifier Therefore Insert Voltage Calibration is not the best choice as it does not give information about the condition of the microphone As an example a short circuited microphone would create a change of about 0 1dB with an Insert Voltage Calibration system while it would be hundreds of times greater with the charge injection calibration method described in the following section 4 8 2 Charge Injection Calibration The Bruel amp Kj r patented Charge Injection Calibration CIC technique enables a complete measurement chain to be verified induding the microphone As the name implies the method uses frequency independent inje
43. L evel Meters IEC 1183 The random incidence corrections for Falcon range microphones have been calculat ed according to IEC 1183 1993 using the free field corrections from 0 to 360 inci dence in 5 steps For other types of Bruel amp Kj r microphone 30 steps are applied according to IEC 651 The random incidence response is determined by adding the random incidence cor rections to the actuator response 3 3 7 Pressure field Response The pressure field response of a microphone refers to uniformly distributed pres sure on the diaphragm This response is often regarded as being equal to the actuator response because the difference between them is small compared to the uncertainty related to most meas BE 1447 11 Microphone Handbook 3 9 Vol 1 Chapter 3 Characteristics of Microphones Directional Characteristics urements The difference is due to the radiation impedance which loads the dia phragm during the actuator response measurement see Section 2 7 The pressure field response may be determined by adding the pressure field correc tion to the individually measured actuator response The pressure field corrections are determined during the intensive analysis that is part of the development of each particular Br el amp Kj r microphone type The pressure field response is measured using the reciprocity method according to the IEC 1094 2 standard This method is the most accurate calibration method availabl
44. O PELUT E eM Reciprocit ri method 6 10 Electromagnetic noise 4 15 Ref p j 294 Frequency Response 4 2 buque ee Gain 3 4 Reference frequency 3 6 High frequency response 4 3 emen inf input impedance 2 32 environmenta influences 2 6 Low frequency response 4 2 ANE 22 Lower limit of dynamic ran 4 8 PCI TOIT FS y ge Resistance 2 32 2 35 2 36 Maximum acoustical signal 4 6 i Damping 2 38 OKUNA CUERE o Reversible changes 3 18 Maximum output current 4 6 RF sianal 4 15 Maximum output voltage 4 6 9 Maximum slew rate 4 6 Noise level 4 12 S Noise spectrum 4 9 Selection Index 4 Microphone Handbook Br el amp Kjaer Vol 1 Index Dynamic range 5 2 U Frequency response 5 2 5 4 Uncertainty 6 2 Type of sound field 5 2 Use of Microphone 5 3 Sensitivity 2 9 2 24 2 45 2 48 2 50 3 3 6 2 6 10 V Ambient pressure 3 19 Diffusefield 2 24 Side vented microphones 2 16 Slew rate 4 4 Small signal response 4 4 Vent position 2 17 Vibration Effect of 3 21 Sound field parameters 2 2 W Sound intensity 2 2 WE640AA microphone 1 3 Sound intensity calibrator 1 4 Wind speed 5 9 Sound intensity microphones 1 4 Wind 5 9 Sound Level Calibrator 6 4 Windscreen 5 8 5 10 Sound pressure 2 2 Sound pressure calibrator method 6 12 Sound pressure level 2 2 2 24 Stability 2 51 3 17 Ageing 1 10 Electret 2 15 Short term 3 17 Temperature 2 54 Variation 1 10 Stability factors 2 52 2 54 Stablity 2 9 Standards ANSI 1 12 Compliance IEC 651
45. Polarisation Voltage Response dB 2 5 Fig 3 7 The sensitivity of the cartridge is essentially proportional to the charge see Fig 3 8 1k Hz 10k 100k Frequency Hz 9406066 Example of the frequency response dependency with polarisation voltage The curves are normalised to the nominal value of 200V This example is for a 7 5 microphone with high sensitivity The dependency is less for 1 6 microphones with lower sensitivity A reduction in polarization voltage and hence sensitivity can be a practical expedi ent to avoid overloading preamplifiers in situations where there is a combination of high levels high frequencies and long cables Bruel amp Kj r instruments supply a positive polarization voltage for externally polar ized microphones This type of microphone therefore produces a negative voltage for a positive pressure 3 9 2 Prepolarized Microphones Prepolarized microphones contain a stable charge in the electret layer on the back plate see Section 2 3 4 All Br el amp Kj r prepolarized microphones are negatively charged They therefore produce a positive voltage for a positive pressure If an external polarisation voltage is accidentally applied to the prepolarized micro phone no permanent harm is done However the sensitivity is significantly reduced by 10dB or more as long as the extern
46. Preamplifiers Effect of Temperature Degrees Frequency Hz 950965e Fig 4 8 Phase responses for two different types of preamplifier A first order input circuit gives a 90 phase shift at low frequencies whereas a second order input circuit bootstrap gives a 180 phase shift The latter can be optimised for very low phase shift for frequencies above 10 Hz materials that allows them to be operated at high temperatures H owever there are always some limitations that should be taken into consideration Firstly at high temperatures the failure rate of the device increases At tempera tures above 100 125 this increase is dramatic and the preamplifier should only be used at such temperatures for short term measurements Another consideration concerns the output power which causes internal heating of the semiconductors In combination with high external temperature this heat can destroy the output transistors The preamplifier can deliver the specified current but if this is done for a long time at a high temperature the heat will destroy the preamplifier If it is necessary to measure at temperatures which exceed the specified values guaranteed by Bruel amp Kj r and if a lower maximum output can be accepted the preamplifier supply voltage may be reduced to decrease the risk of overheating the circuitry In such circumstances the use of a lp preamplifier is recommended as preamplifiers of this size are more efficient at los
47. Stability Factor is the ratio between the stability at a given temperature and that of the Reference Temperature The long term stability is given by the following formula Longrerml Longrerml rer l F stability where SLongTerm T Long Term Stability at the temperature T SLongTerm Tref Long Term Stability at the reference temperature T reg Stability Factor ratio between stability at the temperatures T and Teg F stability T The Stability Factors of the Stainless Steel and Nickel diaphragms are shown in Fig 2 32 and 2 33 respectively Two Stability Factors which are valid for the electret used by the Prepolarised Microphones are shown in Fig 2 34 The factor that is valid for dry air uses 150 C as the Reference Temperature while that valid for humid air 90 Relative H umid ity uses 50 C as the reference The stability at the Reference Temperature should be found in the specification that is valid for the type of microphone Microphone Handbook Br el amp Kj r Vol 1 Chapter 2 Microphone Theory Long Term Microphone Stability Stability Factor 100 150 200 L 300 Temperature C 950898e Fig 2 32 Stability Factor for stainless st l alloy diaphragms Fal con microphones To obtain the stability at any tempera ture multiply the reference stability of 150 C see microphone specification by the stability factor 1070 Stability Factor 2 50 100 150 200 250 3
48. Technical Documentati on Microphone Handbook Vol 1 Theory Bruel amp Kj r WORLD HEADQUARTERS DK 2850 N rum Denmark Telephone 44542800500 Telex 37316 bruka dk Fax 44542801405 e mail info bk dk Internet http www bk dk BE 1447 11 Microphone H andbook Volume 1 J uly 1996 Bruel amp Kj r Microphone Handbook BE 1447 11 Vol 1 Copyright 1996 Bruel amp Kj r A S All rights reserved No part of this publication may be reproduced or distributed in any form or by any means without prior consent in writing from Bruel amp Kj r A S N rum Denmark Microphone Handbook Br el amp Kjaer Vol 1 Preface This volume provides background information on the Bruel amp Kj r microphone product range It gives an insight into the theory behind the development of microphones and pream plifiers and explains the terminology used to describe these products The aim of this volume is to promote a full understanding of measurement microphones and to provide sufficient background information for customers to get the best out of these prod ucts It also gives adequate information for customers to be able to make informed and qualified decisions about the microphone products which are most suitable for their meas urement requirements These products are described in detail in more specific literature such as Volume 2 of this handbook and in product data sheets Chapter 1 gives a brief hi
49. Techniques Disconnected or Broken Cable eje 0 dB 40 dB Fig 4 16 Input signal Hz 960271e Disconnected or broken cable The output leve will change significantly depending on the distance between the conditioning amplifier and the break in the cable A cable broken near to power supply B cable broken near to the microphone Microphone Handbook Vol 1 Br el amp Kjaer Chapter 5 Selecting a Microphone BE 1447 11 Microphone Handbook Vol 1 5 1 Chapter 5 Selecting a Microphone Guidelines on Selecting Microphones 5 1 Guidelines on Selecting Microphones When selecting a measurement microphone it is important to first understand the measurement requirement and how this imposes demands on the performance of the microphone This is necessary because although measurement microphones are precision instruments that are optimised for particular measurement tasks they still offer a wide operational range In fact such is the versatility of Bruel amp Kj r microphones that the user may be tempted into a that one will do philosophy when selecting a microphone simply because a microphone comes within the required general performance parameters If however the user has a good understanding of the measurement requirement then it is possible to choose the optimum microphone for the measurement task in hand For most uses the type of sound field is the main parameter to consider
50. Therefore the phase responses of these two types of microphone differ by 180 degrees Positive pressure leads to positive voltage for the prepolarized microphones while it becomes negative when external polarization is used The electret acts as a series capacitor for the active air gap capacitance This ex plains why a prepolarized microphone has a capacitance which is typically 20 25 lower than that of a corresponding microphone with external polarization Prepolarized microphones which are generally more complex than ordinary con denser microphones are mainly intended for use with battery operated and hand held instruments Because this type of microphone does not require a polarization voltage it is often selected by instrument designers who can save space and power and make their preamplifiers simpler Either of the two types may be selected if it fits with the instruments available However the polarization voltage switch if any must be set correctly For normal microphone types it should be set to the specified voltage in most cases 200 V and for the prepolarized types to OV This will place the output terminal of the micro phone on ground potential Those who make their own preamplifiers and power supplies for Prepolarized Mi crophones should also be aware of this fact and connect a resistor of 1 to 10GQ across the microphone terminals This will eliminate the possibility of any unwant ed charge which might otherwise
51. al the insert voltage directly to the microphone diaphragm housing In the calibra tion the microphone is first subjected to a sound pressure of a known level and frequency say 94 dB at 1kHz This causes the microphone to generate an internal voltage V corresponding exactly to the open circuit microphone voltage which when loaded by the preamplifier produces an output voltage V at the preamplifier output The sound source is then switched off and the insert voltage V4 of the same frequency is applied such as the internal reference voltage of a measuring amplifi er The level of the insert voltage is adjusted so that the voltage measured at the preamplifier output is again V Provided that this voltage V is noted when the microphone and preamplifier are used remotely from the measuring equipment or if for other reasons it is conven ient to apply a direct sound pressure to the microphone the insert voltage method can be used to adjust the sensitivity of the equipment This will provide a system calibration which relies only on the value of V remaining constant BE 1447 11 Microphone Handbook 6 13 Vol 1 Chapter 6 Calibration Calibration Methods Sound source stopped If V is same in both cases then If the microphone were not loaded by a preamplifier the open circuit output voltage would be Vo V4 Vy microphone open circuit output voltage Insert Voltage V applied in series with microphone Preamplif
52. al polarization is sustained The resulting BE 1447 11 Microphone Handbook 3 15 Vol 1 Chapter 3 Characteristics of Microphones Leakage Resistance 5 10 20 50 100 200 500 Polarization Voltage V 950608e Fig 3 8 Change in sensitivity as a function of polarisation volt age at 250 Hz polarisation voltage is the sum of the external and internal polarization voltages which are of opposite sign 3 10 Leakage Resistance Leakage resistance is the electrical resistance between the centre terminal and the housing of the microphone For several design reasons preamplifiers use high resis tors for the polarization voltage supply Therefore to avoid a reduction of the polari zation voltage and so ensure accurate measurements the leakage resistance of the microphone must be at least a thousand times greater than the polarization voltage resistance of the preamplifier This requirement must be fulfilled even under severe environmental conditions for example in conditions of high humidity and high temperature These high values impose stringent demands on design materials and production of microphones Br el amp Kj r microphones are tested to these requirements at 9096 relative humidity Note that the surface of the microphone insulator should not be exposed to contamination for example from dust and hair as this may lead to a low leakage resistance If microphones produced to lower leakage resistance requirem
53. alibration is a solid and definable reference The next definition from the European Space Agency also supports this Calibration A comparison of two instruments or measuring devices one of which is a standard of known accuracy traceable to national standards to detect correlate report or eliminate by adjustment any discrepancy in accu racy of the instrument or measuring device being compared with the stand ard This definition uses the word comparison to explain how something unknown is referred to or compared with something known However the reference is clearly defined as a measurement instrument that is high in the measurement chain and traceable to national standards Why Calibrate A calibration is performed To be sure of making correct measurements To prove that measurement methods and equipment are accurate for example to prove that a measurement complies with the requirements of national legisla tion standards bodies and customers To verify the stability of the measurement equipment including equipment used to perform calibrations To account for local measurement conditions for example variations in ambient pressure and temperature To ensure product quality To build confidence in measurement results Summary Calibration is to determine the sensitivity of a measurement device The fundamental units of measurement volt ampere etc provide the ultimate reference for measurements allowing
54. amp Kj r have invested in fully equipped EMC test facilities Certain tests are conducted at external accredited testing laboratories Bruel amp Kj r products fulfil the toughest generic EMC stand ards for both emission and immunity These standards are EN 50081 1 Generic emission standard Residential commercial and light industry En 50082 2 Generic immunity standard Industrial environment Detailed EMC specifications are given in the Product Data sheets for all Bruel amp Kj r products Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Techniques Cause and Cure of Electromagnetic Introduced Noise The reasons for the sensitivity of devices to electromagnetic noise are often quite simple to identify but the effects can be difficult to avoid The connection cable can pick up signals from the electromagnetic field by acting as an antenna while the semiconductors in the electronic circuit act as rectifiers and demodulate the AM signal from the RF carrier frequency It is then very difficult to separate this demodulated signal from the measurement signal The best solution is to prevent the RF signal reaching the semiconductors This can be done in different ways The most common method is a lowpass filtering of the signal going into the electronic circuit It is of course important that both ends of the cable are connected to devices that are able to a
55. an 10 of the nominal thickness The diaphragm and the front of the back plate form the plates of the active capaci tor which generates the output signal of the condenser microphone see below This capacitance which is typically between 2 and 60pF 10 F depends mainly on the diameter of the back plate The stray capacitance or the passive capacitance be tween the back plate and the housing is kept as small as possible as this makes an undesired load on the active capacitance The back plate is connected to the exter nal contact which together with the housing make the concentric output terminals of the microphone An alternative microphone design is widely applied by Bruel amp Kj r This patented design employs an integrated backplate and insulator see Fig 2 2 In contrast to the first mentioned conventional design of microphone Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design which is mainly assembled by screwing the parts together the integrated back plate and insulator version is assembled by pressing the parts into each other This de sign also deviates from the conventional design by applying a backplate consisting of a metal thin film placed directly on the surface of the insulator Diaphragm Diaphragm Backplate Backplate Housing Insulator Insulator Housing Fig 2 2 Cross Sectional view of microphone types The classic type left is assembled by screwi
56. ance at 10 kHz 2 3 11 Acoustic Impedance of Diaphragm System In cases where microphones are used in small cavities and narrow channels it may be necessary to evaluate and correct for the influence of the microphone on the acoustic system This is usually done by using a model which is even simpler than that shown above in Fig 2 13 The very simple model shown in Fig 2 14 may only be used if the diaphragm not the static pressure equalization vent is exposed to the sound pres sure and if the microphone is essentially unloaded on the electrical terminals Val ues of the circuit elements which correspond to those of the more complex model are given in Table 2 3 Such values are often stated by microphone manufacturers for the purpose of impedance calculations The values of the diaphragm system ds elements may be calculated from those given in Section 2 3 10 by using the formulae below Ca Cc Cas ELEC Lgs La Ls Ras Rs BE 1447 11 Microphone Handbook 2 27 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Acoustic Terminals Fig 2 14 Simplified mode representing the acoustical impedance of a microphone Some manufacturers state the moda parameters for relevant microphone types Diaphragm System Compliance 2 83 x 10719 m9 N Diaphragm System Resistance 56 2 x 109 Table 2 3 Values of the simplified microphone moda shown in Fig 2 14 The mode represents the acoustic diaphragm impedan
57. ance to these influences typically in temperatures from 20 to 70 C and in humidity of up to 90 at 40 C Finally microphones are also tested for resistance to corrosion as proven by the most recent range of condenser micro phones which have been found to be very robust in harsh measurement environ ments It is these skills Knowledge and experience acquired over more than 50 years of development work that allows the Bruel amp Kjaer to handle all the tasks necessary to develop new products from first specification to final calibration Microphone Handbook Bruel amp Kj r Vol 1 Chapter 1 Introduction Development of Microphone Products 950339 1e Fig 15 Microphone undergoing shock tests BE 1447 11 Microphone Handbook 1 7 Vol 1 Chapter 1 Introduction Development of Microphone Products Fig 1 6 Aerial Photo of the Br d amp Kjaer Headquarters Narum Denmark 1 8 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 1 Introduction Production of Microphones at Bruel amp Kj r 1 3 Production of Microphones at Bruel amp Kj r Microphones are precision instruments and while the design of a conventional measurement microphone may appear to be quite simple its production must be very precisely controlled to meet specified tolerances Such tolerances impose great demands on the materials and construction methods used yet the products created must be extremely reliable and rob
58. ances have existed for many years for example through electrostatic discharge lighting transients and mains voltage fluctuations The recent interest in EMC however is due to the huge growth in the use of electronic equipment from microcomputers operating at radio frequencies to mobile telephones using pulse modulation Such devices emit electromagnetic noise at ra dio frequencies which can interfere with equipment that has insufficient immunity At the same time electronic circuits are being integrated in all sorts of electrical appliances from washing machines to burglar alarms making these products more susceptible and vulnerable to electromagnetic disturbance The following sections give a short introduction to EMC requirements with an emphasis on those aspects relevant to microphone preamplifiers 4 7 1 The European EMC Directive Requirements and limits regarding the emission of radio frequencies have existed for many years Until recently little attention was paid to the fact that delicate electronic circuits are easily disturbed by strong external signals In addition to the emission requirements the European Union EMC directive now makes immunity a mandatory requirement The directive states that electrical and electronic equip ment must be sufficiently immune to disturbances of various kinds All electrical and electronic devices must comply with the EMC directive in order that they can legally be sold in Europe 4 7 2 The CE
59. ation voltage 3 9 Polarisation Voltage The working principle of the condenser microphone is based on a fixed charge This charge is established either with a very stable external polarisation voltage via a large resistor or by an electret layer deposited on the backplate The two methods are described below See also Section 2 3 4 for background theory on the polariza tion voltage and the transduction principle 3 9 1 Externally Polarized Microphones Each type of microphone is designed to operate with a specific polarisation voltage at which the specifications are valid The microphone should be allowed to charge properly before a measurement is started In most cases 30 seconds will be suffi cient but this depends on the microphone capacitance and the polarization resistor of the preamplifier The polarisation resistor is kept high typically 10 GQ to ensure a large time constant which will not affect the lower limiting frequency of the microphone and preamplifier combination The microphone should not be operated at polarisation voltages higher than the nominal value as this may result in excessive leakage or even arcing both leading to an unstable situation If the microphone is operated at polarisation voltages lower than the nominal value the sensitivity and frequency response will change see the example shown in Fig 3 7 and Fig 3 8 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 3 Characteristics of Microphones
60. cally 6pF 15 pF and 50 pF While the microphone capacitance is a rather simple and well defined prop erty the preamplifier input impedance can be more complex Normally the input impedance consists of a resistive impedance of approximately 20 GQ in combination with small stray capacitances of about 0 2 pF In conjunction with the microphone this results in a 1 order highpass filter as mentioned above with a minor capacitive attenuation of the microphone signal Some preamplifiers use a boot strapping technique where there is feedback from the output signal to the input of the amplifier This reduces the alternating voltage across the components resulting in lower input currents and subsequently in a higher impedance This technique can be used to achieve a very low cut off frequen Cy as well as good phase linearity Note that with some preamplifiers which use a bootstrapping technique a gain peaking at frequencies under 20Hz can occur In combination with low frequencies and perhaps inaudible signals this can result in an overload of the system Fig 4 1 shows typical electrical low frequency responses for two different types of preamplifier one with a simple resistive input impedance and one with a complex input impedance obtained by bootstrapping Also shown in Fig 4 1 is a solution for lowering the electrical cut off frequency of the preamplifier This is done by simply adding stray input capacitance to the preamplifier Special ada
