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ARTA-HANDBOOK

1. U b 04 aoe 1800 zn 0 T 26 2 100 200 AT 1 2 LIT 0 2 Cursor 1721 1 Hz 78 92 0p 12 3 023 Frequenty Cursor 200 H2 29 22 98 1121 659 Frequenty Ha c Near and farfield with level correction d Nearfield with baffle step correction 44 FR Magnitude dB re 200240 877 1 24 oct Phare 11 FR Magnitude dB re DuPaQ BIV emoothed 1 24 oct Phares 25 0 95 0 isa CIIN iy x 100 200 400 1 2 1 FID 0 100 20 Cursor 2420 Hz 79 36 68 424959 Curzor 2420 Hz 73 10 60 424 929 Frequency Ha e Near and farfield merge overlay f Quasi freefield response Figure 6 7 14 Modelling of the quasi freefield frequency response for a bass reflex enclosure We still need the farfield response to model the complete quasi freefield response The procedure for this is shown in Figure 6 7 14 The level adjustment panel c is carried out as described in Section 6 6 final visual fine tuning Remember to apply the baffle step correction to the nearfield frequency response panel d Then splice the near and farfield responses by using Merge Overlay In the above example the splice is applied at 240 Hz panels e and f Level matching using the volume flow method 131 10 Hz 20 50 100 200 500 Figure 6 7 15 LSP Cad simulation This method ass
2. 14 L e ee 5 Abmessung LS Ain mm B in mm in mm D in min 200 mm 8 in 1350 1650 225 150 250 mm 10 1690 2065 280 190 315 mm 12 in 2025 2475 340 225 400 mm 15 in 2530 3090 430 280 500 mm 18 3040 3715 505 340 Figure 9 2 Dimensions of the IEC standard baffles 9 1 Determination of X max AES2 says the following about X ax Voice coil peak displacement at which the linearity of the motor deviates 10 Linearity may be measured by percent distortion of the input current or by percent deviation of displacement versus input current Manufacturer shall state method used The measurement shall be made in free air at fS This recommendation has been extended through the initiative of W Klippel and 15 now included in the draft standard IEC PAS 62458 28 In Application Note AN4 12 for the Klippel Analyzer a procedure for the determination of X max 15 described The following example with ARTA gives the gist of this procedure 1 Measure the resonant frequency fs of the speaker with LIMP Choose stepped sine as the excitation signal In this example the resonance frequency fs 43 58Hz 2 Runa two tone signal through the speaker under freefield conditions with f1 fs 43 58Hz and f2 8 5 fs 370 43Hz and an amplitude ratio of U1 4 U2 Figure 9 1 1 and run a series
3. essen 163 7 4 Create WAV files for external signal excitation with 00 2 172 8 Dealing with measurement data data files shortcuts etc 173 8 1 Graphical representations in 173 9 1 1 Outputtimb and formatines 173 2 date 174 8 2 Editing measurement data and data 1 179 5 mcCaleand 183 Od Keyboard Shor MS 184 9 Recommended speaker specifications e 185 188 10 DpHeattondNOLU S cete eee liad 192 11 ULM 192 12 LFormulge andcT9 Ure Pa DEED MN 194 This handbook has been written to assist first time users of the ARTA family of speaker measurement programs and is intended to be used in conjunction with the original user manuals issued with the software You can find these together with other user information and application notes on the ARTA website http www artalabs hr While every effort is made to keep the handbook up to date the
4. ce c c d c DO oe ege 1 Xm L2 Lot i a gt i c sn bmt Pw ay arriet 12 3 eta gt FA o e ari LN F 4 Figure 7 3 5 Resonance detection different speaker cabinets Figure 7 3 5 shows measurements a 1m long open middle and closed left transmission line In addition light damping of the line has been measured right All measurements shown frequency phase impedance distortion burst decay CSD are affected by resonance particularly impedance The next example shows the re evaluation of a materials study by Ahlersmeyer 16 The full results are not reproduced here the following 15 restricted to evaluation of the study WAV files with ARTA The impulse responses for the combinations of materials studied are shown below 167 15 mm MDE ent Weichfaser 16mm MDF 2 5mm Kieber 4mm Sperrholz M ren nr rn ES 16 mm MDF 16 mm MDF ADAM eere BUM weve 16 mm MDF 6mm Fliese A WW VA PPS fatuum Pu P LOL OOP DI LLL AI ALE as memre 7 ww 0 VASA M M win r CC TREE AM ABA Add ED 16 mm 16 mm MDF 16mm MDF 4mm Bitument4mm Sperrholz T _ 22mm MDF
5. 27 4 1 3 30 4 2 Soundedtq usse 31 Calibration OF the Measurement 40 2 SoundcardcaliDE JHOD 40 Sll Calibration the outpit C Hannes 41 Calibration of the anpult Chane d den topo dedere va thes 42 32 o HE de DNE 44 22221 UserOr ManULACtURE s eoe oed te nente a on m tn ad eae etnia taut 44 3 2025 Ide OU idee Mtn 45 3 2 9 Tweeter iod 49 5 3 Microphone frequency 51 Calibration using a reference quality microphone gt 200 32 2 92 Calibration below 500Hz with a pressure chamber 55 Sd Tecnos the ae ere mer e Ero ie uod D URN 58 Mea 65 65 6 1 1 e Herm 65 6 1 2 The signal to noise ratio of the measurement 66 6 1 3 TAA fs E ra
6. Microphone sensitivity mV Pa 1 Connect microphone on Soundcard full scale output mV Soundcard full scale input mV 1 Connect electronic voltmeter or scope on left output channel setrange to 2 2 Generate sinus 400Hz Outputlevel 3dB 1 Connect sine generator with known output voltage on Channel gt 2 Enter voltage peak or rms 800 mv rms 3 Estimate Max Input mV Channel left Preamp gain 10 2 Attach sound calibrator Pressure 94 dB 3 Estimate Mic Sensitivity Estimated Current 8 59396 8 59396 mV Pa Accent 3 Enter voltmeter scope value 1000 mv rms 4 Estimate Max Output mV Estimated Current Estimated Current Left 1412 1412 Right 1412 1412 mV Diff 0 0 dB Figure 5 1 1 Calibration dialogue The calibration dialog is divided into three sections a sound card left channel output b sound card left and right channel input c microphone level calibration Note full scale input and output for the soundcard are given as mV peak in Soundcard and Microphone Calibration For the adjustment calculation with the measurement box use mV RMS 0 707 mV peak see Section 3 1 Vs V Peak Ver 0 707 Vs Vss V Peak Peak V mom Current value 5 1 1 Calibration of the output channel Use the following procedure to calibrate the outpu
7. 60 0 20 50 100 200 500 1k 2k 5k 10k 20k Cursor 20 5 Hz 59 98 dB Frequency Hz Figure 7 3 1 In room loudspeaker measurement see also Section 6 4 When in addition to resonance position quality decay duration are also to be determined this may be achieved using CSD or burst decay 163 Specira emoothed 1 6 oct eve PS LA Burst Decay rot ds 50 100 140 Ds 300 Fenods 5D 100 200 200 1 2k Frequency Frequency CHa 13a pr 2009 02 01 21 22 25 Fie 2009 02 01 20 0229 Figure 7 3 2 CSD and burst decay for the determination of room resonances Figure 7 3 2 shows the evaluation for the environment depicted by Figure 7 3 1 with a sampling frequency rate of 48kHz In the burst decay the resonances can be identified easily below 200Hz regardless of sampling frequency With CSD more guesswork is involved Cumulative Specia Decay emoothed 1 5 octave Cumulative Specie Decay emoothed 1 5 octave E Jew Yee m 1 f n rit iii 1 iB ira Frequency Hi 2008 02 04 21 18 04 a _ _ lt Figure 7 3 3 CSD with different sampling frequencies The problem can be solved by reducing the sampling frequency Figure 7 3 3 shows that the low frequency resolution
8. 70 FR dB VA 5 1 50 5 20 Corso 1035 0305 08 10 Fi a on ua FR Magnitude dB vis 20 10 20 60 0 500 qu 90 0 10 0 T Cursor SAHL 87 251 5 Hz cut off Figure 6 2 1 Effect of Pink cutoff at 10 20 50 and 500Hz _ FR Magnitude dB vis a EI 5 i2 20 100 Frequency Cus r 5 9 Bl 5 oH 20 Hz cut cir FR Magnitude dB vis Foo Fraen Hz Cursor 53Hz 43 54 oH EUD Hz ca ctr ak iJ 20 100 210 Hz 1 Framec HI 2 Li The following illustrations show other signals Figures 6 2 2 and 6 2 3 For more detail please see the ARTA manual and Mateljan amp Ugrinovic 5 FR Kisgnriude dB 5 Ba mt zi 3n 100 zu Guo 2231 8 Fia chida 92 10 par whie Pink Noise pl zu a0 100 WE Hz 5132008 Fin pink 206 10 pik zu FR kisgnriude dB 90 0 zu a0 100 Drio 205 H 36150 Fin zg ve PR shite Fregni Hz 1 Z H E 17 064 435 FR kiagnriude dB 5A 70 0 nnl zu a0 100 A Ee H2 S099 im 10 Fr acerca H2 1 Curs oF zAH 17 0414397 Fin zig 10 05 pint 108 Fregan Hz 20 06 147 08 41 10k H2 7 2010 06 17 064 Figure 6 2 2 Difference between random and periodic
9. em 2012 05 04 11 16 15 Overlay files GD zz Excess Gc Figure 6 2 30 Group Delay GD red and Excess Group Delay grey 82 From the excess group delay we calculate the propagation time between the source and the microphone The graph shows GD 0 999 msec at 3kHz which corresponds to the window set in Figure 6 2 15 Note that these observations were made with an idealized modelled tweeter Section 6 3 shows how these compare with real world measurements 6 3 Where to measure the measurement environment Before answering the question of where to measure we should first address the context of that measurement For example a subwoofer or a 3 way floorstander will require conditions that differ from a small full range desktop speaker 4 L Jim v 74 7 404B 100 2900 500 2k Figure 6 3 1 Simulation of 3 way crossover To model the crossover of a 3 way speaker effectively a measurement two octaves below the woofer midrange transition frequency 300Hz in the above example at the appropriate distance should give enough resolution to allow the integration of both drivers and to include enclosure effects A good illustration of the variables to be accounted for when measuring and interpreting results 15 as shown in the following di
10. 67 hagnitude dB re 83 smoothed 1 24 oct Phase 1 180 0 20 50 100 200 2k ok 10k 20 Cursor 20 2 Hz 55 24 dB 107 8 deg Frequency Hz HT Frequenz und Phasengang Figure 6 1 3 Frequency and phase response of a tweeter in a normal living room Individual drivers do not usually cover the entire frequency range Thus a tweeter radiates so little acoustic energy at 100Hz that the transfer function in this region is overshadowed by noise and the phase response calculated in this area is of no use 6 1 3 Averaging As indicated above measurements are rarely made under optimal conditions Traffic noise fans in computers heating or air conditioning wind and general background noise can all spoil measurements To obtain measurements with tolerable accuracy we rely on averaging In IMP mode in the menu Impulse Response Measurement there 15 a field entitled Number of Averages In Linear 2 and SPA see in Averaging submenu in Frequency Response Measurement Setup the field Max averages 32 Averages 68 hagnitude dB 200 os smoothed 1 24 act Img 20 50 100 200 500 1 Zk k 10k 20k Cursor 20 0 Hz 7 7 85 dB Frequency Hz Current file Untitled 2010 09 05 19 46 35 Overlay files em TBAVge ZAV 325 e9 Signale amp Averaging Figure 6 1 4 Averaging in IMP mode These fields sp
11. File Edit Setup LAeq 33 63 dB Time 0 00 08 43 Weighting me TEE Peak Level LCpk 45 84 dB SPL Exponential Averaging LAS 33 75 LAmax 33 75 dB LAmin 23 85 dB Integration time 5low LCpk max 46 16 dB 100 0 dB SPL Leg record Magnitude T a E set A EUM Peste Log eo um 40 0 W was RR ER REM LAI Time zoom 30 0 Mf Nhac aite afe p 4 n Lr Max 00 33 20 01 06 40 01 40 00 30 20 2 38 2 eq 22 26 345 29 94 48 2Cpk dB Mrk 00 35 51 2 3 7 SAeg 28 26 545 31 741 53 4Cpk dB SPL hi SPL history Figure 7 2 3 SPL meter window in ARTA The controls are as follows Linear Averaging LAeg current value of Leq in uppercase Time period relative to the beginning of the measurement Weighting choice of the weighting filter A or Z lin Hours Minutes and Seconds definition of the duration of the measurement maximum 24 hours 59 minutes and 59 seconds allowed SPL Exponential Averaging LAS current value of the time weighted SPL with weighting filter A LAmax maximum value of the time weighted SPL for the entire measurement period LAmin minimum value of the time weighted SPL for the entire measurement period Weighting choice of the weighting filter A C or Z lin
12. 15 D 300 35 FUE 40 45 f Magnitude dB smoothed 1 24 och Phase 7 IRE Of 50 180 0 90 0 M enn 180 0 K 10k Frequency HE 100 200 500 2 Cursor 1331 7 Hz 1 20 dB 67 9 deg 12 dB 1 000 18000 Hz Figure 6 2 11 Simulation of tweeter responses with highpass left and bandpass right filters To answer a frequently asked question the unexpected artefacts that are seen before the start of the impulse are caused by pre ringing 77 te Saco Iu Figure 6 2 12 Impulse response with pre ringing This is caused by the bandwidth limitations of the measurement system and is seen each time the frequency 15 half the sampling rate With current soundcards these frequencies are usually 24kHz 48kHz sample rate and 48kHz 96kHz Pre ringing can be partially corrected by checking the Dual channel impulse response filter in the Impulse Response Measurement Signal Generator window 6 2 2 Phase and group delay An understanding of the use of ARTA for analyzing phase and group delay is important when fully characterizing a sound source The reader is therefore advised to revisit Section 5 briefly and to consult relevant literature 6 as well as the ARTA User Manual 2 Note also the importance of using two channel measurements as this is the only way in which specific phase relationships can be characterized The follo
13. A bass reflex enclosure consists of two sources the speaker cone and the port In the following example port diameter DP 4 80cm and the effective diameter of the speaker diaphragm DD 10 20cm Figure 6 7 8 shows the positioning of the microphone for the driver membrane and the port The distance used should be chosen to keep errors lt 1dB the see Struck amp Temme 6 or Section 6 4 Port 0 26cm speaker cone 0 56cm Figure 6 7 9 shows the impulse responses of the membrane black and the port red The port impulse has a delay of approximately 0 72msec 24 72cm relative to the microphone Figure 6 7 8 Measurement positioning Impulse response TL fpem WIN I 1 87 3 30 494 857 Cur 11 481uV 1 590u 3 188ms 306 Figure 6 7 9 Impulse responses of driver membrane black and port red 128 FR hMagnitude dB 200 os smoothed 1 24 act 135 0 130 0 Im 125 0 120 0 115 0 110 0 85 0 10 20 50 ok Cursor 55 5 Hz 128 20 dB Frequency Hz Vent amp Membrane Figure 6 7 10 Membrane and port nearfield responses without level correction Figure 6 7 10 shows the nearfield response of the membrane and the port The setup with the 5 driver in Figure 6 7 8 RD 5 1 cm has a usable nearfield response up to around 500Hz For the sake of clarity higher frequencies are not shown The differing microphone positions as shown in Figure 6 7 8
14. MOF GE ani pr e SEH 180214 FE HOF EER fr e a 16 04 46 FE HOF EE 1 pi SG 15 05 24 Oy ebay lez Boch 2 HDETGERGn 1 or 4mm Bitumen BurstDeray Burmi Deco J 1 90 200 dk fk 2 B 10 20 dk X 50 100 500 1 Frais Cursor 20 2 Hr 16 07 de Frequency Hz y Hz 1 Current MOF 16 Fest 3 pir 2006 78 21 150924 NDFIEFRES 3 pi 2008 74 21 18 10 28 Fk MOF TSF iene A ot 2041 04 21 12402 Cy ety fhe WEF 2 HOFF eee 3p FR Magnlitude dB wits mooie 1124 cef 6mm Fliese BurstDergy FF Magnitudae dB Wits mooie 1124 cfi Burzi Decay 100 200 dk A XX B 100 20 100 m it Fra Cursor 20 2 Hr 2571 d Frequency Hz ial tHe Current pi am 2006 09 21 151434 FE ADP 1 1 SOG 16 15 18 FRE ADP D 1 gor zDOBD4 z 181528 Oy ey ADEL WHOFTGAEPIO 1 10mm Weichfaser fla p FE Magniude dB viv 1 24 oct BurstDiecay Burst Decay 1n 90 120 13 0 190 An 27 0 au S 100 200 dk ik 2 Vb EM p 100 200 zm di Cursor 20 2 Hr 2244 un Frequency FIRED tHe Curent MDC l BiIpard preme EROS 151 0 Ep rel _2 pir SG 18 12 09 Fe ADP Ep 2 pir zDOB Dw 18153 Cr erbe fee WEF Bech pr BOF SSS pr Bitumen Sperrholz BurstDera FR Magnitude dB vi
15. RR 0 01 0 1 1 0 10 0 11 8 TAD 1 058 Voltage rms 4 1 Ohm vs 8 2 Ohm Figure 5 4 2 THD 1kHz as a function of output voltage with 4 and 8 Ohm loads The frequency response 15 shown in Figure 5 4 3 The lower cut off frequency 3dB is approximately 16Hz while the upper limit is about 60KHz FR Magnitude dB smoothed 1724 act Phase Tm 180 0 5 10 20 50 100 200 500 1k 2k Sk Sk Cursor 16 6 Hz 3 01 dB 60 5 deg Frequency Hz t amp mp Figure 5 4 3 Frequency response of the Thomann t amp based the LM3886 chip 60 6 Spectrum magnitude dBFS Left Avg 26 15 0 gt 4 20 22 30 0 45 0 80 0 5 0 90 0 nmi TNI 120 0 20 50 100 1 Zk 5k 10k 20k Cursor 4853 0 Hz 1 MN p Freaquenc y Hz 5 S dBFsS THO 0 0096 THD N 0 029 t amp mp 1dB Figure 5 4 4 THD N 1kHz and 1dB Figure 5 4 4 shows THD for the t amp The values within the manufacturer s specifications Figures 5 4 5 and 5 4 6 show distortion against frequency for the t amp at 1 and 16W into 8 Ohms The t amp remains stable at outputs up to 16W Distortion 96 0 01 i MAL 0 004 p 0 0001 0 00001 2 20936 SHz 0 00355 2 0 00355 f Hz Figure 5 4 5 Distortion versus frequency at 1W 61 Distortion 951 1 0 0 1 0 01 0 001 0 0001 m 0 00001 2