61. ce of the microphone moda shown in Fig 2 13 The diaphragm system impedance Zqs may by calculated by using the formula bel ow M Lys joc eL ast Ras Ns m 2 3 12 Equivalent Volume of the Diaphragm System The acoustic impedance of a microphone diaphragm may be expressed in terms of equivalent volume The equivalent volume which is complex is a function of fre quency At low frequencies it is essentially real and constant while it varies with frequency at high frequencies where the imaginary part dominates see Fig 2 15 The use of equivalent volume instead of diaphragm impedance often makes it more easy to evaluate the influence of a microphone on the sound pressure of small acoustic systems such as calibration coupler cavities The equivalent volume of a diaphragm is the volume of air which has the same compliance or impedance as that of the diaphragm The acoustic compliance C4 of a volume of gas is given by the following formula 2 28 Microphone Handbook Br el amp Kj r Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design V where y Ratio of specific temperatures of the gas 1 402 for air Ps Static pressure of the gas in the cavity V Volume of cavity Accordingly the compliance of a diaphragm system Cg is where Ve Equivalent diaphragm volume The formula for calculation of equivalent diaphragm volume from diaphragm sys tem impedance is derived from these equations and
62. ce ratio 2 34 CE label 4 13 Charge Injection Calibration 4 15 6 7 Charge injection calibration 1 4 6 5 Charge injection calibration method 6 14 BE 1447 11 Microphone Handbook Index 1 Vol 1 Index CIC 6 14 Comparison Method 6 11 Correcti on Angle of incidence 2 47 Microphone body 2 48 Correction factor 3 5 Corrections 2 45 2 58 Coupler 2 4 2 43 6 10 D DANAK 6 9 Danish Primary Laboratory of Acoustics DPLA 1 5 Dehumidifier 5 10 Design 2 7 Description 2 7 Design parameters 2 7 Diameter Sensitivity 2 22 Diaphragm 2 8 Air stiffness 2 15 Damping resistance 2 21 2 38 Diameter 2 21 2 22 Frequency range 2 23 Mass 2 20 Tension 2 9 2 20 tension 1 10 2 51 thickness 2 8 Diaphragm material 2 9 Diffuse Field 2 49 Diffuse Field Microphone 2 50 Diffuse sound field 3 9 Diffuse field 2 5 correction 2 49 Diffuse field correction 2 49 2 50 2 54 Diffuse field measurements 2 2 Diffuse field response 2 58 Directional characteristics 3 10 Distortion 2 39 2 43 Microphone Distortion 2 40 Preamplifier and microphone system 2 42 Distortion level 3 11 DPLA 6 6 6 8 Dynamic Range 3 11 Dynamic range 2 31 2 32 3 11 5 2 Limits 5 4 E Electret 2 13 2 15 Series capacitor 2 14 Electrical resistance 2 36 Electromagnetic compatibility 3 21 4 13 Electrostatic Actuator 2 54 2 57 Pressure field response 2 58 Electrostatic actuator 2 55 6 12 Phase response 3 7 Response 3 7 Electrostatic Calibration Pressure 2 55 Ele
63. ction of charge into the micro phone and preamplifier input circuit The patent includes the measurement method and the practical realisation of a high quality stable capacitance which is built into the preamplifier The main applications are the monitoring of remote microphones and microphone arrays The principle of operation is shown in Fig 4 11 The built in capacitance is very small typically 0 2 pF This small capacitor makes an attenuator together with the impedance of the combined microphone and preamplifier input circuit A voltage supplied at the CIC input will be attenuated and can be monitored at the pream plifier output The ratio between the output and input voltages can be used to monitor the stability of the whole measurement system including the microphone preamplifier and cables A simplified formula valid in the mid and high frequency range is given in Fig 4 11 A more general formula is given here o Ce 1 al M8 4 l amp C C c 9 ORC FET where e output voltage e input voltage Cc CIC capacitance C input capacitance of preamplifier Cm capacitance of microphone R input resistance of preamplifier g preamplifier amplification 1 As indicated by the formula and as shown in Fig 4 12 the method can be used to monitor the preamplifier input resistance at low frequencies and the microphone capacitance in the mid and high frequency range BE 1447 11 Mic
64. ctrostatic calibration pressure 2 55 Electrostatic pressure 2 55 EMC 4 13 EMC requirements 4 13 EN 50081 1 4 14 En 50082 2 4 14 Environment 5 7 Equivalent diaphragm volume 2 29 Equivalent electric circuit 2 26 Equivalent electric circuits 2 25 Equivalent sound pressure 3 12 Equivalent Volume 2 28 3 13 Equivalent volume 2 31 Diaphragm diameter 2 31 External polarisation source 2 11 Externally polarised microphones 3 14 Externally polarized microphones 5 6 E Falcon Range microphones 5 7 Falcon range microphones 1 4 FET noise 4 9 Field Effect Transistor 4 9 FreeField 2 45 Freefield 2 4 Free field correction 2 49 2 54 3 9 Free Field Microphone 2 48 Free field microphone 2 22 2 38 2 48 2 59 Freefield response 2 49 3 9 Free field sensitivity 2 4 Frequency Response 5 4 Frequency response 2 9 2 14 2 17 2 32 2 48 5 2 6 7 Electrical 2 33 2 34 free field 2 48 Magnitude and phase 2 22 2 26 pressure 2 37 Index 2 Microphone Handbook Bruel amp Kjaer Index Pressure field 2 45 Frozen charge 2 13 H Handling 3 17 High frequency 2 11 2 21 High frequency response 3 7 High frequency roll off 2 34 Highpass filter 4 3 Humidity 5 7 5 9 5 10 Effect of 3 21 IEC 1043 6 8 6 12 IEC 1094 5 7 IEC 1094 1 6 7 IEC 1094 1 2 7 6 10 6 11 IEC 1094 2 3 10 IEC 1094 3 6 11 IEC 1094 4 2 7 3 6 IEC 1183 2 5 3 9 IEC 1183 1993 3 9 IEC 651 3 6 5 7 IEC1043 2 6 IEC 68 2 3 3 21 Impedance analo
65. ctuator stands on isolating studs on the microphone housing in front of the diaphragm The distance between the actuator and diaphragm plates is usually between 0 4 and 0 8mm The actuator is perforated in order to minimize its influ ence on the acoustic pressure produced by the diaphragm when this is displaced by the electrostatic pressure see Fig 2 35 Fig 2 35 Electrostatic actuator used with a microphone When an electrical voltage is applied between the parallel actuator and diaphragm plates a uniformly distributed electrostatic force is produced across the diaphragm The diaphragm reacts on this electrostatic force or pressure as it would react on an equally strong sound pressure 2 7 3 Electrostatic Calibration Pressure The equivalent sound pressure produced by an electrostatic actuator may be de fined by considering the fact that the stored energy on an electrical capacitor may be expressed either in electrical or in mechanical terms This leads to the following equation BE 1447 11 Microphone Handbook 2 55 Vol 1 Chapter 2 Microphone Theory Electrostatic Actuator Calibration where e A zi E electrical voltage between the plates C electrical capacitance between the plates F force between the plates d plate distance e dielectric constant of the gas between plates air 8 85 x 10 14 F m A plate area Rearranging the parameters leads to the following equivalent sound pressur
66. d due to resonances in the room and due the sound absorbtion of the air In cases where diffuse fields are to be used for technical purposes the influence of these effects may be reduced by applying more than one sound source and by mounting reflecting panels which are moved continuously in order to vary the dominating room resonances Sound fields with a close resemblance to a diffusefield may be found in environ ments such as factories where many simultaneous sound or noise sources exist or in buildings with hard walls for example in halls or churches 2 2 Measurement Microphone Requirement Today there are many different areas of application for microphones including tele communications broadcasting recording of music consumer electronics and acous tic measurements For each field and application a certain set of microphone properties is required To meet the needs of a specific microphone application the designer can choose between a number of different acoustic operation and transduction principles A microphone may either sense the pressure the pressure gradient or the particle velocity These may then be converted to electrical signals in several different ways Practically all precision measurement microphones are pressure sensing condenser microphones that use a constant electrical charge for convertion of the diaphragm displacement into an analog electrical signal Pressure sensing condenser microphones are used because th
67. d calibration devices Microphone calibrations are usually performed at calibra tion laboratories on a regular basis As with field calibration this provides evidence of stability and ensures traceability Standards relating to calibration usually define laboratory recalibration intervals of l year which apply to both the calibrator and the measurement system for exam ple sound level meters In other cases the recalibration interval is determined by the users estimate It is advisable to start with a 1 year interval which can then be extended when sufficient evidence of stability is obtained The mode of use of the device should always be taken into account when determining recalibration inter vals the harsher the mode of use the more frequent the recalibration should be because of the increased probability of changes in the performance of the device Not surprisingly the most accurate calibration methods are the most difficult and most time consuming and correspondingly the most expensive Different calibration and test laboratories use different calibration methods However for the customer it is not the method which is so important but the accuracy or uncertainty see Section 6 3 stated by the calibration laboratory and of course the price It is also important for the customer to consider whether traceable calibration see Section 6 5 is sufficient or whether an accredited calibration is required 6 4 1 Primary Calibration Labora
68. d microphones for diffuse field measurements are rare A main reason for this is that the diffuse field correction or the pressure change created by the micro phone itself is so small that many pressure field microphones also have good dif fuse field characteristics Diffuse field characteristics which comply with standards such as ANSI S 1 4 may also be obtained by mounting certain devices on free field microphones These devic es which at high frequencies increase the pressure at the diaphragm might either replace the protection grids or be mounted on them 2 50 Microphone Handbook Vol 1 Chapter 2 Microphone Theory Long Term Microphone Stability 2 6 Long Term Microphone Stability To obtain valid measurements all the instruments applied must be stable and relia ble induding the microphone However the microphone may be exposed to harsh environments outside of the normal environments where the analyser and record ing instruments are generally kept In particular high temperatures imply a risk of a permanent change in sensitivity Depending on the type of microphone there are two parameters which can be ex pected to change One is the mechanical tension of the stretched diaphragm and the other which is only relevant for prepolarised microphones is the electrical charge of the electret Both may decay over the time Carefully controlled artificial or forced ageing procedures are therefore applied during production to s
69. deviation may lead to a measurement error unless a correction is made The ratio between the pressure at the diaphragm and that of the undisturbed sound field is a function of the ratio between the microphone diameter and the wavelength Pressure ratio functions look alike for smaller and larger microphone types but they are shifted within the frequency range depending on the diameter of the microphone body This is illustrated in Fig 2 28 which shows the order of mag BE 1447 11 Microphone Handbook 2 45 Vol 1 Chapter 2 Microphone Theory Microphone Types Dedicated to Different Sound Fields nitude of the pressure ratio valid for sound incidence perpendicular to the front of the microphone zero degrees for different sizes of microphone For most practical measurements the microphone diaphragm needs to be covered by a protection grid This grid will also influence the pressure at the diaphragm as it acts as an acoustic resonator The influence of protection grids is generally very low below 1kHz but it increases significantly with frequency Fig 2 29 shows a typical contribution of a 15 protection grid to the pressure increase at the microphone diaphragm zero degree incidence Undisturbed Sound Field Sound Propagation Direction Measurement Point Disturbed Sound Field Microphone Sound Propagation Direction Fig 2 27 Simplified illustration of the disturbance of a plane sound wave caused by a microp
70. different measurements to be compared The accuracy or uncertainty of the calibration must be known Traceability is necessary but not sufficient Calibration creates confidence in the measurement result Devices throughout the measurement chain must be reliable and stable within a known uncertainty BE 1447 11 Microphone Handbook 6 3 Vol 1 Chapter 6 Calibration Calibration of Microphones 6 2 Calibration of Microphones 6 3 This section explains the common terms and methods used in connection with the calibration of measurement microphones The calibration can either be performed in the field or in a calibration laboratory These calibration situations are described here under separate headings Field Calibration Field calibration is performed at the measurement location using a calibrated refer ence sound source such as a pistonphone This ensures the traceability of absolute sound level measurements The fact that the measurement is traceable is important if the measurement must be recognized by legal authorities or if compliance with international standards is daimed See Section 6 5 for a discussion on traceability It is advisable to perform a field calibration before and after a measurement for example by using a sound level calibrator This gives confidence in the measure ment result by verifying the stability of the entire measurement system Calibra tion before and after a measurement is also somet
71. ds to the threshold of hearing at 1000Hz for a young person with normal hearing ability The pressure has no direction and is thus a scalar 2 1 3 Particle Velocity and Particle Velocity Level The Air Particle Velocity or just Particle Velocity v is the velocity of a small volume of air partide The dimensions of the volume regarded should be very small in comparison with the wave length The unit of partide velocity is metres per second m s The partide velocity depends on the sound pressure and on the sound field conditions Today the recommended 1801683 reference for Particle Velocity Level is one nano metre per second 1nm s or 10 m s However fifty nano metres per second 50nm s is also used as this number was the commonly applied reference in the past To prevent any misunderstanding the reference value should therefore be stated together with velocity measurement results The Partide Velocity Level Ly is defined by the formula below 2 V V Ly 10 log 20 LE where Vre is Lnm s alternatively 50nmys V re ref Particle Velocity and Particle Velocity Level generally refer to the Root Mean Square RMS value of the velocity This is considered if no reference is stated specificly In the propagation direction of a plane progressive sound wave the veloc ity level is practically 34dB above the pressure level when the reference velocity is Lnm s while it is approximately equal to the pressure level if 50nm s is appli
72. e 3 4 Directional Characteristics The directional characteristic is the relative sensitivity variation as a function of angle of incidence This can be represented graphically by polar plots see Fig 3 5 The same information is in principle given in the free field correction curves The difference in the representation is that the free field corrections are given at fixed angles of incidence while the directional characteristics are given at fixed frequen cies The directional characteristic is relevant if the microphone is used for free field measurements UL PH HSN ee ese D e M RS i HARE AAS Fig 3 5 Typical directional characteristics for a microphone normalised to 0 incidence 950600e 3 10 Microphone Handbook Br el amp Kj r Vol 1 Chapter 3 Characteristics of Microphones Dynamic Range 3 5 3 5 1 The directional characteristics of a 1 microphone will resemble those shown in Fig 3 5 but at half the frequency Accordingly the frequency is multiplied by two or four to represent a 1 4 or t g microphone respectively Dynamic Range The dynamic range specified for the Bruel amp Kj r microphones is limited by e the equivalent inherent noise level dB SPL which defines the lower limit e the 396 distortion level dB SPL which defines the upper limit Inherent Noise The inherent noise of the cartridge is determined by the thermal movement of
73. e Fig 5 3 Graph showing relationship b wen upper limiting fre quency range and inherent noise The 4 dots represent the 4 sizes of microphone in order from 1 bottom left to Ig top right The above figures show that if for example measurements at very high frequencies are required this will automatically limit the choice of microphone to one with a fairly high lower limit of dynamic range Once the optimum microphone has been selected in terms of primary selection pa rameters there will most probably be several other considerations which are rele vant to the measurement requirement for example the physical robustness of the microphone for the measurement environment This can be illustrated by compar ing two different types of microphone A T microphone for laboratory standard calibration and a Pd microphone for general use may appear to have similar performance parameters on paper but actually have quite different physical characteristics The mechanical design of the laboratory standard microphone makes it very well suited for pressure reciprocity calibration but the almost unprotected diaphragm makes this microphone far too fragile for general use BE 1447 11 Microphone Handbook 5 3 Vol 1 Chapter 5 Selecting a Microphone What to Consider With this background the following section gives some of the main considerations when selecting a microphone 5 2 What to Consider 5 2 1 Frequency Response Altho
74. e acting on the plates where p electrostatic pressure When a DC and a sinusoidal AC voltage are applied between the plates i e be tween the actuator and the microphone diaphragm the following expression is ob tained for the static and dynamic pressure components 2 1 cos2ot 2 2 Pstatic j qj 0 Sms P ynamic 23 215 Eg GmsSiN ot ej s cos20t where Eg applied DC voltage peak peak value of applied AC voltage Ge rms value of applied AC voltage epa 2 Dctatic produced static pressure Pdynamic produced dynamic pressure O angular frequency t time The equivalent sound pressure produced by the actuator at the frequency of the applied electric signal may be derived from the above formula However it should be taken into account that the area of the actuator due to its perforation makes Microphone Handbook Br el amp Kj r Vol 1 Chapter 2 Microphone Theory Electrostatic Actuator Calibration only a certain fraction of the diaphragm area The simulated sound pressure is therefore reduced accordingly and defined by the following formula e E rms rms 2 area Pdynamic where Rare F ratio between actuator and diaphragm areas typically 0 75 The ratio of the second harmonic component and that of the fundamental frequency becomes d _ Erms 100 96 2 4 Electrostatic Actuator Operation An actuator measurement set up is shown in Fi
75. e frequency range than that of a free field response microphone This is something that can be observed by comparing the random inci dence response of a pressure field with that of a free field microphone using the graphs in Volume 2 of this handbook See also chapter 2 Section 2 5 for an explana tion of how different types of microphone are dedicated to different sound fields 5 2 3 Limits of Dynamic Range The lower limit of dynamic range is dictated by the inherent noise of the micro phone and preamplifier combination The upper limit of dynamic range dictated by the maximum sound pressure level 396 total harmonic distortion Due to the very wide dynamic range of the microphones it is normally either the lower or the upper limit of dynamic range that is of interest Microphone Handbook Br el amp Kjaer Vol 1 Chapter 5 Selecting a Microphone What to Consider An example application for measurement microphones where the lower limit of dynamic range has to be considered is that of the determination of sound power of light armatures for use in office environments Levels as low as 10 12dBA may have to be measured An example application for measurement microphones where the upper limit of dynamic range has to be considered is that of measurements of exhaust systems on engine test cells Here high sound pressure levels 140 150dB may be encoun tered However in addition to high sound pressure levels high temperature may
76. e microphone is equipped with a static pressure equalisation vent The vent which is a narrow air channel ensures that the static pressure of the internal cavity follows the pressure of the environment If no vent were present the static pressure changes might create large and disturbing signals over loads of amplifiers and might significantly displace the diaphragm from its proper working position This would result in a malfunction or significant sensitivity changes The tiny vent channel from the internal cavity leads either to the side or to the rear of the microphone which is named accordingly side vented or rear vented see Fig 2 5 For certain specific applications it is important to select the appropriate type see Section 2 3 7 and Chapter 5 The vent has to be very carefully controlled to equalise the static pressure varia tions without suppressing low frequency components of the acoustic pressure which are to be measured As the nature of these pressure variations is the same this may not always be avoided Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Pressure Equalisation Pressure Equalisation 960291e Fig 2 5 Side and Rear Vented Microphones 2 3 7 Low Frequency Response and Vent Position The time constant of the microphone s pressure equalisation system is typically 0 1s This is a good practical compromise as the equalisation is genera
77. e relative positions of the mechanical parts However these changes will generally not exceed 0 1dB and are therefore negligible for most applications The short term stability of a microphone is very high once the microphone is acdli matised and used in stable ambient environments Usually the microphone will sustain the sensitivity within 0 01 or 0 02dB under such stable environments This is important when the microphone is used as a laboratory reference 3 13 Reversible Changes Reversible changes occur as a result of environmental influences Correction factors are available for the three main influences temperature ambient temperature and humidity 3 13 1 Effect of Temperature The sensitivity of the microphone is only slightly affected by the ambient tempera ture It is usually not necessary to compensate for this influence unless the micro phone is subjected to very high or very low temperatures After quick changes in temperature the microphone should be allowed to acclimatise for at least 15 min utes at the ambient conditions to ensure correct operation 50k Ok Frequency Hz Fig 3 9 Example of reversible changes Typical variation in O incidence freefidd response normalised at 250Hz as a function of temperature relative to the response at 20 C Bruel amp Kj r specify a temperature coefficient at 250Hz and graphs of sensitivity variations as function of temperature These can be used to compensate for the dev
78. easurand To determine the sensitivity is to calibrate the measurement de vice All the information that follows in this chapter is based on this definition For a transducer sensitivity is measured in terms of units of electrical output volts ampere etc per unit of the physical input parameter pressure acceleration distance etc In line with this convention the sensitivity of a microphone is gener ally given in terms of volts per pascal Pascal is defined as one Newton per square metre N m i e a unit of pressure These fundamental units provide a fixed reference for the calibration of a measure ment device This reference is essential as it allows measurements including cali brations to be compared measurements which could have been made by different people in different locations under different conditions The units must be referred to in a known and agreed way This is well defined and monitored on an interna tional basis The accuracy of the calibration must also be known i e the device and method used to calibrate a microphone must perform the calibration with a known uncertainty If these conditions are fulfilled the calibration is called traceable because the cali bration can be reliably traced back through the measurement chain ultimately to the fundamental units of measurement The first or highest link in the measure ment chain is normally a device in a primary calibration laboratory since these establishments us