16. Te matt il ht 16 mm 16 mm MDF 2 VD S MAATA AARAU eat Figure 7 3 6 Decay behaviour of different material combinations 16 Weichfaser softboard Kleber glue Sperrholz plywood Fliese tiles Figure 7 3 6 shows the range of measurement for 16mm MDF with different linings Plain MDF is shown as a reference underneath each trace green Note the scale variations when comparing the different materials Figure 7 3 7 shows an alternative analysis of the measurement files frequency response burst decay burst decay sonogram In the left hand panels 16mm MDF is shown as the reference red The burst decay plots middle and the sonograms right illustrate the effectiveness of different enclosure treatments Accelerometer studies are also planned 168 BurstDergy E FF Magnitude dB uv ts mooie 1124 cf Burzi Decay 1 200 dk 1 XX Bm 10 dk 100 2l it Fra 20 2 Hr 1020 ur Frequency duy Curent NOE TS 2 pir m 2006 09 21 143208 FE ADP Sh 2 pi SOG 150524 FRE ADP Ero 2 pi 2006 79 21 1505509 EFI Bech pe 16 mm MDF BurstDergy FF Magnltude dB Wits mood 1124 cef Burzi Decay 100 200 dk XX B 100 20 dk 100 20 di Cursor 202 Hr 12 28 09 Praquency cim HE Current
17. 10 20 50 100 200 500 1k Zk ok 10k Cursor 7 1 Hz 107 20 dB Frequency Hz Laser black Mic blue red Figure 7 3 9 Frequency response blue diaphragm deflection black and acceleration red FR Magnitude dB re 200 2 23 smoothed 1 24 act Zahm 130 0 125 0 120 0 115 0 110 0 20 50 100 200 500 1k 2k 10k 20k Cursor 308 6 Hz 117 40 dB Frequency Hz 8110 Sicke vs Dustcap mit Impedanz Figure 7 3 10 Surround frequency response black dust cap red and impedance grey Figure 7 3 10 shows two nearfield measurements The black curve is the mid surround the red is the centre of the dust cap In the region around 300Hz the curves start to diverge This can also be seen in the impedance response as an irregularity Artefacts like this can be caused by resonances in the basket membrane or dust cap 171 7 4 Create WAV files for external signal excitation with pra amp Untitled Arta m E E ze 11 Fre Fri PA Multitone Mutitone ue 16 bit PCM 172 8 Dealing with measurement data data files shortcuts etc We all know how frustrating it is to take measurements and then try to remember at some point in the future exactly what we did particularly if certain crucial details like the measuring distance were not recorded We may have an impedance or frequency trace but might then remember that we did not save the impuls
18. ARTA HANDBOOK guide to the ARTA family of programs Based on the original ARTA Manuals Original tutorial in German by Dr Heinrich Weber Original manuals in English prepared by Dr Ivo Mateljan Weber Mateljan Translation into English of Version 2 30D ARTA 1 80 Christopher J Dunn Hamilton New Zealand September 2014 Contents PIES PSUS SV PUE NS DA om Muerto 5 uU UU MENTI MN 5 K 5 1 3 Pin assignment for cables 2 2 2 000600000000000000 0282 8 0 0000010004 10 I Measurement SEMP 11 2 WICK SUP WIth tmc M 13 3 he ARTA Measurement DOX 17 3 1 Two channel calibrated measurements with the ARTA measurement box 19 2 2 Single channel measurement caliDE3UOf ioco Pee a e e Riad 27 A Soundcard setup and LOST Gs Feud EE enu eruta buda bud eue 24 Sounded IS a 24 4 1 1 Windows 2000 XP WDM driver 25 4 1 2 Vista Windows
19. T PONO 68 02 AAPA CK 70 6 2 1 Impulse responses theory and practice oue 74 6 2 2 Phasccand TOU delay tides Dona ech aaah 78 6 3 Where to measure the measurement environment 83 6 4 In room measurement cccecsceccscscscsccscscccscaccscscecscccscscscaccscscscscscecscscacescscecscasesescecaces 88 6 5 Determination of the reverberation time characterization of the measurement space 100 6 5 1 Automatic evaluation of reverberation time 2 2222 2 01 0000 28 105 6 6 Setup for loudspeaker acoustic 5 109 6 6 1 IAG AS it and senses edet tale Sea tale 115 6 7 Scaling and splicing of near and farfield measurements 122 6 7 1 122 6 7 2 Bass toller 128 0 95 133 9 9 137 6 10 Electrical measurements crossovers with 147 4 SSPeClal MCASUreMeNLS ANG e xatbDIeS galas uU edu 154 7 1 Measurement of harmonic distortion with a sine 19 154 7 2 Sound pressure level SPL measurements with 00 158 7 3 Detection of resonance including downsampling
20. red on the left while the circuit implementation is shown on right It is possible to use the microphone input of the ARTA Measuring Box depending on the input impedance of the card this will add approximately 0 5dB of attenuation to the effect of the voltage divider Figure 6 10 1 Setup for electrical crossover measurement The voltage divider shown in Section 1 4 should be suitable under normal conditions With 1 W input the voltage at 8 Ohms will be U V1 8 2 83V The PassFil program offered by Bullock amp White http users hal pc org bwhitejr is recommended for those who wish to examine voltage and current in crossovers The following example shows the mean PassFil voltage curve a highpass 2 way crossover at 15W note that this is close to the limit of the capacity of the 1 10 voltage divider ud LI 1 6 252 uFd Li 0 450 mH 6 252 uFd 11 0 450 Figure 6 10 2 Voltage curve in crossover components as shown by PassFil Figure 6 10 3 shows the frequency response of the tweeter without red and with blue the crossover The crossover is very simple with only 6 8uF capacitor Neither the amplitude response nor the measured slope resemble a first order filter however The high acoustic slope is explained by the superposition of the electric filter on the acoustic highpass of the tweeter Although the slope should be 6dB 12dB 18d
21. ASEO While this subject 15 dealt with detail elsewhere 14 15 16 17 18 we should remember that 1 The acoustic source is but one of many factors affecting simulation results 2 The simulation does not depend on absolute values but on relative differences between drivers 3 The crossover has a significant effect on time behaviour 6 6 1 Measuring and simulating To make sure that our simulated data bear a reasonably close resemblance to real world results we need to make sure that the simulation accounts for measurement conditions and artefacts There are two ways of dealing with this a addition to phase and frequency information other factors such as driver positioning on the baffle and acoustic sources relative to each other are included in the measured data b Measured data include only phase and frequency information from each individual driver Other factors such as driver positioning are added or accounted for separately This section deals briefly with these two options using a broad cookbook approach n b simulations should be based on two channel measurements Option A nU ES z Conditions for option A are satisfied when all drivers are measured from the same microphone position This means that all information pertaining to sound propagation time s phase and acoustic source s are accounted for The two microphone positions shown here are preferred and farfield measurement techniques should be use
22. Magnification of the Y axis shows more detail for the frequency response with a variation of approximately 1dB for the M Audio Transit USB soundcard that was measured FR Magnitude dB smoothed 1 24 act 1 10 100 1000 10000 Cursor 1 5 Hz 29 01 dB Frequency Hz The following traces show responses for several other popular soundcards 37 M Audio Transit Line In 0 1 20Hz bis 20kHz 10000 Cursor 20 2 5 amp 6 eB Frequency Hz Realtek AC97 Audio 2 5 dB 20Hz bis 20kHz Frequency Hz Onboard Karte Intel Mikrofoneimngang 6 5 dB 20Hz bis 20kHz 170 16 2 5 39 JB Frequency Hz For measurement purposes a soundcard should have a low frequency cut off 3dB below 10Hz or preferably 5Hz The card should have a usable range from 20Hz to 20kHz with variations no greater than O 5dB Any noise intrinsic to the soundcard should also be taken into consideration 38 RMS MLS or white noise P 10 log 1 0 5N Crest WN gt 10 110B Crest L D REALTEK Realtek 20Hz D TRANSIT The following example illustrates the effect of high noise levels The Realtek card referred to above has a noise level of about 80 dBFS at 20Hz whereas the M Audio Transit records around 120 dBFS Excitation signals consisting of MLS or white noise were selected with an FFT sequence of N 32768 values This sequence has N 2 16384 spectral co