79. ectra shown in Fig 4 6 as V JHz which is commonly used in electronic engineering but not in the field of acoustics where a representation as shown in Fig 4 7 is more usual Here the total noise is shown for a constant rela tive bandwidth third octave in combination with two different microphone capaci tances 20 Hz to 200 kHz Bandwidth 206 kHz 20 Hz to 20 kHz Bandwidth 23 kHz Thermal generated resistor noise Effect of increase in A Weighting Bandwidth 13 kHz La microphone capacity OD Effect of increase in temperature MONA ML N Flicker noise from FET 0 01 0 1 1 10 100 1000 Thermal resistor noise microphone capacity Effect of increase in cross coupling capacity White noise from FET 10000 100000 Frequency Hz 950961e Fig 4 6 Spectra of noise sources in preamplifiers Microphone Handbook Vol 1 Br el amp Kj r Chapter 4 Characteristics of Preamplifiers Phase Response x x x x x xc x x x x Z mo o nN MO O OMO LO N q 7 de Al T Co an Fig 4 7 Typical third octave noise spectra for preamplifiers noise connected to two different micro phones dummies 6pF and 20pF 10 uV 6 pF 20 pF 1 uV 0 1 uV 22 8 8B Co Oo oO O WO CO LO e OO cO oOo N OO O WO r QO T Tre Tr NN YO st 900 T 630 PL 900 MEME EN 10k 16k 950964e 4 4 Phase Response The phase response is the difference between t
80. ed This is valid for normal ambient conditions i e for a static pressure of 101 325 kPa a temperature of 23 C and for 5096 Relative Humidity 2 1 4 Sound Intensity and Sound Intensity L evel Sound Intensity is acoustic power per unit of area It is the power which flows through a certain area divided by the area Like the partide velocity the sound intensity is a function of the direction of the sound wave propagation The intensity is therefore a vector The unit of sound intensity is Watts per square metre W m The reference value for sound intensity is one pico Watt per square metre 10 1 W m The Sound Intensity Level is specified by the following formula L 10 log dB where Ire 10 1 Ww m re In the propagation direction of a plane progressive sound wave the sound intensity level is practically equal to the pressure level This is valid for normal ambient conditions i e for a static pressure of 101 325 kPa a temperature of 23 C and for 5096 Relative Humidity BE 1447 11 Microphone Handbook 2 3 Vol 1 Chapter 2 Microphone Theory Sound Levels and Sound Fields 2 1 5 Pressure field A pressure field is characterized by a sound pressure which has the same magni tude and phase at any position within the field Microphone Pressure Sensitivity refers to this type of field Pressure fields may be found in enclosures or cavities which are small compared to the wave length Such fields occur in couplers
81. ed by mounting a dehumidifier between the microphone and the preamplifier The function of the dehumidifier is to ensure that only dry air reaches the interior cavity of the microphone thus preventing condensation This is done by allowing air to pass to the back vent of the microphone through a cavity filled with silica gel The dehumidifier therefore only works with rear vented microphones The dehumidifier is normally used in combination with a wind screen The assem bly should be mounted horizontally in order to let possible water droplets run off the microphone diaphragm Long Term Protection For long term outdoor measurements a more comprehensive combination of acces sories is recommended consisting of a raincover a windscreen and a dehumidifier The raincover is designed to be mounted in place of the normal protection grid As well as offering rain protection it serves as an electrostatic actuator which can be employed for remote calibration A special wind screen with stainless steel bird spikes and which allows the use of the rain cover is recommended 5 10 Microphone Handbook Br el amp Kj r Vol 1 Chapter 6 Calibration BE 1447 11 Microphone Handbook Vol 1 6 1 Chapter 6 Calibration Introduction 6 1 Introduction The most important parameter for any measurement device is sensitivity The sen sitivity can be defined as the ratio of the output parameter to the input parameter the m
82. ed due to insufficient output current This is especially the case where voltages of high frequencies are transferred via long cables Possible problems may be solved by either selecting a different preamplifier or by selecting a power supply with a higher current capacity or both The output voltage of the microphone may also be lowered This may done by selecting a microphone with a lower sensitivity or by lowering the polarization voltage for example from 200V to 28V leading to a sensitivity reduction of about 17dB The polarization voltage change causes a change in the frequency response depending on the type of microphone For 1 4 and amp microphones the change is so small that it can general ly be neglected The electrical transfer function should be multiplied or added when expressed in dB to the frequency response characteristic of the microphone itself However in most cases the electrical response is flat within the frequency range of inter est Therefore it is generally sufficient to account for the gain G which is in most cases below unity The measurement of infra sound is an exception to this general rule For this appli cation proper microphones and preamplifiers must be selected because of the acoustic roll off of the microphone itself and the combined electrical roll off of the microphone and preamplifier at low frequencies The flat part of the electrical response may be extended at low frequencies by mou
83. ed to the capacitor terminal preferably with no sound on the microphone The preamplifier output is then measured Changes in the measured outputs reflect changes in the microphone and preamplifi er input combination The method is very effective for detecting small changes in microphone capacitance See chapter 4 for details Microphone Handbook Br el amp Kjaer Vol 1 ndex 4160 microphone 4180 microphone 1 3 1 3 A Accessories 5 8 Accredited calibration laboratories 6 6 Accuracy 6 2 Acoustic compliance 2 28 Acoustic impedance 2 27 2 28 Acoustic resistance 2 36 Actuator method 6 12 Air turbulence 5 9 Air stiffness 2 18 Amplifier gain 2 34 ANSI 1 12 5 7 ANSI S1 4 5 7 B Backplate 1 9 2 8 Integrated 2 8 Backplate to diaphragm distance 2 8 Bird spikes 5 8 Boltzmanns Constant 2 37 Boot strapping technique 4 3 C Calibraation Intercomparison method 6 8 Calibration 2 54 3 2 Accredited laboratories 6 6 Accuracy 6 3 6 9 Actuator 2 58 Actuator method 6 5 Definition 6 2 Field 6 4 Hierarchy 6 8 International standards 6 8 Laboratory 6 5 Measurement channel 6 12 Recalibration interval 6 6 Reference 6 11 Reference units 6 3 Sensitivity 6 3 Sound source 6 11 Traceability 6 2 6 4 6 8 Uncertainty 6 8 Calibration chart 1 2 3 2 calibration chart 1 2 Calibration equipment 1 3 Calibrator load volume 3 13 Capacitance 2 8 2 13 2 31 2 32 2 31 2 43 3 13 6 7 variations 2 11 Capacitan
84. ed when choosing microphones for sound intensi ty measurements and here it is not normally the absolute phase response that is important but the relative phase response between a pair of microphones This is because the phase response characteristics have to be closely matched Special pairs of microphones with matched phase responses are available 5 2 6 Polarisation There are two different types of microphone construction one that employs an ex ternal voltage supply to polarise the backplate to diaphragm air gap externally polarised and one where the polarisation charge is stored in an electret layer on the backplate of the microphone prepolarized BE 1447 11 Microphone Handbook 5 5 Vol 1 Chapter 5 Selecting a Microphone What to Consider Generally there are only small differences between the specifications for externally polarized and prepolarized microphones but these differences make them suitable for different purposes Prepolarized microphones are used for portable sound level meters where their lightweight and lack of a requirement for a polarization voltage supply is an obvi ous requirement Prepolarised microphones also offer slightly better performance in very humid environments see chapter 3 L eakage resistance Alternatively externally polarized microphones are generally more useful for gener al field and laboratory use and for high temperature measurements Also for spe dal measurements external
85. en into account as it would otherwise lead to a measurement error Depending on the type of field and on the type of microphone the influence may be so small that it is insignificant However it may also amount to several decibels which would be an unacceptable error for most measurements Microphone types designed for specific applications may correct for the influence under certain circumstances while in other cases the user must evaluate and if possible correct for the influence Three different types of sound field are generally considered in connection with acoustic measurements namely the pressure field the free field and the diffuse field The influence of the microphone depends on the type of sound field on the microphone dimensions and to a minor degree on its diaphragm impedance Influence of the Microphone on the Pressure Field A pressure field generally occurs in a small dosed cavity see Section 2 1 5 which for example may make a part of an artificial ear used for the testing of telephones or hearing aids To measure the sound pressure in such a cavity the microphone is generally built into the coupler in a way that makes the diaphragm a part of the coupler cavity wall Because the diaphragm is not as stiff as the wall of the coupler it deflects due to the sound pressure and loads the cavity acoustically The influence of the microphone depends on the size and shape of the coupler and on the microphone diaphragm impedance
86. ence is typically smaller The microphone types may in practice be placed in one of two groups depending on their equivalent volume which may either be about 0 010 or 0 045 cubic centimetres Those with a low volume are most frequently applied in couplers Here their influ ence is relatively less than that of the larger microphones on the above mentioned coupler sizes 2 5 2 Pressure Field Microphone and Sensitivity 2 5 3 The frequency response characteristic of a pressure field microphone is optimised to be as flat as possible when its output voltage is referred to a uniform pressure on the outer surface of its diaphragm Ideally the diaphragm should be as stiff as the walls of the coupler within which it is applied As this is not possible its influence may be estimated and taken into account by using equivalent volume or transmission line calculations Influence of Microphone on the Measured Sound Pressure in a Free Field A free field exists where a sound wave propagates in a certain direction without being disturbed by any reflecting objects see Section 2 1 6 However to measure the sound pressure in a free field a microphone must obviously be placed in that field The microphone will change the sound field as it will diffract and reflect the sound wave see Fig 2 27 As a result the pressure acting on the microphone dia phragm will deviate from that of the measurement point in the undisturbed field which was to be measured The
87. ent Microphones Standard Application IEC 651 Sound Level Meters IEC 1094 Measurement Microphones Part 1 Specifications for laboratory stand ard microphones Part 4 Specifications for Working Standard Microphones ANSI 1 12 Measurement Microphones Type L Reference Type XL As is but no outside diam eter specified Type M Sound Pressure Magnitude Type H Small diffraction errors ANSI S1 4 Sound Level Meters Table 5 1 Standards Applicable to Measurement Microphones 5 2 8 Environment When selecting a microphone the environment where measurements are to be made should be considered Some parameters which should be considered are as follows Robustness In a protected laboratory environment all measurement microphones can be used but more robust general purpose microphones are required for field use Temperature At normal temperatures 30 C to 125 C all microphones may be used At high temperatures up to 300 C Falcon Range microphones should be used and at very high temperatures above 300 C a probe microphone should be em ployed The tip of the probe on the probe microphone can withstand up to 700 C Atmosphere In normal air all microphones may be used but for measurements where corro sive industrial gasses exist for example when making measurements in indus trial chimneys the corrosion resistant Falcon Range microphones should be used Humidity In normal humidity all microphones can be u
88. ents are used with high quality Bruel amp Kj r preamplifiers there will be a risk of instability in sensi tivity For prepolarized microphones the leakage resistance is far less critical It need only be about ten times greater than the resistance of the preamplifier polarization re sistance in order not to significantly influence the lower limiting frequency of the microphone and preamplifier combination Microphone Handbook Br el amp Kj r Vol 1 Chapter 3 Characteristics of Microphones Stability 3 11 Stability The stability of a measurement microphone is a very important feature It is one of the features that distinguishes measurement microphones from other microphones Changes in the parameters of a microphone are dealt with here under Irreversible Changes and Reversible Changes 3 12 Irreversible Changes 3 12 1 Long term stability The diaphragm of a microphone has a certain high mechanical tension which will decrease as a function of heat and time A decrease in tension leads to a permanent change in the sensitivity of a microphone At normal room temperature this effect is of the order of 1dB per 1000 years At elevated temperatures the effect is accelerat ed see Section 2 6 for details 3 12 2 Handling The mechanical stability of the microphone is determined by its ability to withstand mechanical effects for example a force applied to the diaphragm clamping ring or unpredictable mechanical sh
89. es BE 1447 11 Microphone Handbook 2 31 Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier 2 4 2 Electrical Frequency Response of Microphone and Preamplifier To minimize the loading effect on the microphone as described at the start of this section the microphone is screwed directly on a preamplifier The preamplifier has a high input impedance which is generally described in terms of input resistance typically 1 100 GQ and input capacitance typically 0 1 1pF The electrical volt age gain is generally very close to unity corresponding to O dB The preamplifier should preferably withstand loading from even very long cables The output resistance is therefore low typically 10 1000 As the amplifier output is resistive and the cable loading is capacitive typically 50 100 pF m the influ ence of the loading if any will be most significant at the highest frequencies Due to the low output impedance of the preamplifier the input resistance and capaci tance of most succeeding instruments 1M 9 and 50 pF can be ignored Modern preamplifiers are able to transfer voltages within a very wide dynamic range from about 1uV to 50V i e more than 150GB The capacitance of the microphone which makes the electrical source may be re garded as being essentially constant when the microphone is used with a modern preamplifier with a low input capacitance The combined electrical circuit of the
90. essure levels exceeding the stated M axi mum Sound Pressure Level peak This corresponds to a peak output voltage of 40 to 5096 of the polarization voltage Above this level the output becomes heavily distorted clipping occurs In addition an externally polarized microphone will temporarily loose its charge and will need some time to recover However even if the maximum sound pressure is exceeded for example by up to 10dB the microphone will not suffer permanent damage provided it is used with a Bruel amp Kj r preamplifier This is because the preamplifi ers are constructed so that the discharge that occurs when the diaphragm touches the backplate does not harm the diaphragm The microphone is always used with a preamplifier therefore the limits for this should also be taken into account The maximum output voltage from the preampli fier depends on the supply voltage To utilise the wide dynamic range of the micro phone the preamplifier should be operated at a supply voltage of 100V Even so the preamplifier may be the limiting factor While the microphone has a distortion proportional to the sound pressure level 396 distortion level is stated the preamplifier has a very low distortion until a level of a few dB below the dipping point where the distortion suddenly increases The maximum output peak level from the preamplifier is usually a few volts lower than half the total supply voltage The actual supply voltages are stated in
91. evel is measured at a high frequency and the frequency is then lowered until the output is reduced by 3 dB Phase Match Intensity Microphone Pair A special acoustical sound chamber has been designed to ensure exposure of equal sound pressure level at all frequencies both at the diaphragm and at the equaliza tion vent of both microphones The phase match of the microphones is then meas ured by the comparison method using a dual channel frequency analyser The phase BE 1447 11 Microphone Handbook 6 7 Vol 1 Chapter 6 Calibration Calibration Hierarchy Traceability and Uncertainty response of the microphones are compared at all frequencies in the specified range to ensure compatibility with IEC 1043 requirements 65 Calibration Hierarchy Traceability and Uncertainty Calibration hierarchy is a representation of the links between the primary calibra tion and the succession of intervening calibrations down to the end user for exam ple a primary calibration at DPLA a secondary calibration at a service centre and a tertiary calibration by the end user Calibration Hierarchy Standards Institutes Physical Parameters Other National National Acoustic Round Robin Test Acoustic Laboratoty Laboratories Accredited Sound and Vibration Laboratories eg Calibration Services Users of Sound and Vibration Equipment 960290e Fig 6 2 Calibration Hierarchy the national acoustical laborato ries use in
92. ey detect what the human ear detects namely pressure Furthermore they can be realized with the high quality and the predictable performance that is necessary for any type of measurement device BE 1447 11 Microphone Handbook 2 5 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Requirement Even if particle velocity or sound intensity are the parameters to be determined pressure sensing microphones can be used Sound intensity and velocity are worked out from pressures measured simultaneously at two points These are typically Spaced 6 to 50mm from each other depending on the frequency range of interest The international standard Instruments for Measurenent of Sound Intensity IEC1043 defines the requirements for pressure sensing microphones and specifies microphone and system requirements As with any other type of measurement the sound pressure that is to be measured Should not be influenced by the microphone applied or if this is not possible then it Should be influenced in a controlled and known way that makes it possible to cor rect for the influence Measurement microphones influence the sound pressure especially at higher fre quencies where the wave length and microphone dimensions are of same order of magnitude However the influence which depends on the type of sound field pres sure field free field or diffuse field is thoroughly analysed for condenser measure ment microphones This is done by labo
93. frequencies are within the production tolerances This test cannot be performed by an actuator because an actuator does not produce any sound pressure at the vent 3 3 3 High Frequency Response To obtain an individual high frequency response type specific and field dependent corrections are added to the individually measured actuator response for the micro phone These actual corrections are measured during the development of each mi crophone type Fig 3 4 illustrates how all frequency responses for each of the three sound fields are obtained for the same microphone This is done by simply adding the type specific correction data to the individually measured actuator response 3 3 4 Electrostatic Actuator Response For a description of the electrostatic actuator and its operation refer to Section 2 7 Calibration by an electrostatic actuator is a convenient and accurate method for determining individual frequency responses To a minor degree the measured re sponse depends on the type of actuator applied The corrections stated in Fig 3 4 are valid only for frequency response measurements using the specified type of actuator The Electrostatic Actuator Phase Response The phase response curves shown in Bruel amp Kj r literature are normalised to zero degrees at low frequencies For all Br el amp Kj r externally polarized microphones positi ve charge the phase difference between the voltage and the pressure at low frequencies
94. from the general equation 1 Z joc The Equivalent Diaphragm System volume is thus Vir Vo where Zgs Diaphragm system impedance JOZq Ve is complex as Zg is complex The equivalent diaphragm volume may be calculated as a function of frequency by using the formula below when the previously mentioned simple C L R model see Section 2 3 11 is considered Y P Ve T EL Jo Y Cy JoL q Ra where Cy Diaphragm system compliance Lgs Diaphragm system mass Rags Diaphragm system resistance BE 1447 11 Microphone Handbook 2 29 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Alternatively the complex equivalent volume may be calculated from another set of parameters as the above equation may be written as fF ay vafi vain i 5 eigo f o where Vellf Low frequency equivalent diaphragm volume fo Diaphragm system resonance frequency Q Diaphragm system quality factor The second set of parameters may be calculated from the first set as L 1 d VAIf y P Ca fy Qn Ca Lge Q C Ra S Both sets of parameters are used in practice The complex equivalent volume is especially applied in connection with the calibration of Laboratory Standard Micro phones and in connection with measurements made on human and artificial ears The calculated equivalent diaphragm system volume of the microphone model shown in the previous section is shown in Fig 2 15
95. g 2 36 The required DC voltage is supplied via a resistor and the AC voltage is supplied via a capacitor These compo nent values should be chosen to give a lower cut off frequency which is 5 to 10 times lower than the lowest calibration frequency Their impedances should be suffi ciently high that the circuit does not create a safety risk for the user DC voltage Supply OO Actuator Microphone AC voltage Supply 5000 pF Preamplifier Measurement Amplifier Recorder Fig 2 36 Schematic diagram of measurement set up for dectro static actuator calibration BE 1447 11 Microphone Handbook 2 57 Vol 1 Chapter 2 Microphone Theory Electrostatic Actuator Calibration Typical voltages are 800V DC and 30V AC rms For a distance between the actu ator and the diaphragm of 0 4 mm and an actuator diaphragm area ratio of 0 75 an equivalent sound pressure of one Pascal or 94dB will be produced The second harmonic distortion will make 1 3 96 It is possible to operate the actuator without any DC voltage In this case the frequency of the simulated sound pressure will be twice the frequency of the sup plied electrical signal see the formula in 2 7 3 which defines the dynamic pressure However this method has two disadvantages firstly a much lower sound pressure may lead to an insufficient signal to noise ratio and secondly the pressure produced depends on the square of the supplied voltage This may increase the uncertain
96. ges The sensitivity can be obtained either as a pressure sensitivity or as a freefield sensitivity by using a coupler cylinder or an anechoic chamber respectively Certain physical requirements of the microphone must be fulfilled to perform the calibration in a coupler for example the mechani cal configuration as described in IEC 1094 1 i LL iy Kern i ae m SS i preety Ul 1 Coupler No 19 i Ll h v a frit YL ti NN HIN Fig 6 3 Reciprocity calibration Microphones are hdd face to face by a spring loaded yoke The reciproaty method is an absolute method which means that it requires the measurement of a number of fundamental physical units such as electrical voltage and impedance length temperature humidity and also ambient pressure But no reference sound pressure is required The method determines the unknown sensitivities of three microphones simultane ously At least two of the microphones must be reciprocal This means that they can be used both as receivers microphones and as transmitters sound sources Microphone Handbook Br el amp Kjaer Vol 1 Chapter 6 Calibration Calibration Methods 6 6 2 The coupler properties must be known to a high degree of accuracy To condude a complete calibration three measurements of receiver voltage and transmitter current ratios must be performed in three different setups inter changed microphones The
97. gy 2 25 nfluence of Microphone Sound pressure 2 6 nfra sound measurement 2 35 Inherent Noise 2 35 Microphone system 2 38 Inherent noise 3 11 Input resistance 2 32 Insert Voltage Calibration 4 15 Insert voltage calibration 6 5 Insert Voltage Calibration method 6 14 Insert voltage calibration method 6 13 Insulator 2 8 2 10 K K andKo 3 5 Ko 3 5 L Large signal response 4 4 Leakage Resistance 3 16 Leakage resistance 2 10 Loaded Sensitivity 3 4 Low frequency adaptors 2 35 Low frequency calibrator 6 7 Low frequency phase response 2 19 Low frequency response 2 18 3 6 Low frequency roll off 2 34 Lower Limiting Frequency 2 18 Lower limiting frequency 2 19 6 7 M Magnetic Field Effect of 3 21 Material 2 9 Maximum output voltage 3 12 Maximum slew rate 4 7 Maximum Sound Pressure Level 3 12 Measurand 6 2 Mechanical tolerances 1 10 Microphone Array applications 5 8 Diskette 3 2 Long term protection 5 10 Short term protection 5 10 Microphone and Preamplifier 2 32 Electrical circuit 2 32 Transfer Function 2 32 Microphone Capacitance 2 31 Microphone holder 5 8 Microphone impedance 2 31 Microphone Modelling 2 25 Equivalent units 2 25 Microphone Noise 2 36 Microphone selection 5 2 Monel 2 10 Multitone calibrator 1 4 6 4 N NIST 6 7 Noise Measurement system 2 36 Microphone 2 36 Microphone system 2 38 Preamplifier 2 36 Low frequency noise 2 37 Noise data 3 12 Noise pressure 2 36 Noise spectrum Microphone 2
98. h 1096 air stiffness Note that the calculation of the curves did not account for the heat conduction effect of the cavity walls The curves are therefore not exact but still provide a good illustration of the influence of air pressure on the low frequency response Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design LH E Oe E A A JIL S ZEB 0 1 1 10 100 Frequency Hz 1k 950908e Fig 2 8 The magnitude of the low frequency microphone response is influenced by the ambient pressure The responses shown are calculated and are all normalized at 250Hz They are valid for a microphone which has 1096 air stiffness at nominal ambient pressure 101 3 kPa a 1 bar b 2 bar c 10 bar d 0 5 bar The low frequency phase response will also change with the static pressure Phase response changes may be more severe than magnitude changes especially in con nection with particle velocity and intensity measurements Closely phase matched pairs of microphone are used for such measurements The microphones of a pair are selected to be essentially equal and to change equally with pressure Their lower limiting frequencies should be the same This also ap plies to the fraction of their air stiffness Fig 2 9 shows the phase characteristics which correspond to the magnitude responses of Fig 2 8 For general purpose types of measurement microphone the lower limiting frequen
99. h information on the preamplifier to calibrate the measurement system The microphone calibration diskette provides individual frequency responses for three types of sound field in Tip octave steps This data can be used to reduce the measurement uncertainty by correcting the measurement data with these respons es by using for example a spread sheet As this implies a microphone optimised for one type of sound field can be used in another type of sound field provided that sufficiently detailed frequency analysis is performed Br el amp Kj r E Microphone Viewer ith microphone data fef Type 4189 NO 4189 06 950337e Fig 3 1 Microphone and preamplifier with calibration chart and diskette The microphone and preamplifier can be stored in the same box 3 2 Sensitivity 3 2 1 X Open circuit Sensitivity So The open circuit sensitivity is defined as the pressure field sensitivity valid with an idealised preamplifier which does not load the microphone The open circuit Sensitivity is a microphone specific parameter The value stated on the microphone Calibration Chart is determined using the Insert Voltage Calibra tion technique with the configuration described in IEC 1094 1 This standard also describes the mechanical dimensions of both microphone and preamplifier BE 1447 11 Microphone Handbook 3 3 Vol 1 Chapter 3 Characteristics of Microphones Sensitivity The open circuit sensitivity which is usually de
100. hange in frequency response corresponds to that below one atmos phere but with the opposite sign Correction dB 500 1k 10k 50k Frequency Hz E Fig 3 12 Typical variation in frequency response normalised at 250 Hz from that at 101kPa as a function of change in ambient pressure 3 20 Microphone Handbook Br el amp Kj r Vol 1 Chapter 3 Characteristics of Microphones Reversible Changes 3 13 3 Effect of Humidity Bruel amp Kj r microphones have been tested for effects of humidity according to EC 68 2 3 standard for Basic Environmental Testing Procedures In general humidity has no influence on the sensitivity and frequency response of the microphone However some microphones have a layer of quartz on the dia phragm which absorbs moisture This leads to a decrease in tension of the dia phragm and a corresponding increase in microphone sensitivity The magnitude of this effect is typically 0 4 dB 100 relative humidity See microphone type specific information for more details The situations where one should be aware of humidity problems are where sudden changes in temperature and humidity occur for example when going from a warm humid environment to a cool air conditioned building The opposite situation is not so critical because any condensation that may occur will only affect the outside of the instrument However if condensation occurs it will usually result in some electrical leakage which obvi
101. he grid acts as an acoustic resonator BE 1447 11 Microphone Handbook 2 47 Vol 1 Chapter 2 Microphone Theory Microphone Types Dedicated to Different Sound Fields 100 1k 10k Frequency Hz 100 k 950943e Fig 2 30 Pressure at diaphragm position referred to the pressure of the undisturbed fied including influence of protection grid O incidence upper curve The free fidd response of the microphone middle curve is flat as the micro phone is designed to have a pressure response lower curve which falls with frequency and compensates for the increase of pressure The curves of the example ap proximate those valid for a 7 5 microphone 2 5 4 Free Field Microphone and Sensitivity The correction for the influence of the microphone body on the sound field can be made by post processing of measured sound spectra but the compensation might also be built into the microphone itself This is the case for the so called Free field microphone types which have a flat free field frequency response characteristic to sound waves that are perpendicular to the diaphragm at zero degree incidence The compensation is made by introducing a heavy damping of the diaphragm reso nance see the lower curve in Fig 2 10 Section 2 3 8 By proper design the degree of damping may be chosen to give a decreasing diaphragm system sensitivity with frequency see Fig 2 30 lower curve which corresponds to the above mentioned in creasing sound pre
102. he measure ment position provided that it is placed sufficiently far away The distance between the source and the measurement site should be at least five to ten times the largest dimension of the source and of the microphone or object which is to be placed in the field In practice sound fields which can be regarded as being free fields may be found at a distance of approximately one to two metres from a sound source This is provid ed that no other sources contribute significantly to the sound pressure and that there are no reflecting surfaces nearby Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Requirement 2 1 7 Diffusefield A diffuse sound field exists at a given location if the field is created by sound waves arriving more or less simultaneously from all directions with equal probability and level The diffuse field or Random Incidence sensitivity of a microphone refers to this type of field even if in most cases the sensitivity is calculated from measurements performed under free field conditions The calculation method is described in the international standard Random incidence and Diffusefidd calibration of Sound Lea Meters IEC 1183 A diffuse sound field may be created within a room with hard sound reflecting walls and which essentially contains no sound absorbing materials Diffuse field spectra created in such rooms may deviate from that of an ideal fiel
103. he phase of the output and the input signals expressed as a function of the frequency All Bruel amp Kj r preamplifiers are designed to keep the phase of the output signal essentially equal to the input signal but for the reasons described in Section 4 2 a phase deviation for low and high frequencies can occur From about 10Hz to above 100 kHz the magnitude response is generally flat whereas the corresponding phase response is close to zero degrees from typically 100 Hz to 10kHz see Fig 4 8 The low frequency phase response is determined by the microphone capacitance and preamplifier input resistance while the cable load in combination with the output impedance determines the phase response at the high frequencies For some applications for example for sound intensity measurements it is impor tant that the phase differences between a number of measurement channels are very small It is recommended that specially developed types of preamplifier are used for this kind of measurement By minimizing the influence of the resistive part of the preamplifier input impedance it is possible to obtain more closely matched phase responses between a number of channels 4 5 Effect of Temperature The specifications of preamplifiers are normally valid within a working temperature range from 20 C to 60 C Some preamplifiers are designed with components and BE 1447 11 Microphone Handbook 4 11 Vol 1 Chapter 4 Characteristics of
104. hing which is prescribed by various sound measurement standards to ensure the validity of the measurement Even when making a relative measurement it is still advisable to calibrate to ensure correct operation of the measurement system This is because calibration is by far the easiest way to check that settings adjustments and postprocessing ana lyzers are correct Most Sound Level Calibrators are portable easy to use and characterized by the production of a well defined sound pressure at a single frequency usually in the range of 200Hz to 1kHz Some calibrators are so called Multitone Calibrators which provide a number of pure tones at single frequencies When using calibrators that produce a single frequency the calibration is strictly only valid at this reference frequency However microphones are generally manufac tured to provide a flat frequency response which means that they will give the same electrical output at all frequencies in the flat frequency range for sound pres sures of equal magnitude Therefore calibration at a single frequency is sufficient in most situations See Section 2 3 9 for more on microphone sensitivity and the reference frequency To perform calibrations across the entire frequency range a Multitone calibrator may be used to check the performance of the measurement system Microphone Handbook Br el amp Kjaer Vol 1 Chapter 6 Calibration Laboratory Calibration Fig 6 1 Sound Levd Calib
105. hone body The pressure at the position of the diaphragm deviates significantly from that of the undisturbed field at higher frequencies This effect must be taken into account to avoid large and unacceptabl e measurement errors The resulting influence of the body of a 1 2 microphone and the protection grid is shown in Fig 2 30 upper curve The influence of the microphone on the pressure of a free field is so great that it needs to be taken into account to avoid considerable measurement errors Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Microphone Types Dedicated to Different Sound Fields The influences of both microphone body and protection grid differ with the angle of 100 1k 10k Frequency Hz 100k 9509606 Fig 2 28 Ratio betwem the pressure at the diaphragm position and the pressure of the undisturbed sound fidd The Same pressure changes occur for bodies of large and smaller body diameter but the frequency range is shifted and is inversdy proportional to the diameter of the mi crophone body sound incidence on the microphone body For all Bruel amp Kj r microphones the in fluence is analysed and stated for a number of specific angles of incidence There fore a correction may be made for the above effect if the angle of sound incidence IS known 950944e Fig 2 29 Typical influence of protection grid microphone sound incidence perpendicular to the diaphragm T
106. hones have higher diaphragm impedances no corrections are necessary 2 6 General Note on Actuator Calibration Bruel amp Kj r give microphone correction values which are to be added to the measured actuator response to obtain the required free field diffuse field and pressure field re sponses Measurement of the actuator frequency characteristic should generally be per formed with the type of actuator specified with the free field diffuse field and pres sure field corrections as this leads to the highest calibration accuracy Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Conclusion 950936e Fig 2 37 Differences between pressure and actuator responses for four different types of microphone 1 pressurefidd 50mV Pa a 1 free fidd 50mV Pa b pressure fidd 12 5 mV Pa c and 1 5 free fidd 12 5 mV Pa d The reason for this is that the presence of an actuator on the microphone modifies the radiation impedance of the diaphragm The modification depends on the shape and dimensions of the actuator As the radiation impedance has an influence on the actuator response the use of other types of actuator may lead to responses which differ from those valid for the published field corrections 2 8 Conclusion This chapter has explained the principles of microphone design and has discussed a number of related subjects This information is given to promote a general under s
107. iaphragm displace ment by typically 10 This effect may be expressed in term of air stiffness by stating that this makes 10 of the total stiffness of the diaphragm system The cavity stiffness depends partly on cavity volume and partly on static pressure Therefore the total stiffness and the sensitivity of the microphone become a func tion of static pressure To minimise the influence of static pressure on the sensitivi ty of the microphone the stiffness of the cavity must be small compared to that of the diaphragm as defined by the following formula A microphone with a low dia phragm stiffness requires a larger cavity volume than one with a high diaphragm stiffness BE 1447 11 Microphone Handbook 2 15 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design 100 S ref P 100 F g F S P S P s ref S P lt microphone sensitivity a function of static pressure P static pressure Pere the reference static pressure at which F is valid F fraction of air stiffness in percent at reference static pressure ratio between air stiffness and total diaphragm system stiffness 2 3 6 Static Pressure Equalization The static pressure may vary within hours or change with the measurement site height above sea level The pressure variations may easily be 109 to 10 times greater than the lowest sound pressures which are to be measured To eliminate the influence of such pressure variations th
108. iation in sensitivity The deviation is read directly from the graphs The temper ature coefficient depends on the frequency Fig 3 9 and Fig 3 10 show how changes occur in the frequency characteristics at various temperatures The magnitude of the influence on the sensitivity at 250Hz is typically 0 002 to 0 008 dB C See microphone type data for more details Microphone Handbook Br el amp Kjaer Vol 1 Chapter 3 Characteristics of Microphones Reversible Changes 4 Response dB 500 1k 10k 50k Frequency Hz re Fig 3 10 Example of reversible changes Typical variation in actu ator response normalised at 250Hz as a function of temperature relative to the response at 20 C 3 13 2 Effect of Ambient Pressure The ambient pressure influences the sensitivity of the microphone The microphone sensitivity and frequency response stated is in most cases valid at an ambient pressure of 1 atmosphere 101 325 kPa The ambient pressure is sometimes re ferred to as static pressure The microphone is designed with a vent to equalise the pressure inside and outside the microphone so it only detects deviations from the equilibrium which is the sound we want to measure The output from the microphone is however affected by variations in ambient pressure This is due to changes in air stiffness and air density which affect the impedance of the cavity behind the diaphragm The ambient pressure varies with altitude and i
109. ical model The parameters of the model correspond to those valid for a series of 1 ls and 4 microphones which make a part of the Bruel amp Kj r program The sensitivities and frequency ranges therefore correspond to those valid for some real microphone types The simple models discussed have been chosen to illustrate the influence of the main parameters which determine the microphone response Other less important parameters should be taken into account to relate the models to microphones of the real world In particular 1 4 microphones generally cover a wider frequency range than estimated from the simple models This is because a resonance between the diaphragm mass and the compliance of the air in the slit behind the diaphragm may increase the response at higher frequencies and extend the frequency range of these microphones In principle any microphone could be improved with respect to frequency range if the diaphragm mass could be reduced A lower mass would increase the resonance frequency and thus also the highest operation frequency H owever this would mean that a thinner diaphragm foil should be used and that the tension in the dia phragm material itself would be increased accordingly This might lead to sagging and instability in the diaphragm Therefore very strong diaphragm materials are needed when good acoustic performance and good long term stability are required as is the case for measurement microphones BE
110. ich is necessary To minimize the loading and to ensure that even very long cables perhaps several hundred metres may be used specific mi crophone preamplifiers are employed Some microphone system properties are determined by the preamplifier in combina tion with the microphone This applies for the electrical frequency response func tion especially for the lower limiting frequency and for the dynamic range which is limited by inherent noise and distortion 2 4 1 Microphone Capacitance The microphone is an electrical source with an impedance which is determined by its capacitance The capacitance is mainly made up by the active capacitance be tween the diaphragm and the backplate and by the stray capacitance of the micro phone housing However the microphone capacitance is also influenced by mechanical properties of the diaphragm system This is especially the case for those microphone types which have the highest sensitivities At low frequencies the electro mechanical coupling adds to the capacitance by up to 1596 while it subtracts capacitance above the diaphragm resonance 1 296 where the phase of the diaphragm movements is op posite Due to the low input capacitance of the preamplifiers used these rather small capacitance variations are generally ignored The capacitance of condenser measurement microphone types is a function of micro phone size and it varies from about 3pF for 1 s microphones to 70 pF for 1 micro phon
111. ier Open circuit sensitivity No P If the microphone were not loaded by a preamplifier the open circuit output voltage would be V 820196 1e Fig 6 4 Principle of the Insert Voltage Calibration method When the method is used for verification of measurement channels an initial cali bration is usually required after which a reference level is measured with insert voltage Each time a verification of the channel is required a new measurement with insert voltage is compared with the reference level The disadvantages of the method are that the system is subject to disturbance from electrical noise and that faults in the microphone cartridge cannot be detected 6 6 7 Chargelnjection Calibration M ethod This method is developed for monitoring of microphone channels and requires a preamplifier with a small extremely stable built in capacitor which makes it possi ble to apply an electrical signal to the preamplifier and microphone input termi nal The Br el amp Kj r patented charge injection calibration method is based on detection of changes in impedance at the input terminal Each verification measure ment is compared to an initial reference measurement The pin used for the CIC method must be connected to ground potential or to the preamplifier output when the microphone is used for normal measurements To use the charge injection calibration facility a test signal for example an electrical broad band noise signal is appli
112. ight PCTS TRENT 5 8 Low Wind Speed Random Direction ss ssssssssrrrssrssrrrrrnrrrsrrssrrrnnrrrerresrnesnrereneer 5 9 High Wind Speed Known Direction aa diana aaa oresssecssanccntaiedoness 5 9 Turbulent Pressure Fluctuations TT 5 10 O s a kann na 5 10 SE 2 Oe m 6 1 6 1 diiecdeusi 6 2 6 2 Calibration of Microphones nennen nennen nennen KR 6 4 6 3 Fiela UU rm 6 4 6 4 Laboratory Calibration PRI DT UU T m 6 5 Primary Calibration Laboratories risin nnne nennen nnn nnns 6 6 Accredited Calibration Laboratories rannsaki 6 6 Calibration at the Bruel amp Kj r F actory eeeeeeeeeeeeee enne 6 7 6 5 Calibration Hierarchy Traceability and Uncertainty eeeeeeeeee 6 8 6 6 ls Sr 4104 t 6 9 Reciprocity Calibration Ms a 6 10 I ON MAA tesa nce grec E 6 11 Weg A 4 M oii 6 11 Sound Pressure Calibrator M ethod nakrar krk 6 12 a Pe creepers enm sec niae EEEa DIIS 6 12 Insert Voltage Calibration Method ooo root ester eee PP a 6 13 Charge Injection Calibration M ethod eeeeeeeeeer eene nnne nnn 6 14 Index 0 4 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 1 ntroduction BE 1447 11 Microphone Handbook Vol 1 1 1 Chapter 1 Introduction Hist
113. ing heat than a preamplifiers For preamplifiers working at high temperatures an increase in the noise level is caused by an increase in the bias current in the preamplifiers This very small bias current of a few pA produces a noise spectrum equal to the resistor noise At nor mal temperatures this noise is negligible but a temperature rise of 7 C causes a doubling of the noise contributions and so a change from 25 C to 75 C will increase the noise level by approximately 40dB That is enough to make it comparable to the resistor noise At even higher temperatures this noise is the most dominant component This effect of temperature is shown in Fig 4 6 High temperature noise data are given in Bruel amp Kj r type specific documentation Microphone Handbook Bruel amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Effect of Magnetic Fields 4 6 Effect of Magnetic Fields The design of the preamplifier ensures that the influence of external magnetic fields is negligible In practice this means that no influence can be detected within a measurement accuracy of 0 0dB when the preamplifier is exposed to a magnetic field of 100 A m 4 7 Electromagnetic Compatibility The electromagnetic compatibility EMC of a product which contains electronic components is the ability of the product to function as intended without disturbing and without being disturbed by other electronic equipment Some electromagnetic disturb
114. ion which represents the difference between the diffuse field and the pressure field sensitivity may be measured but this is usually calculated The international standard Random incidence and diffuse field calibration of sound level meters IEC 1183 prescribes how this should be done The calculation employs a weighted power based summing of the correction values valid for the different angles of incidence The weighting accounts for the non equal solid angles of incidence which are represented by the equally stepped angles at which the corrections are measured More weight is thus put on the corrections of the angles close to the 90 degrees than on those valid for angles dose to zero degrees IEC 1183 defines the calcula tion formula and the weighting factors BE 1447 11 Microphone Handbook 2 49 Vol 1 Chapter 2 Microphone Theory Microphone Types Dedicated to Different Sound Fields 1k 1 0k Frequency Hz 100k 950959e Fig 2 31 Freefidd corrections 0 to 180 in 5 steps valid for a 175 microphone without protection grid The diffuse fidd correction bold curve is calculated according to IEC 1183 As with free field characteristics it is very easy to determine the diffuse field char acteristic of a microphone by adding the diffuse field correction to its individually measured pressure field response or electrostatic actuator response 2 5 6 Diffuse Field Microphone and Sensitivity Dedicate