23. processing and manipulation of measured data Access to these functions is available via three menus Be aware that these functions are somewhat different to commands in the time and frequency domains that have very similar names Edit View Record Analysis Setup Tools view Record Analysis Mew Ctrl M Set Marker Open Delete Marker Save CEri4 5 Save 45 Info Invert Riokate cursor Scale E ce Copy Import P Colors Load and sum New clears the memory Invert inverts the impulse response see 8 1 Open opens PIR data files Inv Save saves PIR data files Rotate at cursor cuts the impulse response Save as saves PIR data files under another before the cursor start position name Note ARTA overwrites files without asking you first If you summed or scaled Scale Scales the impulse response by means of a modified PIR file always save with this mathematical operation see example command Pir Scaling Info plenty of space provided for comments on the measurement Add your text here Enter number or arithmetic expression to scale PIR 3 18 2 88 File 36cm pir Samplerate 48000 Hz Length in samples 32768 Length ms 682 667 ms Input Device voltage probe User supplied additional informations Hier kann beliebiger Text stehen Export export in another data format ASCII File ML554 ASCII File AWAY File 179 Import
24. significant effect on the simulation Validation of the acoustic source simulation Simulated and measured data should also be validated when using option B before simulating crossovers As for option the measured and simulated summed frequency responses should be compared 126d5 116dE 200 Figure 6 6 13 CALSOD simulation results Figure 6 6 13 shows results obtained using CALSOD The right panel show the superimposition of the simulation coordinates over the measured data The difference between the two acoustic sources can be seen under the woofer s Z axis Overall the simulation looks very similar to the measurements While not as good a fit as the data shown in Figure 6 6 9 these results are perfectly acceptable as they vary by only around 14 In contrast to option A Figure 6 6 14 illustrates what happens when the measured and simulated coordinates do not match FR amp Minimum Phase path difference 2 2cm 119 kA Y THAB 268 Figure 6 6 14 CALSOD minimum phase with acoustic centres simulated at 80cm 119 The simulation is set at 80cm solid line while the broken line represents the measured data for 80cm whereas the measurement was actually taken at 40cm We can see that agreement is not perfect but the data are nevertheless usable and represent a significant improvement over the traces shown in Figure 6 6 10
25. which is very effective when using variable inductors and capacitors Smoothed frequency response Misea VB mW pir x File Overlay Edit View Smoothing Phase averaged in 1 24 act E Fit Range Set Smoothing phase estimation x 124 Enter delay For phase estimation ms 1 3535 Update Cancel ox 108 0 144 0 High Fr 180 0 Low Fr 200 500 10k Cursor 200 4 Hz 28 8 deg Frequency Hz Mag ad Record Overlay Copy p Figure 6 9 4 Target and measured crossover phasing Targeting can also be applied to the phase response Figure 6 9 4 Target functions can be used in conjunction with phase estimation Edit then Delay for phase estimation By adding delay the measured phase can be approximated to the target function Figure 6 9 5 The source data remain unchanged and the added delay is taken into account when the data are exported FF rd Fh inm inm myeragail in el i myeragad in 154 ni tenn ERU i160 od n n 1440 Fr 144 0 144 0 F 106 D T T T A 106 0 A 106 0 A r2 r2 36 0 36 0 a i H i ll i zn Ms M40 Tun Aon n Aon n a n 200 5n It ik 200 5n It ik 200 5n 2L 1 TK Curmor 20
26. 2 Calibration of the input channel You can use an external generator or the output channel of the soundcard to calibrate the input channels If using the output channel of the soundcard as a calibrated generator 1 Set the left and the right line input volume to maximum 2 Connect the left output to the left line input 3 Press the button Estimate Max Input mV and monitor the input level at bottom peak meters If the soundcard input is clipping lower the level of input volume 4 Enter the value of signal generator voltage in the edit box but only if it differs from value used during output channel calibration 42 5 Press the button Estimate Max Input mV 6 If you are satisfied with the measurement press the button and the estimated value will become the current value of the LineIn Sensitivity 7 Repeat 1 6 for the right input channel Note This procedure is recommended as it guarantees that you can connect the soundcard in loopback mode If you want to calibrate input channels with input volume set to maximum many soundcards require a reduction of the level of the output channel Note also that the standard calibration sampling rate was previously 44 1kHz Because of problems with some soundcards when using this rate 468kHz has been available since release 1 8 Sampling rate Hz 44100 L 7 50 A 10 de m t 43 5 2 Microphone calibration To calibrate the
27. 250 Ex 200 e o 705 150 1 00 1 0 0 50 0 00 0 5 125 250 500 1000 2000 4000 8000 Frequency Hz Figure 6 4 13 Reverberation time blue and reverberation radius red in a gym with dimension 27m X 15m x 5 5m The results suggest an overall reverberation radius of about 1 40m which means that up to this distance the influence of the room on measurements should be low but is this true 98 Gated red Burst decay Ungated 1 3 oct blue 1 24 oct grey Ovew 105 6 40 te Que ve 561921 dp NUS DS Ove fux 1 n 004 ON 1 100 d 1 1 9m Oa P8 o 20 9 v pureed 1 24 et AM Ours 9401 106 22 460 Cpe te oped Levee pres meor 42040 Ovin fen 124 Cite 100m 12 OM 2 4 oo 1 04 109 me a Orr e 107 P3 40 Fo Cpe Vue etate we 24 008 124 12 Mem Gett 12208 j 1 e PR Mose ge NIV qormemes 1 24 00 we 1 4 Curve 114 96 12 26 0 Care 9025 Lette I we 2 Ove Hee 37 jon 124 cht 2T m Oe 2 XmO we C201 gt LY IM 1 kD 36 03 27 1932123 Gpr 7096 93 27 Figure 6 4 14 Measurement of a Solo 20 in gym at various microphone distances Esti
28. 30 0 40 0 100 200 500 1k 2k Sk 10k 20k Cursor 101 2 Hz 53 61 dB 119 6 deg Frequency Hz Cursor 101 2 Hz 53 61 dB 108 6 deg Frequency Hz Pre delay 0 5 ms Pre delay 0 8 ms Figure 6 2 16 Phase with pre delay 0 5msec Figure 6 2 17 Phase with pre delay 0 8msec FR Magnitude dB VIV smoothed 1 24 Phase 144 0 108 0 720 36 0 0 0 36 0 72 0 108 0 P 144 0 180 0 100 200 500 1k 2k 5k 10k 20k Cursor 101 2 Hz 101 4 deg Frequency Hz Current file Dirac_Butteryy 3_ HP amp O0 pir 2012 05 03 22 46 09 rot Target berechnete Phase des HP 3 Ordnung Cursor 101 2 Hz 53 61 dB 101 4 deg Frequency Hz Pre delay 1 0 ms Figure 6 2 19 Black phase with 0 998msec pre Figure 6 2 18 Phase with pre delay 0 998msec delay black red calculated ideal HP phase Removal of the excess phase associated with the flight time between the source and the microphone in this case by using pre delay should reveal the pure phase response of the highpass filter Figure 6 2 20 Comparison of the calculated ideal highpass phase red with the phase corrected using 0 998msec pre delay confirms this Figure 6 2 19 FR Magnitude dB smoothed 1 24 och Phase T FR Magnitude dB v fsmoothed 1 24 och Phase 1 00 10 0 20 0 180 0 30 0 0 0 40 0 180 0 100 200 500 1 2k 10k 20k 100 200 500 ik 2k 5k 10k 20k Cursor 101 2 Hz 53 61 dB 101 4
29. Balance Balance Balance Volume Volume Volume Yolume Volume ut ids SERES Mute all Mute Mute Mute Mute Advanced Advanced Intel r Integrated Audio Figure 4 3 Typical soundcard output mixer settings in Windows XP 26 H aufnahme _ Qptionen CO Audio Mikrofon Waveausg Mis Balance Balance Balance p F glp F 4p F g Lautstarke Lautstarke Lautstarke Ausw hlen Auswahle Ausw hlen Conexant HO Audio input Figure 4 4 Typical input mixer settings in Windows XP German n b Most professional soundcards have their own software for input and output channel adjustment or have their own hardware to control input monitoring together with input and output volume controls 4 1 2 Vista Windows 7 WDM driver setup Microsoft has changed its approach to the control of sound devices in Vista Win7 The operating system Sometimes in conjunction with the control software for professional soundcards is now responsible for setting the soundcard native sampling rate and bit resolution The OS changes the native resolution to the floating point format for high quality mixing and ultimately for sample rate conversion ARTA users should therefore select the Float resolution setting and set the sampling rate to the native format Access to these values is gained via the Windows sound control panel Control Pa