115. is 180 For all Bruel amp Kjar prepolarized microphones which use a negative charge the shift is 0 The 90 phase lag relative to the phase at low frequencies determines the reso nance frequency of the microphone BE 1447 11 Microphone Handbook 3 7 Vol 1 Chapter 3 Characteristics of Microphones Frequency Response ZH Aouenbes4 4 001 401 yL 004 zH Aouenbes4 y 001 401 AL 004 G vog esuodsey pjalj ainssaid UO 1991109 pJai eJnssaJd ZH Aouenbel4 ZH Aouenbes4 ZH Aouenbes4 001 01 1 004 01 xl 001 pd TEE HEN MEE L2 RES NN EK ERN MX FS A gp nd URN 5 NS T s LE 9g 3 2 Oo aooo S esuodsey eouepiour uopueH esuodsay Jojenjoy UOI1294102 e2uepiour ui opued ZH Aouenbel4 zH Aouenbes4 001 4 OL 4 001 0c po RN dL ee Se ak i G eouepiour 0 asuodsay pjar e9JJ u01 391109 pJ91J 99JJ esuodsey UOI109 109 t esuodsey 10jenjoy 950389 1e Br el amp Kjaer Vol 1 the individually measured actuator response The response valid for different types of sound Microphone Handbook fidd is obtained by adding the corrections shown in bold to the actuator response Fig 3 4 Thefrequency response curves valid for an individual microphone are obtained on the basis of Chapter 3
116. is included in an electrical circuit it may be consid ered as being a purely capacitive component The capacitance is determined by the distance between the diaphragm and the backplate and by stray capacitance between the backplate and the microphone housing The capacitance of the microphone and its variation with frequency is a function of the polarisation voltage see Fig 3 6 The stated values of microphone capacitance are valid for nominal polarization voltage at 250 Hz generally 200V for externally polarised microphones and OV for prepolarized microphones The capacitance of the microphone can be used for the evaluation of loaded sensitivity Sc lower limiting frequency and preamplifier noise The stability of the microphone is dosely related to its capacitance i e changes in capacitance reveal changes in microphone sensitivity and frequency response This relationship is exploited in the Bruel amp Kj r patented Charge Injection Calibration technique for monitoring the condition of a microphone see Section 4 8 BE 1447 11 Microphone Handbook 3 13 Vol 1 Chapter 3 Characteristics of Microphones Polarisation Voltage Capacitance pF A similar capacitance variation with frequency occurs with prepolarized micro phones LL NN Frequency Hz BARBA Fig 3 6 Example of the variation of capacitance with polariz
117. is related to the active part of the amplifier input stage which consists of a Field Effect Transistor FET The FET noise has three different origins channel noise creating white noise materi al impurities creating pink noise and leakage current which in combination with the input resistors creates white noise coloured by the input impedance The in fluence of the leakage current will be mentioned under Section 4 6 Effect of Tem perature BE 1447 11 Microphone Handbook 4 9 Vol 1 Chapter 4 Characteristics of Preamplifiers Dynamic Range Under normal environmental conditions only the white noise source is relevant This white noise source has the same nature as the previously mentioned resistor noise but here the resistance is typically 1 to 2k The microphone capacitance does not have any practical influence on frequency distribution of this noise and as seen in Fig 4 6 this noise component becomes responsible for the noise from about lkHz and above Due to coupling capacities shown in Fig 4 8 as Cc a fraction of the FET noise is coupled forward to the high impedance input of the preamplifier resulting in an increase in the noise level The noise contribution from this mechanism depends on the capacitative attenuator formed by the microphone capacitance C and the cou pling capacities Cc This explains why the higher frequency noise also depends on the capacitance of the microphone Note that the noise sp
118. is will be discussed in connection with inherent noise of microphone systems Microphone Handbook Br el amp Kj r Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design 2 3 10 Microphone Modelling by Equivalent Electric Circuits Analysis of electric circuits has been a well known and widely applied discipline for many years Today very effective computer programs have become available for the purpose This makes the technique very interesting for both microphone designers and for designers of acoustic systems that include microphones To use these tools in designing microphones a model must be made by converting acoustic circuit elements to equivalent electric circuit components and by connec ing these in series and parallel corresponding to the acoustic circuit There are two main analogies the impedance and the admittance analogies The impedance analo gy is the generally applied analogy for modelling microphone circuits An acoustic compliance which corresponds to reciprocal stiffness is converted to an electric capacitance The acoustic mass corresponds to an electric inductance and acoustic damping is represented by an electric resistance In such a model pressure corresponds to voltage acoustic volume velocity to electric current and acoustic displacement to electrical charge Combined acoustic mechanical and electric constructions like the condenser micro phone may be modelled by simply connecting the
119. ition is usually covered by na tional acoustical calibration laboratories such as DPLA Calibration laboratories op erating at lower levels use comparison or substitution methods based on reference standards calibrated by higher ranking laboratories As seen in Fig 6 2 DPLA obtains traceable accredited calibrations of physical units from other calibration laboratories DPLA is also accredited by the Danish Trade and Industry body DANAK which accredits laboratories according to DS EN 45001 General Criteria for the Operation of Test Laboratories 45002 General Criteria for the evaluation of Test Laboratories and 45003 General Criteria for Organisations which issue laboratory accreditations 6 6 Calibration Methods This section gives an overview of calibration methods These are listed in Table 6 1 with an estimate of the expected uncertainty that can be obtained using the various methods Reciprocity Calibration 0 03 to 0 05 dB Comparison Method 0 06 to 0 14dB Substitution Method 0 06 to 0 14dB Pistonphone 0 07 to 0 3 dB Sound Level Calibrator Actuator Frequency Response 0 1 to 0 5 dB Table 6 1 Uncertainties two standard deviations related to various calibration methods BE 1447 11 Microphone Handbook 6 9 Vol 1 Chapter 6 Calibration Calibration Methods 6 6 1 Reciprocity Calibration Method This is the most accurate calibration method for determination of the open circuit sensitivity of the microphone cartrid
120. ity refers to the pressure at the dia phragm only by definition The freefidd sensitivity and diffusefidd sensitivity are defined for pressure applied at both the diaphragm and pressure equalisation vent Condenser measurement microphone types generally have a sensitivity between lu V and 100uV per Pascal When designing and selecting a microphone for a certain application the expected sound pressure level and microphone output volt age must be taken into account A microphone with proper sensitivity should be selected The sensitivity should not be so high that the microphone output signal overloads the preamplifier and it should not be so low that it is exceeded by the inherent noise of the succeeding amplifiers In general the sensitivity may be used for ranking microphones with respect to their ability to measure low and high sound pressure levels The higher the sensi tivity the lower the sound pressure levels that may be measured and conversely the lower the sensitivity the higher the sound pressure levels that may be meas ured The sensitivity is thus not only linked to the applicable frequency range as previ ously discussed It is also linked to the dynamic range The microphone designer must work with this fact and compose a programme of microphone types which meets the needs of the user Analysis of microphone system limitations at low levels shows that inherent noise of the microphone itself must also be taken into account Th
121. label The presence of a CE label on a product indicates that it complies with all relevant European Union Directives The CE label is affixed by the manufacturer or an authorised European representative and indicates that product complies with the requirements of the EMC directive and other directives as applicable Precisely what the relevant requirements are will depend on the product but for all electron ic devices the requirements indude electromagnetic compatibility Microphone preamplifiers are CE labelled but devices such as microphones and cables are pas BE 1447 11 Microphone Handbook 4 13 Vol 1 Chapter 4 Characteristics of Preamplifiers Electromagnetic Compatibility l z e ES E AR End Fig 4 9 GTEM Gigahertz Transversal Electromagnetic test cell at Br d amp Kj s EMC test labora tory measuring radio frequency signals emission and immunity sive components and therefore do not need to be CE labelled They are however subjected to thorough EMC testing together with their associated equipment for example cables are tested with their associated preamplifiers In many cases the cables have been specially developed for example with braiding patterns to obtain the specified EMC properties as defined by the manufacturer 4 7 3 EMC Test Facilities at Bruel amp Kj r In recognition that EMC requirements are now an important part of the develop ment of high quality electronic equipment Bruel
122. lead to an erroneous sensitivity If a prepolarized microphone is accidentally connected to an external polarization voltage source of 200V no permanent harm is caused to the microphone However while the voltage is applied the sensitivity of the microphone will differ significant ly typically by 10dB to 40dB or even more from its nominal value In addition the frequency response relative to the reference frequency might change by typical ly plus one decibel in the frequency range around half the diaphragm resonance frequency Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design To ensure accurate calibrations and measurements after lengthy exposure to an external voltage 4 200 V the microphone should be stored with zero volts between its terminals for at least 24 hours This will minimize the risk of errors due to intermittent charge displacements and shifts in sensitivity which might otherwise be up to a tenth of a decibel Bruel amp Kj r introduced prepolarized measurement microphones in the late seven ties and showed by experiments and by extrapolation of measurement results that such microphones could be made very stable and that they could meet all the re quirements set for most applications Electret microphones intended for consumer applications were well known at that time They used polymer foil diaphragms The polymer foil served as both a dia phragm
123. lly fast enough to eliminate any disturbance from changing static pressure It also gives the microphone a flat magnitude response down to less than 5Hz which is sufficient for most applications Below 10Hz the frequency response of the microphone is greatly influenced by the pressure equalisation time constant and by the position of the external vent open ing The vent opening might either be exposed to or be outside the sound field see Fig 2 6 The response is very different in the two cases 800114 1e Fig 2 6 Pressure Equalisation Vent positioned inside B and outside A the sound field The different situations lead to different microphone responses at low frequencies BE 1447 11 Microphone Handbook 2 17 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Fig 2 7 0 1 1 10 100 Frequency Hz 1k Under general field measurement conditions the vent is exposed to the sound field When this is the case the vent will tend to equalise the sound pressure at low frequencies This reduces the pressure difference between the front and the rear of the diaphragm and leads to a smaller diaphragm displacement and a lower micro phone sensitivity The lower the frequency the more significant the effect becomes The sensitivity will continue to fall with frequency At very low frequencies the slope reaches a maximum of 20 dB per decade see Fig 2 7 lower curve 950906e Low frequency response valid for situations
124. lly stable and its electrical leakage resistance should be as high as 10 at normal ambient conditions In order to match these requirements the insulator is normally made of either sapphire ruby or mono crystalline quartz All insulators are machined to a high degree of flatness across the two plane surfaces typically to within O 5um All surfaces are also highly polished to achieve the high electrical resistance that characterises Br el amp Kjaer microphones Finally a thin and invisible layer of silicone is applied to the insulator by a high temperature process This ensures proper operation in tropical and other hot and humid environments Further requirements are imposed on the microphone design and construction by the polarization voltage that is applied across the diaphragm to backplate gap As the distance across the gap is 20um and the voltage is typically 200V the field strength becomes 10 kV mm This exceeds the normally considered break down field strength for air by a factor of 3 to 4 However the microphone operates be cause the distance between electrodes is small enough to prevent the normally occurring electric ion multiplication from reaching a significant level The ion multi plication that does occur does not escalate and lead to any significant discharge or detectable noise Due to the high field strength the diaphragm and the back plate must have flat high quality surfaces which are dean and free from particles Thi
125. ly polarised microphones offer a broader range to choose from 5 2 7 Standards Compliance When selecting a microphone it is normally a requirement to consider whether the microphone fulfils certain standards for example the ANSI 1 12 standard for Lab oratory Measurement microphones or the IEC 651 standard for sound level meters including microphone As can be gathered from the above example standards gen erally relate to the type of application for the microphone In addition a further range of levels specified by type numbers exist within a standard for example the IEC 651 standard for sound level meters has types O to 3 Type O relates to labora tory reference standard requirements while type 3 is mainly for field applications Most Bruel amp Kj r microphones are type 0 and type 1 The standards and their associated type levels also specify the performance toler ances to which microphones and associated equipment must conform This conform ance is stated in product literature Bruel amp Kj r microphones are designed to come within 50 to 70 of the required tolerances depending on the application In the case of tolerances relating to the use of sound level meters the effect of the sound level meter in the sound field is also taken into account when designing and producing microphones Microphone Handbook Br el amp Kjaer Vol 1 Chapter 5 Selecting a Microphone What to Consider Standards Relevant to Measurem
126. mbi ent pressure than that of other microphones Other major design parameters are the diaphragm diameter and the diaphragm damping resistance For condenser microphones in contradiction to many other types of transducer an optimal diaphragm damping may be obtained and main tained over time Therefore such types of microphone may be used in the frequency range around and even above the diaphragm resonance frequency The damping is caused by the movement of air in the slit between the diaphragm and the back plate Diaphragm movements lead to air movements in the slit which cause viscous loss The damping resistance may be controlled by holes in the back plate By changing the number and size of holes and by varying the back plate s distance to the diaphragm various degrees of damping may be obtained 100 1000 10000 Frequency Hz 100000 950792e Fig 2 10 Influence of damping on the high frequency microphone response magnitude The damping is due to movement of the air in the slit between the diaphragm and back plate The damping depends on the microphone design Examples of a low a critical b and a high c damp ing are shown The influence of damping is illustrated by the curves shown in Fig 2 10 In relation to these the resonance frequency was kept constant at 10kHz which is the typical resonance frequency for 1 and for high sensitivity BO mV Pa or 26 dB re 1V per Pa 7 microphones The illustrated degrees of damping may
127. microphone and the preamplifier is shown in Fig 2 16 In addition a set of typical circuit element values valid with a 1 2 micro phone is given in the table below Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier Open circuit microphone voltage Voc Microphone Capacitance 18 pF 2 microphone Preamplifier Input Capacitance Preamplifier Input Resistance OR m ee Preamplifier Output Resistance DOR 899 A Cable Capacitance 3nF corresponding to 30 m Table2 4 Values of Electrical Circuit Elements of the simplified modd see Fig 2 16 which repre sents the microphone capacitance preamplifier and cable of a typical microphone system 157 The mode may be used for calculation of the electrical frequency response of a microphone system Often only the acoustic response is taken into account as the dectric response is generally flat see Fig 2 17 and Fig 2 18 Microphone Preamplifier Fig 2 16 Simple mode for calculation of the electrical frequency response of a microphone and its preamplifier loaded with a cable The eectrical response should be combined with the acousto mechanical response of the microphone to obtain the over all system response The electrical frequency response of the circuit is given by the formula below M 7 Ja Cy CR 1 O m Voc CmtCi l Je Cq COR 1 joC Ro G cm g eM io BE 1447 11 Microphone Handbook 2
128. microphone systems The larger and most sensiti ve microphones should be used for low levd measurements The microphone noise is most significant in such systems The preamplifier noise is most significant in systems with small and less sensitive mi crophones Microphone Distortion Microphone distortion in the frequency range where the diaphragm displacement is stiffness controlled is caused by passive capacitance in parallel with the active diaphragm capacitance This passive capacitance is made up by the backplate hous 2 40 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier Fig 2 23 b Preamplifier Measurement Amplifier c Measurement Amplifier E 8 16 31 5 63 125 250 500 1k 2k 4k 8k 16k 31 5k Frequency Hz 951156e Equivalent levels of inherent noise 1 3 octave bandwidth produced by the denents of a low noise system with a special 1 freefidd microphone 100 mV Pa and low diaphragm damping The microphone and the preamplifier 20dB gain which contains a frequency response equalization network form the main noise sources The system noise leve is 2dB A ing capacitance preamplifier input capacitance and to some degree by parts of the diaphragm capacitance which are less active than others Passive capacitance leads to a dominating second harmonic distortion which increases proportionally with the sound pressure and a less significan
129. n microphone products However at Bruel amp Kj r this in vestment is seen as essential because it allows the design parameters and physical properties of microphones to be based on a solid foundation of knowledge skills and experience As a result of its pioneering nature development work often enjoys a symbiotic relationship with standards organisations New product developments contribute new standards to the research and development field while Bruel amp Kjaer products benefit from a free flow of information on the most current and industry wide re quirements for microphones Improved standards and more advanced Br el amp Kjaer products result Similarly this collaborative approach to development includes work with universi ties and knowledge centres with benefits to both sides A good example of this approach is the cooperation between Bruel amp Kjar and the Technical University of Denmark to set up and run the Danish Primary Laboratory of Acoustics DPLA At Bruel amp Kj r the more theoretical aspects of microphone development involve both mathematical modelling and measurement techniques using an anechoic chamber with the most recent technology available Research and development work also encompasses a number of areas that reflect the different aspects of micro phone design and construction in particular where highly accurate measurements need to be performed These include acoustic measurements such as pressure
130. nals Vent Common Fig 2 13 Microphone modd The components of the equivalent dectrical circuit represent stiffnesses masses and damp ing of the eectro mechanical system The response of the mode depends on the exposure to sound pressure of the static pressure equalization vent exposed 3 connected to 1 unexposed 3 to 2 Such a model may be used for calculation of the sensitivity and the frequency response magnitude and phase which may be found with and without sound pres sure at the static pressure equalisation vent It may also be used to determine the compl ex acoustic diaphragm impedance as well as the electrical impedance In addi tion the inherent electric noise of the microphone may be calculated as this may be referred to the noise of the resistance elements also known as Nyquist and J ohn son Noise This type of model is commonly used for calculation of the response of the micro phone as previously described in this section The model is also used for describing other microphone properties This is discussed in the following sections Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design 9me Wet ent we omo ow qe I Electrical Capacitance when diaphragm is blocked Table 2 2 Values of the microphone model eements which are shown in Fig 2 13 The mode corre sponds to a microphone 50mV Pa having a critically damped diaphragm with reso n
131. nce Zgg and by the low frequency value of its equivalent vol ume Vellf respectively stiffnesses of the source the coupler cavity and the microphone diaphragm see Fig 2 26 b In such cases it is very practical to work with equivalent volumes of the source and the loading microphone Most couplers are equipped with a static pressure equalisation vent The impedance of this should be taken into account if the model is also to be used at the lowest frequencies The sound pressure attenuation due to the microphone is given by the following formula V V V If attenuation dB 20 joo pe eee V cou pler HN Ve source Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Microphone Types Dedicated to Different Sound Fields where Vcouple Coupler volume Vesource equivalent volume of source Ve If equivalent volume of microphone diaphragm system In practice 1 microphones are used with larger couplers 3 10 cubic centi metres which may be used for headphone testing while 1 2 microphones are used with smaller couplers of 1 to 3 cubic centimetres for example those designed for insert earphone testing As the equivalent diaphragm volume of a 1 microphone is typically 0 16 cubic centi metres the microphone will in practice attenuate the sound pressure produced in such a coupler by 0 5 dB to 0 15 dB depending on its size For smaller couplers with 7 microphones the influ
132. nd noise is to mount a wind screen on the microphone and pream plifier 5ound Level Meter The wind screens are made from a porous polyurethane foam They will attenuate the wind noise by 10 to 12dB at those wind speeds gen erally considered acceptable for outdoor testing 0 m s to 6 m s High Wind Speed Known Direction Acoustic measurements in wind tunnels are normally very difficult due to aerody namically induced noise around the microphone When the microphone is exposed to high wind speed in a known direction the disturbance can be reduced by using a nose cone The nose cone replaces the normal protection grid It has a streamlined shape with a highly polished surface that gives the least possible aerodynamically induced noise 9509706 Fig 5 5 Nose cones in different sizes to fit J g ly yt 11 y and 1 microphones BE 1447 11 Microphone Handbook 5 9 Vol 1 Chapter 5 Selecting a Microphone Selecting The Right Accessories 5 3 3 Turbulent Pressure Fluctuations The turbulence screen is designed to attenuate noise from turbulence when measur ing airborne noise in situations where turbulence may occur such as in ducts and in wind tunnels 5 3 4 Humidity The microphone may be protected against humidity in different ways depending on the type and duration of the measurements Short Term Protection Short term protection against humidity when making outdoor measurements in unfavourable weather can be perform