30. Currant 16 TT Bde pir B dB gt Fre 20 1 Hz han BT 64 dB 02 2852 0B Des 3D 06 dG 04 44 T5 dB Currant il TT 1 248 12 58 Figure 7 1 5 Comparison of Farina and STEPS methods at four different levels Frequency Hz 2027 3 He Wagi 3 27 D3 48 55 DX 47 22 D T2 2 de 2010 09 05 22 31 44 STEPS Single 1 dB pmi 5k 1 20k 20 Fragua HT 425 3 Magni 90 84 GB 02 50 64 D 51 55 Dic 77 An des 2010 09 05 21 35 27 STEPS Singje 3 cB 10k ia 2m Frequency 146333 Hz Magn 68 26 cB 20 58 33 D3 33 05 Duc TO 3 i 7040 00 05 21 36 19 Single Peqelhorrektur y J Magnitude re 30uPa 5k 10k 206 Fraquancy Hz 204353 Hz Magn 50 52 98 D3 210 37 dB 2010 09 05 21 37 58 STEPS Singe 12 dB mit PHY 7o m cn eg 7n m wan 157 7 2 Sound pressure level SPL measurements with ARTA Music is not always perceived as beautiful because it may be regarded simply as noise but how do we define loud or quiet Some guidance is available from official directives technical manuals and standards e g Directive 2003 10 EC or DIN 15905 5 event equipment sound equipment Part 5 Measures to prevent the risk of hearing loss by the audience from high noise level emissions from electroacoustic sound systems Veranstaltungstechnik Tontechnik Teil 5 Ma nahmen zum Vermeiden einer Gehorgefthrdung des Publikums durch hohe Schall
31. Drivers 1 Dimensions and weight 2 Dimensioned line drawings 3 Mounting information 4 List of accessories 5 Description of electrical connections 6 Additional descriptive information 7 Physical constants piston diameter moving mass voice coil winding depth and length top plate thickness at voice coil minimum impedance and transduction coefficient 8 Thiele Small parameters 5 5 10 VAS QES QMS RE SD 9 Large signal parameters PE max X max VD 10 Frequency response 0 45 in standard baffle 11 Distortion second and third harmonic swept at 10 rated power 12 Impedance response free air 13 Power handling free air 2h 14 Displacement limit 15 Thermal rise after power test 16 Recommended enclosures Notes High Frequency Drivers 1 Dimensions and weight 2 Dimensioned line drawing 3 List of accessories 4 Description of electrical connections 5 Additional descriptive information 6 Description of diaphragm and diaphragm construction 7 Frequency response on plane wave tube PWT 8 Distortion on PWT swept second and third harmonics at 10 rated power 9 Impedance on PWT swept 10 DC voice coil resistance 11 Power handling on appropriate acoustic load 12 Displacement limit of diaphragm 13 Thermal rise after power test For the dimensions of standard baffles see Figure 9 2 This recommendation has now been extended see STE
32. Integration time selection of the time weighting F fast S Slow or I pulses Peak Level e LCpk current peak level C weighted time interval 1 s e LCpk max maximum peak level C rated for the total measuring time Audio Devices e Sampling rate choice of sampling frequency 44 100 48 000 or 96 000 160 Rec reset starts measurement or sets all values to zero Reset Stop stops the measurement OK closes the SPL Meter window dBFS Peak meter dBFS displays the current peak operating level relative to the full range of the sound card in dBFS Record SPL history enables data recording in graphics mode level recorder There are 5 recorded values Leq LSlow Lfast Lpeak and Limpulse Graphics are as elsewhere in ARTA Figure 7 2 4 SPL graph setup E X Magnitude axis Magn range dB Lslow v Lfast Thick plot lines Show local time Figure 7 2 4 SPL Graph setup The controls are as follows Magnitude axis e Magn top db the maximum value of the Y axis e Magn range dB the value range of the Y axis Time axis e Graph max Definition of upper time limit e Graph min Definition of lower time limit figures are relative time values no actual time entered Show curves Leq LSlow Lfast LPeak LImpulse enables disables the curves to be displayed e Thick plot lines line style thickness e Show local time set
33. Record Analysis Setup impulse peak The length of the gate is v Toolbar shown below the chart We can use this Status Bar to calculate the distance of measurement W y Gate time im 344m5 Cancel d 0 91715 0 344m speed of sound 0 3154m Note that since release 1 2 has been doing the calculation for you it is shown below the chart e Alternatively calculate the distance as d c peak position 300 sample rate which in this case would be d 344 344 300 48kHz 0 3154m Enter this value in the Pir Scaling dialog Edit Scale amplitude 3 Go to the menu item Overlay Generate Target Response and select target that resembles the roll off response illustrated by the manufacturer e g see Reference passband sensitivity 92 dB Figure 5 2 6 Various filter options are ren ue Butterworth ander available ranging from first to sixth order Low pass eal pm TP Nerio Wor OF Filter type sensitivity and cut off IV High pass Beussl ILorder Linkwitz II order frequency are entered by the user Fleira pass Dutiermorih 11i ander Bessel III order Butterworth IV order Bessel IV order Figure 5 2 8 shows the measured frequency response together with the response esse Linkwitz IV order corrected to metre alongside the target Q factor 1 Butterworth V order function 12dB Butt
34. Theoretically when the measured phase is adjusted accurately using the correct pre delay the response should be identical to the calculated minimum phase Phase 7 averaged m 1 26 21 100 w0 E ut 101 2 Hz 101 4 deg Frequency Pri 101 2 H2 44 8 deg Frequency Hz Current tix Drot Aww 2 0900 e 20 20506 105238 Current Direc _Butierw 3 9800 pr 2012 05 04 1055 22 vM Se Raan TTE verin fies Mina Exceed a ent ee eee oe Minimum Phase vs Phase with 0 998 ma Pre Delay Figure 6 2 24 Minimum phase Hilbert Figure 6 2 25 Excess phase blue real phase transformation green and real phase red red and minimum phase green In reality there may be differences caused for example by diffraction effects Figure 6 2 24 The calculated minimum phase green and adjusted real phase red differ by the excess phase blue as shown in Figure 6 2 25 20 50 100 200 500 dk 2k 5k 10k 20k 20 50 100 200 500 dk 5k 10k 20k Cursor 20 1 Hz 20 8 deg Frequency Hz Cursor 20 1 Hz 20 8 deg Frequency Hz 81 Figure 6 2 26 Wrapped phase Figure 6 2 27 Unwrapped phase The menu item Unwrap Phase can be used to switch between the images shown in Figures 6 2 26 and 6 2 27 In Figure 6 2 26 the phase response 15 forced into a 360 degree range The repeated flipping between 180 degrees and 180 degrees indicates how often the phase goes through a full 360 degree cycl
35. To explore this comparison further let s see what happens when we place the simulation 10 degrees above the tweeter axis Figure 6 6 15 amp Minimum Phase path difference 2 2cm 11605 Figure 6 6 15 Simulation 10 degrees above the tweeter axis at 40cm BoxSim We know that BoxSim deals with the Z axis acoustic source somewhat differently to CALSOD so what happens if we repeat the experiment in Figure 6 6 14 FR amp Minimum Phase path difference 2 2cm 0 0000 0 0000 100 Cursor 112 3 Hz 96 56 dB Frequency Hz BoxSim Messung vs Simulation Figure 6 6 16 Simulation results BoxSim minimum phase with acoustic centre difference Figure 6 6 16 illustrates the comparison between measured and simulated results for a microphone distance of 40cm Evidently the simulation has not worked very well and the results would not be of any use Admittedly under these circumstance we would probably use option A anyway Experimentation with a path length difference of 3 93cm which matches the propagation time difference captured by a single point measurement without geometric correction gives the results shown in Figure 6 6 17 120 Minimum Phase Match SEO 3 93 cm 110 0 70 0 LIH 0 0000 0 0000 Cursor 200 4 Hz 88 74 dB Einpunktmessung MinPhazse Match Pos 1a Frequency Hz Figure 6 6 17 Simu
36. What do these results tell us about the utility of the soundcard under test As a general rule THD N lt 0 1 usable soundcard THD N lt 0 01 good soundcard To check the frequency response of the soundcard use impulse measurements IMP Tre Fre Fri S mm Use the single point mode click gt and make sure the dual channel measurement checkbox is empty 34 Impulse response measurement Periodic Morse Sweep MLS Penodic nose generator Recorder Sequence length EN Prefered input channel Left Sampling rate Hz 48000 Dual channel measurement mode Time constant bee By m Invert Phase of input channel Mose spectrum Pink Number of averages i Output volume dB d Frequency domain 2Ch averaging Fink cutoff Hz 50 Filter dual channel impulse response Generate Record L 70 50 30 i0 S 50 40 20 Default Abbrechen Check by clicking on Generate whether the soundcard line in is being overdriven The levels are shown in the peak level meter db If levels enter the red or yellow zones reduce the output volume until the bar is entirely green Click Record and wait for the measurement to complete 1 e until the peak level meter shows no sound Click OK and you should see something that resembles the following impulse 35 fe Untitled Arta File Edit View Record Analvsis Setup Tools Mode He
37. amplification gain 1s the ratio of the output and input voltages ZUA UE This 15 measured with a sinusoidal alternating voltage typically at IKHz A precise voltage divider between the generator and the amplifier facilitates the measurement at high gain e g microphone preamp If a voltage divider is used measure the voltage UE before the voltage divider then calculate the voltage divider ratio R1 R2 R2 Then V UA u UE V UA UE Example t amp UE 0 8493V UA 18 539V UA UE 18 539 0 8493 7 V 21 83 26 74B UA Figure 5 4 10 Measurement of amplifier gain Output impedance is the internal impedance of the output side of the amplifier and can be determined from a load resistance RL The output voltage of an open circuit UO is reduced by a load to the load voltage UL Under these conditions RA RL UO UL 1 Example t amp UO 5 47V UL 5 462V RL 8 20 8 2 5 47 5 462 1 0 0120 Figure 5 4 11 Measurement of output 63 impedance The measured values as shown in Figures 5 4 9 to 5 4 11 match the manufacturer s specification 64 6 Measurement with ARTA 6 1 General After the calibration of the equipment is complete and everything is ready we can start actual measurements Be sure to check all cable connections and settings thoroughly and carefully before each measurement session 3 USB soundcard A Figure 6 1 1 Measuring eq
38. can be found on the Earthworks homepage in the article How Earthworks Measures Microphones 3 For frequencies above 500Hz Earthworks uses the substitution method in which test driver is measured an infinite baffle with a reference microphone The problem is that it is difficult to find a suitable large enough anechoic chamber for the measurement of low frequencies To solve this problem Earthworks uses a small pressure chamber for calibration at lower frequencies see Section 5 3 2 52 Grerz schalldruckpegel f r 3 Klirrfaktor bei 1 KHz Max SPL for THD 3 at 1 kHz Eigenrauschen mit Vorverstarker MV 203 Inherent noise with preamplifier MV 203 Polarisationsspannung PolarZation voltage Kapazit t mit Polarisationsspannung bei 1 KHz Polarized cartridge capacitance at 1 KHz Arbeitstemperaturbereich Operating temperature range Temperaturkoeffzient Main ambient temperature coefficient Statischer Druckkoeffizient Main ambient pressure coefficient Durchmesser Diameter mit Schutzkappe with protection grid ohne Schutzkappe without protection grid H he Height PTB Zulassung Nr zur amtlichen Eichung 3 5 Hz 20 kHz 2 dB 50 mV Pa 146 dB 15 dBA 200 V 19 pF 50 100 C 0 01 dB K 1x10 132 0 02 mm 12 7 0 02 mm 16 4 mm Figure 5 3 2a The MK221 Reference Microphone by Mikrotech Gefell Figure 5 3 2b shows responses for the reference microphone and the test microphone MB
39. components still causing aliasing effects f Sampling frequency a FF Magnitude dB v fsmoothed 1 24 act 25 0 30 0 35 0 40 0 45 0 50 0 55 0 60 0 65 0 70 0 20 50 100 200 500 Cursor 20 2 Hz 38 15 dB Fre Original 96 kHz doweneampled ta kHz redz n 35 black 0 5 Figure 7 3 4a Effect of the anti aliasing factor 0 5 black 0 95 red 165 9 199 D DO Vv D O00 5 wr FR Magnitude dB Viv 1 24 oct sp 10 20 Cusor 200 H2 1335 dB 96 kHz zm Frequency HZ Ce sini 28 kHz 19 Curt DOES ev 0 O00 08 Organ 8 ec Icd leg EU 200 202g 42 0 Cowon ho 411 Cumulatve Spectral Decay smoothed 1 12 octave do 2 N Frequency Original 95 kHz down caelo to d kHz Figure 7 3 4b Comparison of PIR FR and CSD before left and after right downsampling to 4kHz Speaker enclosures The above considerations also apply to speaker enclosures because in this respect they can be regarded effectively as small rooms that simply resonate at higher frequencies 166 Open undamped Open damped EEE 00 0 TROC EN austus 158 gt upto db gt 24 wt Frequency phase Impedance E n oy hue lie ee 4s pe om n r9 i 54 4
40. deg Frequency Hz Cursor 101 2 Hz 53 61 dB 99 8 deg Frequency Hz Pre delay 1 0 ms Predelay 1 043 ms 2 Sample Figure 6 2 20 Phase with pre delay 0 998 msec Figure 6 2 21 Phase with pre delay 1 043 msec If the delay is longer than the flight time increased for example to 1 043 msec which corresponds to two samples the phase graph reverses back on itself as shown in Figure 6 2 21 If we were to achieve this effect by placing the cursor two samples behind the impulse peak as shown in Figure 6 2 22 the frequency response would become distorted as in Figure 6 2 23 80 Impulse response mVIV 81 FR Magnitude dB V smoothed 1 24 oc 7 gt 621 62 696 29 70 96 445 63 320 29 194 96 iz 68 53 55 70 181 04 6 49 6 54 6 83 7 01 7 19 ms 4 00 200 500 1k zk Sk 10k 20k Cursor 179 240 6 3401 302 Cursor 101 2 Hz 3 24 dB 177 9 deg Frequency Hz Cursor 1 043 ms 2 Sample Cursor 1 043 ms 2 Sample Figure 6 2 22 Cursor placed two samples after Figure 6 2 23 Cursor placed two samples after impulse peak impulse peak The difference between using pre delay and moving the cursor is caused by the software not applying the FFT transformation to the part of the pulse that lies before the cursor If we tick Minimum Phase under the View menu the software calculates the minimum phase response of the tweeter by using a Hilbert transform
41. direct comparisons between different crossovers and enclosures etc Overlays are used chiefly in the frequency domain but can also be useful in the time domain 174 inl x amp mnnkher frequency response TT TT 1 0 mH 10uF pir File dit View Smoothing i FR Magnitude dB re 21 2 83 smoothed 1 24 act T Top Fit Pange F Set 105 0 1000 Smoothing 124 95 0 90 0 95 0 80 0 15 0 70 0 65 0 High Fr 60 0 Low Fr 50 100 200 500 1k Zk 5k 10k 2k 4 Cursor 242 0 Hz 85 53 dB Frequency Hz m Record 2 Figure 8 1 4 Smoothed frequency response window with overlay In the Smoothed frequency response window the current curve or filter targets are defined as overlays Further manipulation is possible with menu items as follows Set as overlay saves the current graph as an overlay gt Smoothed frequency response HT HT File Overlay Edit View Smoothing Set as overlay Ctrl 4 Set as overlay Below cursor Set as overlay Above cursor Load overlays Save overlays Manage overlays Ctrl M Delete last overlay Delete all overlays Generate target response Load target response Delete target response Load impedance overlay Delete impedance overlays Set as overlay Below cursor stores the part of the curve to the left of the cursor as an overlay Set as overlay Above cursor stores the part o