133. nd then implemented precisely once the correct tolerances have been adjusted Similarly the tension of the diaphragm is dosely monitored and computer controlled Fig 1 8Diaphragm press in use during production Forced ageing using temperature and humidity is employed to ensure stability It does this in two ways Temperature releases tension inherent in the materials after manufacture This achieves the required diaphragm tension which will then remain highly stable High temperature in combination with high humidity also stabilises the polarisation charge applied to pre polarised microphones The stability of the microphones is such that Bruel amp Kj r can state an accuracy variation from the recorded values of just O 1dB over a period of 50 years for a typical 1 2 condenser measurement microphone Microphone Handbook Br el amp Kjaer Vol 1 Chapter 1 Introduction Product Support Fig 1 9 Microphone calibration at the end of production using an eectrostatic actuator All such measures play an important part in achieving the renowned reliability and accuracy of Bruel amp Kj r microphones This reliability is based on the known per formance parameters of the microphone These parameters are measured at the end of production using highly accurate calibration equipment The results are recorded on individual calibration charts and data diskettes which are supplied with micro phone products 1 4 Product Support Br
134. ng the parts together The new type right is assembled by pressing components together The de sign is patented by Br el amp Kj r In practice the first mentioned type implies more freedom for the designer to opti mise the frequency response while the second is advantageous during production The main choice which must be made in respect to the two different design types is one of more narrow frequency response tolerances offered by the conventional de sign as opposed to reduced production costs for the alternative design 2 3 3 Material and Process Requirements A microphone which is to be used for measurements must be stable over time and its properties should preferably not vary with variations in ambient temperature pressure and humidity Therefore carefully selected high quality materials must be used even if they are relatively difficult to machine The sensitivity of the microphone is inversely proportional to the diaphragm ten sion The tension must therefore be kept stable Normally it is a requirement that a measurement microphone has a broad frequency range and a high sensitivity This creates a requirement for light weight diaphragms with high internal tension and thus a very high loading of the diaphragm material This is achieved by applying a tension of up to 600 N mm which would break most materials to the diaphragms BE 1447 11 Microphone Handbook 2 9 Vol 1 Chapter 2 Microphone Theory Measurement Micr
135. niques The Insert Voltage Calibration method is primarily intended for use in calibration laboratories for determining the open circuit sensitivity of condenser microphones The open circuit sensitivity is the sensitivity V Pa of the microphone working to an infinitely large electrical impedance i e the same as that of an ideal preamplifier The Insert Voltage Calibration technique may also be used to provide a field check of a measurement system including a preamplifier and cables However the method does not account for the mechanical parameters of the microphone cartridge which determine the acoustical properties of the measurement setup The method is suffi cient to verify the electrical part of a measurement system but it is not satisfactory for verifying the microphone cartridge To be able to verify the complete system Bruel amp Kj r have developed and patented a technique called Charge Injection Cali bration CI C Microphone Preamplifier Generator Fo _ C n Jd rss Typical Valves C 15 20 pF C 2 0 3 pF 940528 1e Fig 4 11 Chargelnjection Calibration and Insert Voltage Calibration The formulae are valid for the mid and high frequency range However the IVC technique is still the standardised system that should be used for calibration of laboratory microphones The Charge Injection Calibration CIC technique represents a great improvement for remote testing of a measurement setup compared to the
136. nting a capacitor in parallel with the microphone This should be at least a few times larger than the microphone capacitance but it should not be too large as it also reduces the system sensitivity The modification corresponds to an increase in the input capacitance C see the formula stated above Note that low frequency adaptors containing a capacitor are available and can be mounted between the microphone and the preamplifier Inherent Noise of Microphone Systems Both condenser microphones and preamplifiers produce noise In particular the equivalent noise pressure depends on the size of the microphone The noise from both microphone and preamplifier adds to the measured signal However in most measurement situations the sound pressure produces signals which are so high that the inherent system noise can be ignored When measurements are to be done at lower levels the system noise must be estimated and if necessary taken into ac count Typically the inherent A weighted noise corresponds to 10 and 4OdB for systems equipped with 1 and 1 microphones respectively Special microphone sys tems with inherent noise as low as OdB A are also available The measured inherent noise level depends on the bandwidth The broader the measurement bandwidth the higher the noise level and conversely the narrower the bandwidth the lower the noise level Filtering may therefore make it possible to BE 1447 11 Microphone Handbook 2 35 Vol
137. ocks such as if the microphone is dropped onto a hard surface Br el amp Kj r microphones are designed and tested to withstand such ef fects However it should be noted that microphones are delicate precision measur ing instruments especially those designed for laboratory use and should be treated carefully Special care should be taken when the grid is removed avoid touching the dia phragm as it is easily damaged by sharp points or particles Some microphones have screwed on diaphragms and in this case touching the damping ring may change the diaphragm tension causing subsequent changes in sensitivity and fre quency response n addition care should also be taken not to stress the diaphragm by having different static pressure on the front and the back side of the diaphragm normally avoided by having the Pressure Equalisation Vent in the sound field This may also happen when mounting and dismounting the microphone from a small coupler or cavity Here the microphone may be subjected to a large vacuum causing heavy loading of the diaphragm This could cause changes in sensitivity and frequency response 3 12 3 Short term Stability Over the lifetime of a microphone it is very likely that some minor variations in sensitivity will occur due to thermal or mechanical shock These changes occur due BE 1447 11 Microphone Handbook 3 17 Vol 1 Chapter 3 Characteristics of Microphones Reversible Changes to settling of th
138. ophone Design made from very fine grained nickel foil or special stainless steel alloy These differ ent diaphragm materials are used for the traditional and for the newer Bruel amp Kj r microphones respectively As a result only slight and insignificant sagging occurs in the Br el amp Kj r diaphragm foils Carefully controlled heat treatments during the manufacturing process also contrib ute to the high stability of the microphones For most types of microphone the systematic sensitivity change over time is thus predicted to be less than 1dB in 500 years at room temperature For information about the stability characteristics of microphones see also 2 6 and 3 11 Diaphragm tension should also be unaffected by temperature changes To ensure this the thermal expansion of the housing and the diaphragm foil should balance each other by being essentially equal This puts strong ties on the selection of these materials In particular Monel a high Nickel alloy is the most frequently em ployed housing material as it matches the diaphragm materials and has a high resistance to corrosion The distance between the diaphragm and the back plate is another critical parame ter In order to keep this distance constant within the operation temperature range the thermal expansion of the back plate must match that of the housing Strong restrictions are thus also related to the choice of back plate material The insulator must also be mechanica
139. ophones and sound calibrators in duding pistonphones leave the factory in a calibrated state This is documented on an individual calibration chart stating traceability to DPLA and NIST National nstitute of Standards and Technology U SA The methods used for factory calibration of microphones are as follows Open circuit Sensitivity The open circuit Sensitivity is determined using a preamplifier with an Insert volt age calibration facility An unknown microphones is calibrated by the comparison method using a reference standard microphone in a small acoustical coupler The reference standard microphone is calibrated by DPLA using reciprocity calibration according to IEC 1094 1 Frequency Response The frequency response relative to the sensitivity at 250Hz is measured for each microphone using the actuator method see Section 6 6 5 To obtain other responses the corresponding correction for example the free field correction is added to the actuator response Capacitance of Microphone Cartridge The capacitance of the microphone cartridge when mounted on a preamplifier is measured using the patented Charge Injection Calibration technique The measure ment system is checked regularly with a calibrated reference capacitor Lower Limiting Frequency The Lower Limiting Frequency is measured using a low frequency calibrator which exposes the equalization vent of the microphone to the same sound field as the diaphragm A reference l
140. orical Background to Microphone Development at Bruel amp Kj r 1 1 Historical Background to Microphone Development at Bruel amp Kja Br el amp Kj r began producing microphones in 1945 By the late 1950s they were established as a leading supplier of measurement microphones due largely to the inspirational leadership and enthusiasm of Dr P V Bruel in the field of microphone development In parallel Bruel amp Kj r also conceived designed and developed com plete acoustic measurement systems Measurement microphones formed an impor tant part of these systems in combination with instruments such as analysers recorders and sound level meters From this beginning Bruel amp Kj r gained an increasingly good reputation amongst users of microphones both in the acoustics industry and in the field of academic research This was achieved as a result of providing a high standard of service and reliable well built products The product range was also enhanced by a programme of research and development that ensured continuous improvements in the accuracy and performance of new instruments Today this approach continues to deliver in novative measurement instrumentation induding a comprehensive range of meas urement microphones in sizes from t g to LL Together these microphones cover all aspects of measurement microphone usage Fig l IRange of condenser measurement microphones By the early 1970s Br el amp Kjaer s strong p
141. ory sette nanna 2 1 PA Sound Levels and Sound FIO GB uisoroiisvI GINE YMISS RE S3RVEPTPIPEPIUT NEN E S SNAP PSTUPFPULIPPNEIIR S SUP 2 2 Sound Field POOLE Bored sivi sav oPpE scars TRU PE e qvis aa Poss UPsIS QD ove XR EPPPPIM IERI RU Peto Us 2 2 Sound Pressure and Pressure L evel seesseeeeeeeee enne nenne nnne nnne nnns 2 2 Particle Velocity and Particle Velocity Level eene 2 3 Sound Intensity and Sound Intensity Level seeeeeeeneenenennne nnns 2 3 e a MEET 2 4 4i rrE X m 2 4 I PEL IEEE TE 2 5 Ji Measurement Microphone Requirement sssssssrrssrrrrerrernnsrnrrnrrrorrrernrennrresrresnrene 2 5 Za Measurement Microphone DeSIQN ccccccccssseeessseeeseseeesseeeesseeeesaeeessaneesseeeessaeesnanees 2 7 Bstigoe BLEUS RETO 2 7 Design DESCI DUON CL 2 7 Material and Process REqUIrEMEN LS i 2 9 Tamaua ey gl a 0 RETI TET 2 11 Diaphragm and P Gm NIE RN 2 15 Static Pressure Equalization oc nccc dives rn 2 16 Low Frequency Response and Vent Position eese nnne 2 17 High mitad top 2 20 MRT OpNONE EN Rg I ER SR VR 2 24 Microphone Modelling by Equivalent Electric Circuits 2 25 Acoustic mpedance of Diaphragm System eeeeseeeeeeeeee nne nnn 2 2
142. ously results in a malfunction of the microphone and preamplifier The moisture will attenuate the sensitivity of the microphone and as a side effect in crease the inherent noise level see Section 3 10 3 13 4 Effect of Vibration The vibration sensitivity of the microphone normal to the diaphragm is well de fined as it is determined by the mass of the diaphragm The vibration sensitivity is much smaller in all other directions Preamplifiers and electrical adaptors and alike may also contribute to the vibration sensitivity of the measurement channel The vibration sensitivity usually varies with the vibration direction The magnitude of this effect is of the order of 65dB SPL for 1 ms 3 13 5 Effect of Magnetic Field The Bruel amp Kj r microphones are designed with materials that provide a very low sensitivity to magnetic fields The latest Falcon Range microphones have signifi cantly lower sensitivity to magnetic fields than earlier microphones 3 13 6 Electromagnetic Compatibility A microphone cartridge is a passive and well shielded component Therefore the electromagnetic compatibility entirely depends on the equipment connected to the cartridge See Section 4 7 for more information on this subject BE 1447 11 Microphone Handbook 3 21 Vol 1 Microphone Handbook Vol 1 Br el amp Kjaer Chapter 4 Characteristics of Preamplifiers BE 1447 11 Microphone Handbook Vol 1 4 1 Chapter 4
143. ponse The opposite large signal response will be discussed in Section 4 3 Dynamic Range The small signal electrical high frequency response of the preamplifier is deter mined by the low pass filter created by the output impedance of the preamplifier and the capaatative load of the connection cable The output impedance is determined by protection components built into the preamplifier output circuit stage This protection consists of a current limiting re sistor and for some preamplifiers a filter that protects the preamplifier electronic circuit against high frequency electromagnetic noise picked up by the cable This filter indudes a series inductor which in combination with the cable capacitance can cause gain peaking in the 100 kHz to 200 kHz frequency range as illustrated in Fig 4 2 The advantage of this system is that it enables the preamplifier to be used where radio interference can be expected Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Dynamic Range 10k 100 k Frequency Hz 1M 950966e Fig 4 2 Electrical high frequency responses for small signal of preamplifiers with different cable capacitances The cable capacitance is typically 100 pF m 4 3 Dynamic Range The dynamic range for the microphone and preamplifier combination is also dis cussed in Section 2 4 and 3 5 This total dynamic range is primarily limited by the preamplifier This section concentra
144. pposite polarity which is moveable A certain fraction of this lies on the conducting part of the backplate while the other part is on the inner surface of the diaphragm The relative distribution between the positions is determined by the ratio between the capacitance of the electret and the capacitance of the air gap BE 1447 11 Microphone Handbook 2 13 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Thus two electrical fields are produced in the microphone One across the air gap and one across the electret These fields must stay essentially constant during the microphone operation which means that any resistance loading on the microphone must be so high that the voltages produced by the microphone do not lead to any significant interchange of charges The microphone sensitivity is a function of the field strength in the air gap This also applies to external polarization see the formula above Typically the field strength and equivalent polarization voltage correspond to those used for micro phones with external polarization namely 200V The main microphone specifica tions do not depend on the polarization principle applied There are some minor exceptions For design reasons all prepolarized microphones made by Bruel amp Kj r have a negative charge applied to the electret the virtual backplate while the Bruel amp Kj r power supplies apply positive charges for exter nally polarized microphones
145. ptors may be used to obtain this effect but this obviously also has the effect of lowering the sensitivity for the system Such a microphone and preamplifier system is useful for measuring low frequency sound 4 2 2 High Frequency Response Normally the high frequency response of a microphone and preamplifier combina tion is determined by the acoustical high frequency cut off of the microphone H ow ever the electrical high frequency response of the combined preamplifier and cable loading must also be considered BE 1447 11 Microphone Handbook 4 3 Vol 1 Chapter 4 Characteristics of Preamplifiers Frequency Response Fig 4 1 950962 1e Electrical low frequency response of preamplifiers with various microphone and adaptor combinations a preamplifier with bootstrapping b c and d preamplifiers with 1st order high pass filter Microphone capacitance a 6pF b 20 pF c 6pF d 20pF in combination with a 80pF stray capacitance adaptor The electrical high frequency response is determined by different properties of the preamplifier in combination with the capacitive load of the connection cable These properties are the output impedance the maximum output current capacity and the maximum output slew rate The normally mentioned Frequency Response will be considered here as the re sponse obtained when the dynamic properties of the preamplifier are negligible This response is also known as the small signal res
146. r at any frequency The noise spectrum of a microphone looks like its pressure frequency response This may be realized from the equivalent circuit diagram As the noise pressure genera tor of the diaphragm resistance is in series with the diaphragm compliance mass and resistance it can be regarded as if it were connected to the acoustic input terminals like any external signal The flat noise spectrum of the damping resist ance will therefore lead to an output voltage noise spectrum at the microphone output terminals that is shaped like the pressure response of the microphone As most acoustic measurements are made by using filters of constant relative band width microphone noise data are generally presented for third octave bands see Fig 2 20 Preamplifier Noise The preamplifier noise may be regarded as a noise composed of two main parts Namely a low frequency noise which originates from the input circuit and is a function of the microphone capacitance This noise voltage is inversely proportional to the frequency and to the transducer capacitance The other source which is relat ed to the amplifier has a flat voltage spectrum over the entire operation range Third octave filtering of the preamplifier noise leads to a spectrum which falls by 10 dB decade at low frequencies and raises by 10 dB decade at high frequencies see Fig 2 21 BE 1447 11 Microphone Handbook 2 37 Vol 1 Chapter 2 Microphone Theory Combination of
147. rator used for fidd calibration of measurement microphones In multi channel microphone systems or where microphones are difficult to access an acoustical calibration is sometimes impractical to perform In these situations a verification method can be used to provide a quick and easy method of checking the function of each measurement channel All the verification methods have their own advantages and disadvantages Three common verification methods are The Actuator Method Thelnsert Voltage Calibration Method The Charge Injection Calibration Method See chapter 4 and Section 6 6 for details 6 4 Laboratory Calibration Laboratory calibration is a common expression for the type of calibration that can not be performed in the field Laboratory calibration is an indoor method prefera bly performed in a dedicated and well controlled environment The laboratory calibration methods are normally more accurate than the field cali bration methods This is partly due to the type of equipment used for calibration and partly due to the stable laboratory environments Calibrations in the field are especially affected by temperature variations wind and humidity While field calibration is usually applied to an entire measurement system labora tory calibration also covers the calibration of separate devices such as microphones BE 1447 11 Microphone Handbook 6 5 Vol 1 Chapter 6 Calibration Laboratory Calibration an
148. ratory measurements and confirmed by calculations which are possible due to the simple cylindrical shapes of the condens er microphone The simple diaphragm system and the simple geometry of a condenser microphone ease accurate measurement calculation and description of the microphone proper ties under various sound field and environmental conditions These are very impor tant features of microphones which are to be used for accurate measurements under the many different measurement conditions which may occur Corrections for the influence of the microphone on the sound field may be incorpo rated in the microphone design Alternatively it may be made by post processing of the results A list of general measurement microphone requirements is given below e good acoustic and electric performance wide frequency range and flat frequency response wide linear dynamic range low inherent noise and low distortion low influence on the sound field to be measured e minor influence from environment low influence from ambient pressure temperature and humidity low influence from vibration magnetic and electromagnetic fields etc good mechanical robustness good bump and shock resistance good chemical resistance good corrosion resistance e high stability of sensitivity and frequency response small short term fluctuation random changes Microphone Handbook Bruel amp Kjaer Vol 1 Chapter 2 Microphone Theor
149. re be used for infra sound measurements However in practice the use of the methods is rare due to their complexity lack of stability and the relative ly high inherent noise levels that these methods imply Only the constant charge principle will be discussed in the following External Polarization Source The constant charge of the capacitance between the diaphragm and the backplate may be applied either from an external voltage source as employed for externally polarized microphones or from a permanently charged polymer known as electret as employed for prepolarized condenser microphones Today the newer pre polariza tion principle is widely used especially for microphones used with hand held instru ments such as sound level meters The transduction principle of a condenser microphone using external polarization is illustrated in Fig 2 3 The capacitor of a condenser microphone is formed by two plates the diaphragm and the back plate These plates are polarized by an external voltage source which supplies a charge via a resistor The resistance of this must be so high that it ensures an essentially constant charge on the microphone even when its capacitance changes due to the sound pressure on its diaphragm The charge must be constant even at the lowest operational frequencies The value of the resistor is typically in the range 10 to 10100 or 1 to 10GQ One of the plates the diaphragm may be displaced by the sound pressure while the
150. re non linear The change in distance is negative for a positive pressure Therefore for a positive polarization voltage which is most commonly used the phase of the output voltage is opposite to that of the sound pressure A positive pressure creates a negative output voltage and vice versa 2 12 Microphone Handbook Br el amp Kjeer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Electret Polarization Prepolarized microphones contain an electret The electret consists of a specially selected and stabilised high temperature polymer material which is applied to the top of the backplate see Fig 2 4 The electret contains trapped or frozen electrical charges which produce the necessary electrical field in the air gap The frozen charge remains inside the electret and stays stable for thousands of years Diaphragm Image charge gt Air gap III i Frozen charge er QOO E Electret Layer l i i O U o Image charge GJ G Backplate Diaphragm Backplate 950601e Fig 2 4 Electra polarization The electre amp consists of a polymer which contains a permanent or frozen electrical charge This and an image charge create the necessary fidd between the diaphragm and the back plate The frozen charge is located near to the surface of the polymer which faces the air gap behind the diaphragm The frozen charge attracts an equally large image charge of o
151. resence in the measurement microphone field had become firmly established with the development of high sensitivity l microphones These microphones benefited from new techniques allowing less ten sion in the diaphragm leading to an increase in sensitivity of approximately 12 dB During this period Br el amp Kj r also improved techniques to accurately measure and document the performance parameters of their measurement microphones and supplied this information in the form of calibration charts Microphones could sub sequently be relied upon according to certain stated parameters Microphone Handbook Br el amp Kjaer Vol 1 Chapter 1 Introduction Historical Background to Microphone Development at Bruel amp Kjaer Calibration equipment was also made such as reciprocity equipment for laboratory calibration and the pioneering hand held pistonphone This convenient calibration device effectively improved the accuracy of everyday microphone usage by allowing users to check measurement accuracy in the field In 1973 Bruel amp Kjar consolidated their position as the leading microphone produc er by meeting a request from Western Electric to supply microphones to replace their successful but ageing WE 640AA microphone The Br el amp Kjaer solution took the form of the classic Type 4160 microphone The 4160 together with the 4180 the l version were quickly established as reference microphones for laboratory stand ard use
152. rom lg 3 16mm to about 1 23 77 mm A newer internal design invented and patented by Bruel amp Kj r is also described For both designs the microphone properties such as sensitivity frequen Cy response and inherent noise depend mainly on the diameter and on the dynamic properties of the microphone diaphragm system The Br el amp Kj r microphone pro gram is composed of a number of microphone types with properties optimised for many different applications 2 3 2 Design Description A condenser microphone consists of a metal housing inside which an electrical insu lator with a back plate is mounted behind a delicate and highly tensioned dia phragm see Fig 2 2 a The flat front surface of the back plate is positioned dose to the front open section of the housing The diaphragm rests across the housing in a way which makes the diaphragm parallel to the backplate A controlled mechani BE 1447 11 Microphone Handbook 2 7 Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design Fig 2 1 Classic Design of a Condenser Measurement Microphone cal tension in the foil gives the diaphragm the required mechanical stiffness The distance between the backplate and the diaphragm is typically 20um 0 8um The nominal distance may vary between microphone types from about 15 to 30um The thickness of the diaphragm may vary from about 1 5 to 8um depending on the microphone type The tolerance is typically less th
153. rophone Handbook 4 17 Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Techniques 33 10 100 1k 10k 100 k Frequency Hz 960273e Fig 4 12 Measured ratio ey as a function of frequency for a 1y 2 microphone 50mV Pa and preamplifier with and without polarization voltage The acoustical equivalent level to the signal introduced through CIC can for the mid frequency range be calculated according to SPL PE BG 94dB 20109 e UNS Sc Po Cc Ca where Sc loaded sensitivity for the microphone e g 12 5 mV Pa po pressure level for stated microphone sensitivity 1Pa Cm microphone capacitance eg 15pF Ce CIC capacitance typically 0 2 pF The CIC method shows that the microphone capacitance varies with frequency This phenomenon is explained in Section 3 8 Use of the CIC System For acoustical systems frequent use of a precision acoustical calibrator would be the ideal means of verification Although this can involve practical and economical disadvantages for example difficulty of access the time involved and disassembl y Used in the right way the CIC technique has the advantage that it can be used to increase the interval between costly acoustical calibrations H owever an acoustical calibration can never be completely replaced by an electrical test facility
154. rophone cartridge Fault Diagnosis with CIC Those who do the monitoring of the measurement system do not need to know the reasons for the observed changes in the ratio between the output and the input signals However those who perform the maintenance and fault finding may find the following examples of the use of CIC helpful The following examples are obtained using a white noise signal as the CIC input Normal Working Condition eye 0 dB Input signal Output signal oo 40 dB Hz 960268e Fig 4 13 Normal working condition Notice the attenuation of ap proximatedy 40dB in the mid frequency range The low frequency roll off is caused by the preamplifier input re sistance Microphone Handbook Bruel amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Techniques Diaphragm Torn or Missing eje Input signal 0 dB E E d Output signal N 40 dB Hz 960269e Fig 4 14 No microphone attached or diaphragm torn or missing The output leva is significantly increased due to reduced mi crophone capacitance Microphone Short Circuited eye Input signal 0 dB E 40 dB Output signal Hz 960270e Fig 4 15 Microphone short circuited The output levd is signifi cantly reduced rdative to the normal condition BE 1447 11 Microphone Handbook 4 21 Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration
155. rresponds to change in sound pres sure which is due to the presence of the microphone body in the sound field The free field correction may therefore be used for determining the individual free field response characteristic of a microphone This is done by adding the correction to the individual pressure response or to the so called electrostatic actuator re sponse n practice this is a great advantage as those responses are less costly to determine This is especially the case for the actuator response Most Bruel amp Kj r free field and diffuse field corrections refer to the electrostatic actuator response Influence of the Microphone on the Measured Sound Pressure in a Diffuse The sound pressure of a certain point in a diffuse field is created by waves which over a certain time impinge to that point from all directions To measure the pres sure a microphone must be placed in the field However the microphone is not equally sensitive to sound waves coming from different directions as the pressure change due to the presence of the microphone and a possible protection grid is different for different angles Fig 2 31 shows free field corrections for a 4 2 microphone without protection grid These were measured for the angles between zero and 180 degrees in steps of 5 degrees The above free field corrections may be used for calculation of the diffuse field cor rection see the bold curve in Fig 2 31 The diffuse field correct
156. ruel amp Kjaer s response to customer requirements has created several innovations in the related acoustic measurement field including Sound intensity microphones phase matched for accurate low frequency meas urements see Section 2 3 7 Sound intensity calibrator simulating a free field environment for calibration of sound intensity equipment Charge injection calibration patented for verification of complete measurement systems see Section 4 8 Feedback calibration Multitone calibrator allowing calibration of microphones up to 16 kHz New intensity measurement microphones with patented pressure equalisation vent system giving much improved low frequency response Fig 1 3 Sound Intensity probe using a phase matched pair of mi crophones Microphone Handbook Br el amp Kjaer Vol 1 Chapter 1 Introduction Development of Microphone Products These and other innovations reflect a dedicated and uncompromising approach to the creation of measurement microphones where reliability accuracy and quality are the key considerations This philosophy continues to find ways of meeting the needs of a growing global market through intensive research and development work 1 2 Development of Microphone Products Development of microphone products is performed by a dedicated team of engineers This is something which is not easy to provide given the relatively small and spe cialist market for precisio
157. s is necessary to avoid noise due to arcing within the gap Therefore special grinding polishing deaning and test procedures are applied to the key components Assembly is per formed in a dean room environment Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Measurement Microphone Design From this brief description of microphone construction it can be seen that although the design of the condenser microphone may appear to be very simple the materi als that must be used the mechanical tolerances and the required properties make production of high quality microphones a task which requires significant knowledge and experience 2 3 4 Transduction Principles The condenser microphone cartridge converts sound pressure to capacitance varia tions These variations in capacitance may be converted to an electrical voltage in two ways The most simple conversion method makes use of a constant electrical charge which is either permanently built into the microphone cartridge or applied to it via the preamplifier Today this method is used for practically all sound meas urements However it should be mentioned that the capacitance variations may also be con verted to voltage by using high frequency circuits high frequency conversion may imply frequency or phase modulation and may use various types of bridge cou plings In principle such methods may work to very low frequencies even to DC and may therefo
158. s what the human ear detects Measurement microphones have therefore been widely used for analysis and recording of sound pressure This still applies today even though modern technology has made it possible for sound intensity and particle velocity measurements to be quite easily and commonly per formed Sound pressure as well as particle velocity sound intensity and their respective levels relative to defined references are thus the sound field parameters which are measured and stated in most measurement reports today The levels are expressed in decibels dB 2 1 2 Sound Pressure and Pressure L evel The sound pressure at a certain point is the difference between the instantaneous pressure and the ambient mean pressure The unit of Sound Pressure p is Pascal Pa which is equal to Newton per Square Meter N m The reference value for Sound Pressure Level SPL is twenty micro Pascal 20uPa The Sound Pressure Level Lp is defined by the formula below p p 20 log dB where P ef 20uPa Pref ref Ly 10 log Sound Pressure and Sound Pressure Level generally refer to the Root Mean Square RMS value of the pressure The RMS value is considered if no specific reference is stated A pressure equal to the reference value is thus equal to zero dB while 1Pa Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Sound Levels and Sound Fields equals 94dB 93 98dB The zero dB value correspon
159. se Individually measured frequency response calibrations are supplied with most Bruel amp Kj r microphones The frequency response of a microphone depends on the type of sound field in which the microphone is used In acoustic measurements there are three main types of sound field e Free field e Pressure field e Diffuse field equivalent to random incidence response BE 1447 11 Microphone Handbook 3 5 Vol 1 Chapter 3 Characteristics of Microphones Frequency Response The microphone is optimised to have a flat frequency response in one of these sound fields This response is called the optimised response The differences be tween the responses are only evident at higher frequencies Below 1000Hz the responses differ by typically less than O 1dB The determination of all responses is based on corrections added to individually measured electrostatic actuator responses above 200Hz All measurement micro phones have a relatively flat frequency response from 10 Hz to 1000Hz independ ent of the sound field According to IEC 651 and IEC 1094 4 standards one frequency in the 200 Hz to 1000 Hz range is selected as the reference frequency see Fig 3 3 Bruel amp Kj r normally use 250Hz 102 4 Hz as the reference frequency The sensitivity at the reference frequency is used to represent the overall sensitivity of the microphone Low frequency Response High frequency Response Reference frequency 10 100 200 1k
160. se is characterised as white noise i e the same magnitude for all frequencies and is proportional with both resistance and absolute 4 8 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Dynamic Range Fig 4 5 temperature However in the preamplifier this resistor noise is shunted by the capacitance of the microphone Cm This has the effect of low pass filtering the noise signal resulting in a noise spectrum as seen in Fig 4 5 which shows the noise spectrum for different combinations of capacitance and resistance 107 VV Hz 107 107 1078 0 01 0 10 1 10 100 1k 10k 100 k Frequency Hz 960272e Noise spectral density v Hz for different combinations of microphone capacitances and input resistances In the frequency range where the preamplifier is used this low pass filtering caus es a noise decrease when the input resistance increases due to the low pass filter ing effect For this reason modern preamplifiers are designed with high input resistances The input resistance is determined by two resistors one used to establish the polar isation voltage for the microphone and the other to set the DC working point for the preamplifier Resistors of too high a value can give practical problems both with respect to DC stability and with inconvenient long stabilisation times for the complete measuring system The second noise source the transistor noise NET
161. sed but for high humidity prepo larized microphones offer greater reliability This is because they are more re sistant to the attenuation of the polarization voltage which can occur at high humidity levels BE 1447 11 Microphone Handbook 5 7 Vol 1 Chapter 5 Selecting a Microphone Selecting The Right Accessories 5 2 9 Microphone Array Applications When many microphones are to be used for example for spatial transformation of sound fields the size and price per channel are the most important parameters For this purpose a specially designed microphone with built in preamplifier can be used as an integral part of an array 5 3 Selecting The Right Accessories 9602426 Fig 5 4 Range of microphone accessories windscreens micro phone holder turbulence screen windscreen with bird spikes rain cover and nose cone 5 8 Microphone Handbook Br el amp Kj r Vol 1 Chapter 5 Selecting a Microphone Selecting The Right Accessories 5 3 1 5 3 2 For most types of microphones the use of appropriate accessories can effectively extend the application range of the microphone particularly when making outdoor measurements Accessories are therefore described here with respect to the two main threats to effective microphone operation wind or air turbulence and humid ity Low Wind Speed Random Direction Outdoor measurements are often disturbed by wind noise An easy way of reducing the effect of wi
162. ssure on the diaphragm see Fig 2 30 upper curve This leads to a frequency response characteristic which is essentially flat in a free field when the output voltage is referred to the sound pressure of the undisturbed free field The produc or the sum when decibels are applied of the relative pressure increase and the diaphragm system sensitivity is constant over the frequency range see the middle curve shown in Fig 2 30 The pressure increase on the diaphragm may be used for extending the frequency range of free field microphone types at higher frequencies Therefore free field mi crophones cover wider frequency ranges than equally large and equally sensitive Pressure field microphone types which have a flat pressure field response The ex tension typically equals one octave The ratio between the diaphragm pressure and the undisturbed free field pressure is generally expressed in decibels This ratio is essentially the same for all micro Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Microphone T ypes Dedicated to Different Sound Fields 2 5 5 phones of the same type as their dimensions are the same although the ratio depends slightly on the diaphragm impedance as well Therefore it is determined in connection with the development of any new Bruel amp Kj r microphone type The measured number is generally called the free field correction As described above the free field correction co
163. story of microphone development at Bruel amp Kj r and provides an insight into the research and development invested in microphone products An overview of the production of microphones is also given Chapter 2 gives a general introduction to microphone theory and explains the decisions that are made about the design and construction of microphones Chapter 3 provides more detailed information about the characteristics of microphones with the aim of allowing an informed view on the specifications applied to microphone products Chapter 4 introduces the main characteristics of preamplifiers and explains the categories of information given in Volume 2 Chapter 5 provides information on the selection criteria commonly used to identify the most suitable microphone for different applications and also the most applicable accessories that should be used Chapter 6 discusses the requirement for the calibration of microphones and offers an over view of calibration methods BE 1447 11 Microphone Handbook Vol 1 0 ii Microphone Handbook Br el amp Kj r Vol 1 Contents 1 Introduction ttti ttt 1 1 1 1 Historical Background to Microphone Development at Bruel amp Kj r 1 2 1 2 Development of Microphone Products eeseseeeseeeeeeeeee nennen nennen kr 1 5 1 3 Production of Microphones at Br el amp Kjaer ou sasssa 1 9 1 4 dtt aw s 02 QC 1 11 2 Microphone The
164. t also varies over time at the same location With the exception of few locations these variations do not usually exceed the range 80 to 120kPa only 20kPa relative to one atmosphere Those micro phones with the greatest sensitivity to ambient pressure will rarely give rise to a correction of more than 0 4 dB See microphone type specifications for more details Fig 3 11 shows an example of the variation in sensitivity as a function of ambient pressure at the reference frequency of 250Hz The slope of the curve at 101kPa is specified as the pressure coefficient and is determined by the ratio between the diaphragm stiffness and the stiffness of the air in the internal microphone cavity The pressure coefficient may be used to add corrections to the sensitivity of the microphone H owever if a sound calibrator is used to calibrate the entire measure BE 1447 11 Microphone Handbook 3 19 Vol 1 Chapter 3 Characteristics of Microphones Reversible Changes ment system the change in sensitivity of the microphone is automatically taken into account 100 l 1 Ambient Pressure kPa 950615e Fig 3 11 Example of the variation in sensitivity as a function of ambient pressure The graph in Fig 3 12 shows the variation in frequency response due to changes in ambient pressure below one atmosphere These graphs may be used to make correc tions of the frequency response For small variations in ambient pressure above one atmosphere the c
165. t of Temperature 4 11 4 6 EI HMMM 4 13 4 7 Electromagnetic OY FR SRS 4 13 The European EMC IS sannana hika anna sveesnwuncasseceninitiasa es 4 13 SS ss RU T Sanncaueeeeesaticueqn satan taiaaeseseca 4 13 EMC Test Facilities at Bruel amp K Jaer eeeeeeeeeene eene nnne nnns 4 14 4 8 Monitoring and Calibration Techniques eeeeeeseeeeeneeee nnne nnns 4 15 inseri gees lt 2 isles dit RITTER 4 16 A ete sk PI COT 4 17 CIC Input Signal Requirements sanna beicdddaiidsdeaieaibls Ahl a ab 4 19 5 Selecting a Microphone 5 1 5 1 Guidelines on Selecting Microphones 5 2 5 2 What to Consider lseeeeseeeeeeeee nennen REKNIR 5 4 Frequency FR I c E 5 4 Type of Sound Field ecccccccecsseeecsaeeessaeeeseeesaeseesaeaessaaeessaseesaeseesaaeeesaseeesaeeeenaes 5 4 Limits of Dynamic Rangg eeeeeeeee eene nnn nnne nenne nnn nian nani n naa ann n nan 5 4 Microphone Venting eeeeeeeeee eene nnn nnne nn nhan nn nian rana nana aan n nan 5 5 Phase RESPONSE 5 5 P E a E S 5 5 Sihe fis geo 8s e li c r 5 6 znugcian i A es eee 5 7 BE 1447 11 Microphone Handbook 0 3 10 3 97 Vol 1 Contents Microphone Array Applications 5 8 5 3 Selecting The R
166. t third harmonic component which increases by the square of the pressure The microphone distortion is practically proportional to the parallel or stray capaci tance Br el amp Kjaer has patented a method for reduction of microphone system dis tortion which implies the application of negative preamplifier input capacitance for equalization of the passive part of the microphone capacitance Microphone distortion is proportional to diaphragm displacement Therefore small er and less sensitive microphones produce less distortion than larger microphones do at the same sound pressure However as the parallel capacitance of a small microphone is relatively larger than that of a larger microphone the reduction in BE 1447 11 Microphone Handbook 2 41 Vol 1 Chapter 2 Microphone Theory Combination of Microphone and Preamplifier distortion does not fully follow the sensitivity reduction This can for example be seen in the sensitivity and distortion differences between a 1 2 and a 4 micro phone The difference in sensitivity is 22dB while the difference in distortion is only 15dB Distortion 96 ond Harmonic Pd ar Harmonic 125 135 145 SPL dB 155 940498e Fig 2 24 Distortion of microphone M and of microphone pream plifier combination C The preamplifier distortion which plays the minor role is just noticeable at
167. tabilise these param eters The stabilisation is not only relevant for high temperature applications but also in connection with laboratory standard microphones which are calibrated at room temperature to within a hundredth of a decibel They must therefore be very stable A permanent decrease of the diaphragm tension implies an increase of the micro phone sensitivity while a decrease of the electret charge causes a decrease of the sensitivity The speed of the relaxation processes which lead to permanent sensitivity changes depends on temperature At high temperatures the speed at which changes ocaur is so high that it can be measured Under normal ambient conditions the speed at which changes occur is very low and is not directly measurable The stability at room temperature may be estimated from high temperature measurements by ex trapolation using an Arhenius plot This displays stability as a function the recipro cal Kelvin temperature Within a particular type of microphone there may be a great spread between the units The high temperature stability of some units may be up to ten times better than the estimated and specified value Very often the permanent and systematic changes which are to be expected are less than the random changes which may occur due to mechanical shocks and heat transients This is especially the case for microphones with stainless steel dia phragms Falcon range microphones as they are significantly less sensi
168. tanding of measurement microphones and in so doing to allow the reader to select and use the correct type of microphone for a specific purpose More detailed information on specific types of microphone and microphone products is provided in Volume 2 of this handbook on individual microphone calibration charts and in product data sheets Chapter 3 provides further background on the way this data is commonly presented BE 1447 11 Microphone Handbook 2 59 Vol 1 Microphone Handbook Vol 1 Br el amp Kjaer Chapter 3 Characteristics of Microphones BE 1447 11 Microphone Handbook Vol 1 3 1 Chapter 3 Characteristics of Microphones Introduction to Characteristics 3 1 Introduction to Characteristics The previous chapter of this handbook has described the theory and principles be hind measurement microphones In doing so the way in which different micro phones are designed for different purposes has been explained The suitability of a microphone for a particular task is commonly described in terms of a set of characteristics This chapter describes these characteristics in some detail to give a background to how and why different characteristics are measured These characteristics are referred to in product specific literature such as Product Data sheets and Volume 2 of this handbook Characteristics are normally described under the following headings The Calibration Chart Sensitivity Freq