42. easy to make a measurement microphone see ARTA Hardware and Tools guide for details 14 UN 2 n Power Amp Microphone ait 1 lt e J LJ t P LAs 4 4 a With the minimum equipment for acoustic measurements computer with onboard soundcard power amplifier measurement microphone and the above basic settings you can now perform your first measurements Easy test setup for impedance measurement with LIMP For impedance measurements onboard soundcards are not usually suitable see also Section 4 1 If you have a soundcard with stereo Line In and a headphone output use the headphone output test setup shown above You will need a 100 Ohm reference resistor and some shielded cable In the absence of a headphone output use the following depending on whether the input jack on your soundcard is RCA or 3 5mm for example take a proprietary cable and cut off the end that is not required You will also need a banana plug a socket a 27 Ohm 5W resistor and two reference resistors of 8 2 Ohm and 1 0 Ohm 0 25W Assembly is as shown below 15 Three further settings are required before proceeding further The reference channel must be set to Right under Measurement Config in the Measurement Setup menu and the reference resistor value set e g to 27 Ohms The exact value of the resistor should be known and should be in the range 10 47 Ohms Measurem
43. eliminating reflections the environment provided by an chamber corresponds to the outdoors at a distance far from the ground with the signal unaffected by reflections Anechoic Visaton lt Wavelength A m 20 10 5 2 1 0 2 0 1 0 05 10 20 50 100 200 500 1k 2k 5k 10 k Frequency f Hz Figure 6 3 5 Relationship between frequency and wavelength The low frequency limit of the anechoic chamber is determined by its dimensions and lining Cut off frequencies are usually in the range of 70 125Hz and room volume roughly 350 360 The length of the absorption wedges should normally be about 1 4 of the wavelength of the lower frequency limit Figure 6 3 5 In order to effectively absorb down to the above mentioned cut off frequencies wedge lengths of approximately 1 metre are needed Ground plane measurements For ground plane measurements GPM no towers or specially lined and insulated large rooms are needed just a large reflecting area An asphalt parking lot playground or gym providing they not normal use are all suitable Loudspeaker ee am MM n o Microphone n 53 B n a Mirror source 86 Figure 6 3 6 Ground plane measurement There should be no reflecting obsta
44. for the calibration see also baffle in Chapter 9 5 3 Microphone frequency compensation The use of a good measurement microphone with a linear frequency response is recommended Suitable DIY microphones are described in Section 5 2 When you purchase the microphone or the microphone capsule ensure in addition that it has a smooth frequency response and omnidirectional polar pattern ARTA and STEPS offer the means for correction of the frequency response of your microphone but bear in mind that the correction will be limited to the on axis response Off axis frequency response errors will not be accounted for in the correction Follow the steps under the menu item Frequency Response Compensation as follows 1 Load the appropriate compensation file mic Figure 5 3 1 This should be a normal ASCII file that has been renamed from txt to mic It should have the following structure Frequency Hz Magnitude dB 172922 0 99 17 714 0 95 17 902 0 91 18 093 0 87 18 286 0 83 If necessary you read the correction values from the reference frequencies and enter them yourself into an ASCII file with no formatting After the file is loaded the frequency response of the microphone as in the above example 15 displayed n b It is important that you enter the frequency response and not the already corrected values If you have only a few measured values ARTA can interpolate intermediate values automatically by mean
45. frequency response at 45 a polar response diagram for representative frequencies 15 shown 186 Nennbelastbarkert Rated power 60 w Musikbolastbarkeit Maximum power 90 Nennimpedanz 2 Nominal impedance 2 Ohm bertragungsberesch Frequency response 8000 Hz fu untere Grenzfrequenz abh ngig vom Gehituse fu Lower cut off frequency depending cabinet Mittlerer Sc 87 dB Moan sound pressure level 1 W t Abstrahlwinkel 6 dB Opening angle 6 dB 947 4000 Hz Grenzauslenkung Excursion 8 5 mm Resonanzfrequenz fs Resonance frequency fs 43 Hz Induktion Magnetic mduction 0 95 T Magnetischer Fluss Magnetic flux 450 uWb Obere Polplattenh he Height of front pole piate 6 mm Voice coil diameter 25 mm Wickethohe Height of winding 18 mm Schalwand ffnung Cutout diameter 115 mm Gewicht netto Net weight 1 kg Gieschatromwiderstand Ade resistance 5 0 Ohm Mechanischer Q Faktor Qms Mechanical Q factor Qms 4 37 Elektrischer Q Faktor Electrical factor Oos 0 42 Gesamt Q Faktor Total factor Ots 0 38 Aquivalentes L umen Vas 131 Equivalent volume Vas Effektive Membranfliche Sd piston area Sd 79 Dynamische bewegte Masse Mms Dynamically moved mass Mms 99 Antriebelaktor Bx Force factor 5 6 Tm Schwingspuleniduktivitat L Inductance of the voice eol L 0 9 mH 187 P LA
46. measurement Dual channel measurement Impedance measurement setup Loopback for soundcard testing Voltage probe setup The soundcard left line output channel is used as a signal generator output The left line input 15 used for recording a DUT output voltage and the right line input is used for recording a DUT input voltage In a single channel setup only a DUT output voltage is recorded In a semi dual channel setup the right line input is used to measure the right line output voltage In a loopback setup the left line output is connected to the left line input and the right line output is connected to the right line input Acoustic measurements power amplifier E loudspeaker microohane soundcard Right input Left input preamplifier power amplifier microahanae soundcard loudspeaker preamplifier Right input power amplifier microphona soundcard Right input Left input loudspeaker voltage probe preamplifier Single channel measurement A single signal from the DUT is detected Soundcard and amplifier artefacts are included in the measurement as they cannot be compensated for Semi dual channel measurement The right channel serves as a partial reference compensating for soundcard artefacts Dual channel measurement Soundcard and amplifier artefacts are compensated see also the ARTA measurement box in Chapter 3 11 Impe
47. microphone you need a level calibrator The procedure is as follows Microphone sensitivity mv Connect microphone on Channel Left Preamp qain 10 77 sound calibrator Pressure 94 dB 3 Estimate Mic Sensitivity Estimated Current 9 79485 9 79485 Use of a reference tweeter 1 Connect the microphone pre amplifier to the soundcard line in left channel 2 Enter the gain of the preamplifier preamp gain and the SPL value of the calibrator Pressure 3 Present the calibrator to the microphone bi 4 Press Estimate mic sensitivity 5 If the measurement is acceptable press Note If the gain of the preamplifier is unknown you can set a default value but this value must also be entered as the gain in the Audio Devices Setup see also Figure 5 2 3 If you do not have a level calibrator you can use one of the following methods a Use of manufacturer s specification b Calculation of Thiele Small parameters and nearfield These methods are not substitutes for a proper level calibrator but they are suitable for DIY use in most cases 5 2 1 Use of manufacturer s data If you have a microphone with a reliable data sheet you can use the manufacturer s specifications Below you will see values for common microphones and electret capsules For data relating to the ARTA measurement box see S
48. necessitate a level correction xj The correction factor is PNF PD SP SD PP nearfield level PD driver level PP Enter number ar arithmetic expression to scale PIR port level SP port area 18 01cm SD driver 18 01 82 0 5 membrane area 82 00cm Figure 6 7 11 Entering the scaling values ca 129 FR hMagnitude dB 200 os smoothed 1 24 act 135 0 130 0 Im 125 0 120 0 115 0 110 0 90 0 85 0 10 ok Cursor 55 5 Hz 121 70 dB Frequency Hz Yent 5 5dB Figure 6 7 12 Nearfield membrane and port responses with level correction FE hagnitude dB re ZlHuPa Z os smoothed 1 24 act 125 0 120 0 115 0 110 0 105 0 10 Cursor 10 1 Hz 83 27 dB Frequency Hz Summe Figure 6 7 13 Summed frequency response black 130 Use Load and sum to get the combined nearfield frequency response Figure 6 7 13 The response up to around 500Hz can be used in this setup T FR Magntude Dufan BTV 1 24 oct FR Magnitude dB 20uPa0 37 emoothied 1 24 oct 0 10 0 80 100 0 2 0 a 2 00 200 500 1 2 jk 0 FAL Cursor 1422 6 Hz 91 06 0B 6 4 929 Frequency H2 Cursor 3899 9 Hz 11292 29 24 2029 Frequency 42 a Farfield frequency response calibrated b Near and farfield without level correction FR Magnitude 2B re uP ad 827 1 24 oct hace m Phace