169. ted to the same signal which in turn means that there is no phase difference between the signals at the microphones and hence no sound intensity should be detected A measurement in this situation represents the false intensity generated by the measurement system itself due to the phase mis match between measurement channels Comparing the actual sound pressure level with the lowest measurable intensity represents the measurement capability of the intensity instrument see IEC 1043 6 6 4 Sound Pressure Calibrator M ethod The purpose of using a sound pressure calibrator is to get a well defined sound pressure with a certain microphone This makes the calibrators equally suited for calibration of single microphones as well as entire measurement channels In most cases the calibrator has only a single tone frequency in the range 200 to 1000 Hz at which the calibration is performed IEC 942 Sound Calibrators but multitone calibrators are also available usually with frequencies in steps of one octave When using the calibrator for microphone calibration the output from the micro phone is measured with the well defined sound pressure from the calibrator applied to its front The sensitivity is determined by dividing the output voltage by the sound pressure When the calibrator is used to calibrate the entire measurement channel the well defined sound pressure is applied to the front of the microphone and a proper adjustment is made to give the
170. tercomparison methods Round Robin to en sure their traceability Traceability is defined in the International Vocabulary of Basic and General Terms in Metrology as the property of the result of a measurement or the value of a standard whereby it can be related to stated references usually national or interna tional standards through an unbroken chain of comparisons all having stated un certainti es Microphone Handbook Br el amp Kjaer Vol 1 Chapter 6 Calibration Calibration Methods The uncertainty of a measurement is defined as the parameter associated with the result of a measurement that characterises the dispersion of the values that could reasonably be attributed to the measurand The accuracy or a measurement is the doseness of the agreement between the result of a measurement and a true value of the measurand Note that accuracy is a qualitative concept The term precision should not be used for accuracy In the ISO publication Guide to the Expression of Uncertainty in Measurement it is recommended that uncertainty is stated in terms of 2 standard deviations 2 sig ma This means that 95 of the calibrations will be within the stated uncertainty range for a normal distribution function Normally the uncertainty decreases the higher up in the hierarchy the chain the calibrations are performed with absolute calibration methods based directly on the physical units at the top of the hierarchy This pos
171. termined at 250Hz or 1000Hz is used for calibration and monitoring of the microphone cartridge When the entire measurement system must be calibrated the influence of the preamplifier should be taken into account see 3 2 2 3 2 2 Loaded Sensitivity Sc When the microphone cartridge is connected to the preamplifier its input voltage is attenuated by the preamplifier input capacitance Cj This effect is valid over a wide frequency range therefore the capaciti ve loading of the cartridge has no effect on the relative frequency response The gain of the microphone and preamplifier combination G is also influenced by the gain of the preamplifier itself g which is measured by connecting the genera tor directly to the preamplifier input The loaded sensitivity Sc can thus be de scribed using the open circuit sensitivity so S S G dB Microphone Preamplifier 950574 1e Fig 3 2 Circuit diagram of preamplifier and microphone combi nation Cm where Cm polarized microphone capacitance as stated on the microphone calibration chart Ci preamplifier input capacitance as given in the preamplifier specifications G gain of the microphone and preamplifier combination g gain of the preamplifier as given in the preamplifier specifications Microphone Handbook Br el amp Kjaer Vol 1 Chapter 3 Characteristics of Microphones Frequency Response The internal gain g of the Bruel amp Kj
172. tes on the preamplifier and its associated meas urement chain The dynamic range of the preamplifier is the ratio between the maximum and the minimum output voltages The lower limit is defined by the self generated noise of the amplifier and the upper limit is set by the distortion created at high output levels due to dipping of the signal The typical dynamic range of a preamplifier varies from about 1 to 4uV A weight ed to approximately 35V with maximum preamplifier supply voltage This gives a dynamic range of up to about 140 150 dB The location of this range with respect to sound pressure level is mainly dependent on the sensitivity of the selected mi crophone The selection of the microphone depends on the actual measurement re quirements see Chapter 5 The remainder of this section describes some of the different mechanisms which set the limits at both ends of the dynamic range 4 3 1 Upper limit of Dynamic Range The parameters that determine the combined frequency response of the preamplifi er and cable also determine the response and behaviour at high signal levels These BE 1447 11 Microphone Handbook 4 5 Vol 1 Chapter 4 Characteristics of Preamplifiers Dynamic Range parameters are the maximum output voltage the current and the slew rate of the signal 4 3 2 Maximum Voltage The limitation set by the maximum output voltage is due to a dipping of the signal which occurs when the peak of the output
173. the high est measurable levels The distortion is valid for a fr fidd microphone 1 7 50mV Pa It is measured at 90Hz and is representative for that part of the frequency range where the diaphragm displacement is dominantly controlled by stiffness Preamplifier and Microphone System Distorti on The preamplifier distortion may generally be ignored as this is much lower than the microphone distortion This is the case at levels up to a few decibels below the upper operation limit where dipping occurs and makes the system unusable see Fig 2 24 The results shown in the figure are obtained with a Br el amp Kjar High Pressure and Low Frequency Calibrator Type 4221 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Microphone T ypes Dedicated to Different Sound Fields Distortion Due to Large Parallel Capacitors Most often the preamplifier has only a minor influence on the microphone distor tion as it only adds 0 2 to 0 5 pF to its passive parallel capacitance However when a large passive capacitor is used for extending the frequency response to lower frequencies this will have great influence on the distortion which may increase by a factor of five or more see Section 2 4 2 2 5 Microphone Types Dedicated to Different Sound Fields 2 5 1 When a microphone is placed in the sound field which it is to measure it will influence the field and change the sound pressure This change needs to be tak
174. the product data sheet for the power supply Often it is convenient to express the preamplifier maximum output in terms of equivalent peak sound pressure level The equivalent peak sound pressure can be found from the following equation Speak SPL pea max 20 of 5 esk dB C ref where SPLneak equivalent peak sound pressure level of the preamplifier peak Maximum output voltage of the preamplifier V Microphone Handbook Br el amp Kjaer Vol 1 Chapter 3 Characteristics of Microphones Equivalent Diaphragm Volume Se oaded microphone sensitivity V Pa Pref reference sound pressure 20 10 Pa 3 6 Equivalent Diaphragm Volume 3 7 The equivalent diaphragm volume is that volume of air which has the same compli ance as the diaphragm at a static pressure of 101 3 kPa Equivalent diaphragm volume is used in connection with coupler calibration of mi crophones and for evaluating the loading which the microphone presents to small couplers See Section 2 3 12 for more detailed information Calibrator Load Volume The calibrator load volume is the sum of the equivalent diaphragm volume and the air volume enclosed by the outer surface of the protection grid and the diaphragm See the relevant data sheet for typical values The calibrator load volume is used to evaluate and correct the loading of micro phones when used on calibrators and pistonphones 3 8 Capacitance When a condenser microphone
175. tive to long term heat exposure than the Nickel diaphragms used by other types of microphone Minor random changes in sensitivity may occur due to irreversible mutual displace ments of the microphone parts Therefore microphones which are used as reference standard microphones for example for national standards should always be stored at room temperature For an externally polarized microphone the diaphragm tension may change over time It will decrease slightly and lead to a small increase in the microphone sensi tivity The rate at which this occurs depends on the temperature and follows the general rule for a material process BE 1447 11 Microphone Handbook 2 51 Vol 1 Chapter 2 Microphone Theory Long Term Microphone Stability Q tr kx eP RT where tz is the time for a certain charge at the absolute temperature T Q is the process activation energy R is the universal gas constant and k is a constant Transformed to logarithmic form this gives 1 logt Ki K x where K1 and K2 are constants which characterise the material process The equa tion shows a linear relationship between log tr and 1 T For most Bruel amp Kj r microphone types the long term stability is described by the stability valid at a Reference Temperature see microphone type specifications and by a Stability Factor which is a function of temperature The Stability Factor de pends on the type of material and relaxation process The
176. to produce a well defined calibration pressure over a wide frequency range without the special facilities of an acoustics laboratory However the actuator method cannot be used for determination of the microphone sensitivity as its absolute accuracy is not sufficiently high Therefore an actuator frequency response calibration is in most cases combined with an absolute sensitivi ty calibration at a reference frequency This may be performed by using a piston phone a sound level calibrator or other means The actuator simulates a pressure field or a pressure acting directly on the micro phone diaphragm It cannot simulate a free field or a diffuse field Free field and diffuse field corrections must therefore be available if calibrations valid for those types of sound field are required Microphone Handbook Br el amp Kjaer Vol 1 Chapter 2 Microphone Theory Electrostatic Actuator Calibration The electrostatic force or pressure produced by the actuator is practically independ ent of environmental factors This makes the actuator method well suited to envi ronmental testing of microphones which may be performed at various temperatures pressures and even in gasses other than air 2 7 2 Electrostatic Actuator An electrostatic actuator is a stiff and electrically conducting metal plate which forms an electrical capacitor together with the microphone diaphragm This must be made of metal or of a metal coated material The a
177. tories Primary laboratories are nationally recognised laboratories which have the respon sibility of maintaining developing and promulgating the highest levels of metrology 6 4 2 Accredited Calibration Laboratories Accredited calibration means that the calibration has been performed by an accred ited calibration laboratory To achieve accredited status the laboratory must be approved by an external accreditation body which also performs an audit on a regular basis The laboratory must have their quality assurance manual calibration procedures uncertainty budgets and technical personnel approved by the accreditation body This means that accredited laboratories can only offer accredited services which are within the range approved by the accreditation body Examples of accredited calibration labo ratories are the Danish Primary Laboratory of Acoustics DPLA which is jointly run by Br el amp Kjaer and the Technical University of Denmark and the Br el amp Kjaer Calibration Laboratory Microphone Handbook Br el amp Kjaer Vol 1 Chapter 6 Calibration Laboratory Calibration 6 4 3 Calibration at the Bruel amp Kj r Factory Bruel amp Kj r performs thousands of calibrations every year as part of the produc tion of measurement equipment These are performed under laboratory conditions although they are often referred to as factory calibrations because they are an integral part of the production process All micr
178. ty of the measurement The calibration of a microphone using an electrostatic actuator implies a high AC voltage on the actuator and often a very low output voltage from the microphone being calibrated This creates a risk of making calibration errors due to cross talk in the measurement set up This risk is especially great at high frequencies The calibration set up should therefore be carefully checked and modified if necessary The check may be done by either setting the polarization voltages of the micro phone or actuator to zero volts The displayed signal should then fall by 40dB or more depending on the required calibration accuracy If this is not the case the cross talk signal might be reduced by proper selection of instrument ground connec tions 2 7 5 Actuator and Pressure field Responses In principle the actuator and the pressure response of a microphone are different In practice the difference may be insignificant It may vary between less than 0 1dB and about 1dB depending on the acoustic impedance of the microphone dia phragm system and on the radiation impedance which loads the outside of the diaphragm The higher the diaphragm impedance the smaller the difference For the newer and for some of the older Bruel amp Kj r microphone types corrections are available for determination of pressure field responses from measured actuator responses see examples for 1 and 1 2 microphones in Fig 2 37 As smaller micro p
179. ually have the most accurate measurement equipment Normally the establishment to which a calibration can be traced is stated as a reference for a calibration and as already mentioned the terms of reference are monitored between calibration establishments See Section 6 5 for more information on traceability It is important to note that traceability is not in itself an indication of high accura Cy but if the uncertainty is known then the calibration can be compared with other valid measurements Therefore a calibration is not useful unless the related uncer tainty is known Other Definitions Definitions are available which may provide a variation on the above However they generally have some main principles in common as seen by the following examples The first comes from the ISO publication The International Vocabulary of Basic and General Terms in M amp rology Calibration The set of operations that establish under specified conditions the relationship between values of quantities indicated by a measuring in strument or a measuring system or values represented by a material meas ure and the corresponding values realised by standards Microphone Handbook Br el amp Kjaer Vol 1 Chapter 6 Calibration Introduction The important point in this definition is the reference to corresponding values realised by standards in the last part of the rather long sentence This supports the argument that fundamental to c
180. uency Response Directional Characteristics Dynamic Range Equivalent Volume and Calibrator Load Volume Capacitance Polarization Voltage Leakage Resistance Stability Effect of Temperature Effect of Ambient Pressure Effect of Humidity Effect of Magnetic Field Electromagnetic Compatibility Similar headings are used in this chapter to provide a reference for the information about specific microphones that is given in product specific literature Information about the characteristics of preamplifiers is not given in this Chapter but is induded in Chapter 4 3 1 1 The Calibration Chart and Diskette The calibration chart is an important piece of documentation providing essential information about the performance characteristics of the microphone A calibration chart is supplied with each microphone delivered by Bruel amp Kj r and each calibra tion chart is individual to that microphone The calibration chart states both typical 3 2 Microphone Handbook Br el amp Kjaer Vol 1 Chapter 3 Characteristics of Microphones Sensitivity and individual data that applies to the microphone cartridge only i e not a micro phone cartridge and preamplifier combination The open circuit sensitivity of the microphone cartridge is an example of an individ ual and valuable piece of information given on the calibration chart It allows the user to verify the performance of the cartridge and together wit
181. ugh particular types of microphone are optimised for particular purposes they still have a wide operational range as explained by the Frequency Response Dy namic Range inter relationship in Section 5 1 Frequency response should therefore be considered in relation to other selection requirements such as the type of sound field 5 2 2 Type of Sound Field A good way of narrowing down the choice of microphone is to consider the type of sound field in which measurements are being taken For measurements to be made away from reflecting surfaces or in acoustically well damped indoor environments for example when making outdoor measurements with a sound level meter or indoors in an office offering a lot of natural acoustic damping a free field microphone offers the best choice M eanwhile for measurements to be made in small dosed couplers or dose to hard reflective surfaces a pressure field microphone is a more appropriate choice An application which illustrates such usage is where a set of pressure sensing micro phones are positioned at different points across an aircraft wing A complete picture of the pressure variations across the wing surface can then be established For measurements in enclosed areas where reverberations are likely pressure field microphones adapted for random incidence measurements offer the best choice This is because the random incidence response of a pressure field microphone is much flatter or constant across th
182. um sound pressure level the frequency and cable load for a given current capability is given by the following formula Microphone Handbook Br el amp Kjaer Vol 1 Chapter 4 Characteristics of Preamplifiers Dynamic Range peak S PL peak ImaX 24 atl p e e G where pek maximum current capacity of the preamplifier or if lower of the pow er supply in A C total capacitative load presented by the connection cable in F Typically 50 to 100 pF m Po pressure level for stated microphone sensitivity 1 Pa f applied maximum frequency 4 3 4 Maximum Slew Rate The third limitation is due to the slew rate which is defined as the rate of change of output voltage i e de dt The slew rate limitation is caused by the internal cur rents and capacities inside the preamplifier This is typically the limiting factor for the output voltage and frequency when short cables are used The slew rate only requires attention in special situations for example where com bined very high frequencies and signal levels occur The limitation is described by S PL peak max 94 20l0g 5 n B ES where de dt maximum slew rate V s fmax maximum frequency of interest The influence of these three limitations voltage current and slew rate are shown in Fig 4 3 The upper limits are defined here by a distortion level of 3 Common to all these limitations is the fact that an excess will create a sudden rise
183. ust Many of the dosely controlled production processes stem from this requirement At Bruel amp Kj r the emphasis is therefore on quality rather than mass production Some examples from the different stages of production illustrate this approach Two components which receive a lot of attention during production are the micro phone diaphragm and backplate During production the surfaces of these compo nents are made extremely smooth as a very high electrical field strength must exist across the diaphragm to backplate gap Any unevenness in the surfaces can lead to arcing All surfaces must also be thoroughly dean to avoid contamination from dust and dirt that would accentuate the detrimental effect of humidity and in turn the performance of the microphone A very precise and dean construction environment as well as rigorous quality control are therefore employed This indudes assembly in a dean room environment where potentially harmful particles of dust in the air are kept to a minimum 950863e Fig 1 7 Prepolarized microphones nearing the end of production BE 1447 11 Microphone Handbook 1 9 Vol 1 Chapter 1 Introduction Production of Microphones at Bruel amp Kjaer Another critical area of production is the distance between the microphone dia phragm and backplate which must be constructed to very small mechanical toler ances Typically this is set to 20u with a tolerance of 0 5 u The required distance is monitored a
184. void demodulation of the RF noise This means that connecting a preamplifier with high immunity to an old measuring amplifier that is not con structed to achieve RF immunity will not give the expected immunity for the system Fig 4 10 shows the immunity improvement that can be obtained with a preamplifier constructed to fulfil the EM C requirements compared to an earlier version These measurements show the noise signal generated in the preamplifier when it is ex posed to an EMC field as described in the EMC standards a field strength of 3 10 V m a modulation of 80 AM and a carrier frequency of 80 1000 M Hz LM E LLL M MA WV LLL T Pe iL 1 MN CoP Pri 810 MHz 3 4 56 5 6 8 10 MHz 950956 1e Fig 4 10 Preamplifier noise when exposed to electromagnetic fidd before left and after EMC improvements right The OdB line corresponds to the inherent noise levd of the preamplifier The x axis corresponds to the carrier fre quency 20M Hz to 1GHz 4 8 Monitoring and Calibration Techniques Two different verification techniques are available for Br el amp Kj r preamplifiers These are either the Insert Voltage Calibration IVC facility or the Charge Injec tion Calibration CIC facility Each has a particular area of application The princi ple of both these techniques is shown in Fig 4 11 BE 1447 11 Microphone Handbook 4 15 Vol 1 Chapter 4 Characteristics of Preamplifiers Monitoring and Calibration Tech
185. where the static pressure equalisation vent is inside B and outside A the sound fied The frequency at which the response is down by 3dB is called the Lower Limiting Frequency of the microphone At Br el amp Kjaer 250Hz is normally used as the Ref erence Frequency as this frequency lies well within the flattest and most well defined part of the frequency response characteristic In some measurement circumstances only the microphone diaphragm is exposed to the sound field This is often the case when the microphone is used for measure ments in small endosures such as various types of acoustic couplers When this is the case the response does not fall with decreasing frequency In fact it increases with falling frequency because the fraction of stiffness which is due to the reactive pressure in the internal cavity becomes smaller as this is equalised through the vent see Fig 2 7 The low frequency sensitivity increase is smaller for microphones having a low fraction of air stiffness The lower limiting frequency is a function of static pressure as this determines the compliance of the internal cavity Generally this effect can be ignored but under specific circumstances the response may change significantly This may be the case in pressurised tanks diving bells and inside some aircraft Examples of calculated magnitude and phase responses are given in Fig 2 8 for an ambient pressure of 0 5 1 0 2 0 and 10 bar and for a microphone wit
186. y Measurement Microphone Design small long term drift systematic change small high temperature drift systematic change e high suitability for measurement and calculation of properties Suitable for calibration using practical and accurate methods simple shapes and easy to describe dynamic system parameters e comprehensive specifications and performance descriptions availability of measured and calculated microphone type data performance documentation by individual calibration chart availability of service for periodic recalibration The International Electrotechnical Commission I EC has worked out two standards which prescribe performance requirements for types of Laboratory Standard and Working Standard microphones respectively These are IEC 1094 1 Laboratory Standards and IEC 1094 4 Working Standards The standards are available through national standard organisations 2 3 Measurement Microphone Design 2 3 1 Introduction The pressure sensing microphone as introduced in the previous section can be designed for use in different types of sound field i e in a Pressure field Free field and Diffuse field The influence of the different design parameters is discussed in this section The dassic design of a pressure sensing measurement microphone is shown in Fig 2 1 The microphone shown in Fig 2 1 is a 1 7 microphone 12 7 mm but microphones of a similar design exist with housing diameters f

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