49. not recommended Some ASIO control panels express the buffer size in samples while others use time msec When the latter is used the buffer size in samples can be calculated as follows buffer size samples buffer size msec sample rate KkHz number of channels Some ASIO drivers allow buffer sizes in samples that are a power of 2 256 512 1024 etc in which case ARTA adjusts the buffer size automatically ARTA always works with two input and two output channels treating them as stereo left and right As ASIO supports multichannel devices the user has to choose the pair of channels to be used in the ARTA Audio Device Setup dialog i e 1 2 3 4 etc 4 2 Testing the soundcard The easiest way to test the quality of the soundcard is in the Spectrum analyzer mode Fre Fei oo gt 9 Click on the SPA symbol as shown in the above toolbar Connect the line inputs of the soundcard to the signal outputs loopback connection an example is shown below soundcard Click Generator gt Setup or click the toolbar icon You will get the following dialog Enter the values shown in the red box 31 Sine square generator Multitone and noise generator Level dB FS z Level dB FS RMS Voltage 0 568 V Dither Level 16 v Multitone Wideband v Two sine generator Freqi Defi 13 kHz Def2 250Hz User Now enter the values
50. of measurements with varied amplitudes from Ustarr lt U1 lt Signal Generator Setup Sine generator 43 58 Peak Level dB 12 16 bit Frequency Hz Dither Level Signal generator type PN pink Spectrum made FH made Two sine generator Freg Frege Magn Def Mikk fahe 1 1 t Def2 2 4 1 ms W gt User generator Output volume dB E Fink cut off Hz 20 Maultitone Wideband Default Cancel Figure 9 1 1 Settings under Signal Generator Setup 3 Measure sound pressure in the near field and perform a spectral analysis to measure the amplitude of the fundamental P f1 and P f2 of the harmonic components P k f1 with k 2 188 3 K and of summed tone component P f2 n 1 f1 difference tone components P f2 n 1 f1 with n 2 3 versus amplitude U1 4 Measure the peak displacement X f1 versus amplitude U1 A simple method for determining the deflection is as follows Using a vernier caliper with depth gauge the distance to the dust cap is first measured without a signal and the value recorded as zero Then the speaker is excited with a sine signal generated by ARTA in SPA mode at fs and the depth gauge pushed carefully towards the dome until contact noise is heard The value of the excitation voltage value subtracted from zero gives the corresponding deflection 5 Determine THD with A
51. of the cursor while the farfield response is erased in this B W backgeeund coles area while continuing to appear to the right of the cursor 7 he per Figure 6 7 6 Use the options under the Overlay menu to delete any remaining overlays to see the overall frequency response The trace in Figure 6 7 6 shows clean transition with clear verte phase response vas Merge Overley above poo 4 Dele to phave mae v 126 FR Magnitude dB smoothed 1 6 act Phase SAS ALMA PTT Lu LL 1 i SY A LLL 4001 20 100 10k 20k Cursor 1322 7 Hz 12 96 E 3 1 TEE Hz iauazi Freifeldfrequenzaganda Figure 6 7 6 Overall frequency response quasi farfield To export the spliced frequency response to a simulation program go to File Export ASCII This gives two options e Export as an ASCII file with comments etc Export in FRD format ASCII file with no headers or comments G Smoothed frequency response 96cm pir View Smoothing Overlay FR Magnitude dB V V smoothed 1 3 act 10000 Cursor 297 0 Hz 24 99 dB 41 6 deg Frequency Hz Figure 6 7 7 Exporting the combined frequency response 127 6 7 2 Bass reflex enclosure
52. on the current spl file see Figure 7 2 5 With Copy the data will be copied to the clipboard Edit Copy copies the chart to the clipboard B W background color switch to black white Calibrate audio device opens the calibration menu Setup audio devices opens the soundcard setup menu 162 7 3 Detection of resonance including downsampling Resonances whether from the room speaker enclosure or speaker diaphragm are not wanted in most cases but they are impossible to prevent altogether so must be minimised This assumes however that they can be identified In some cases this can be achieved with simple means in others more effort is required More detailed discussion can be found in the publication Detection of Audible Resonances 10 The examples presented here are intended to serve as a primer only Room resonances In rectangular rooms the formula for room resonances 1s f frequency of the mode in Hz c speed of sound 344m s at 21 C n order of room length mode ny order of room width mode order of room height mode nx ny 0 1 2 3 etc L B H length width and height of the room in metres The following is an example based on a room measuring L 5 00m B 3 90m 2 20m Compare the calculated and measured positions of room resonances Figure 7 3 1 815 Hz 85 2 H 111 9Hz 122 4Hz 129 1Hz FR Magnitude dB re 200 2 8 gt 2
53. otherwise set the gain to unity LR channel diff enters the difference between the levels of the left and the right input channels in dB Power amplifier gain the power amplifier voltage gain is needed for calibrated results if you connect the power amplifier to the line output in a single channel setup The best way to determine these values is to follow the calibration procedure as described in the nex chapter Microphone section Sensitivity specifies the sensitivity of the microphone in mV Pa Microphone used check the box if you use a microphone and want the plot to be scaled in dB per 20uPa or dB per Pa In addi the microphone input channel from the drop down menu the ition select default setting in ARTA is the left channel use this setting wherever possible Setup data may be saved and loaded using Save setup and Load setup The setup files have the extension cal n b Remember to mute the line and microphone channels in the output mixer of the soundcard in order to avoid positive feedback during measurements If you use a professional audio soundcard switch off any direct or zero latency monitoring on the line inputs 4 1 1 Windows 2000 XP WDM driver setup After selecting the soundcard Figure 4 2 disable mute the line in and microphone inputs in the output mixer In addition select the input to be used for recording Line In or Microphone
54. shown below the toolbar Gen Sine Fs Hz 48000 FFT 16384 wind Kaiser Avg None 4 Nu Or select Setup Spectrum Analysis from the menu Setup then Measurement Spectrum Analysis 5etup Input channel FFT resolution FFT size 16384 Window Kaiser Sampling rate 42000 averages 32 Default Cancel Choose the left input channel then prepare the Windows sound mixer enable the line input channel mute the line in channel in the output mixer set the line out volume at maximum and set the line in volume at a low level Use Setup Spectrum Scaling or or right click in the graph title area to get the spectrum scaling dialog Use this to set magnitude scaling power weighting and distortion measures 32 Spectrum Scaling Scaling Power dB FS dBY SPL PSD Weighting Hone Voltage units dEv v Pressure units re 20 uPa Distortion THO Normalize with full power T THU M Low cut off Hz 20 IMO 2nd and 3rd order Show AMS Level Default Update Cancel Multitone Frequency weighting Check THD THD N and Show RMS Level Start recording by clicking the toolbar icon gt or via menu gt You should get a response like the one shown below This figure c
55. speaker The microphone is place on axis to the tweeter while the woofer is measured at positions and B The corresponding on axis woofer reference positions are shown as A and B Not surprisingly the shorter the measurement distance the greater the angle of measurement for the woofer and consequently the deviation from the on axis response If we input these frequency measurements into simulation software and simulate for other distances we will inevitably encounter further errors 110 FR Magnitude dB re 20uPa 2 83V smoothed 1 3 oct 100 200 500 1k 2k 5k 10k 20k Cursor 1797 6 Hz 91 05 dB Frequency H2 Current file AL 2 BHT pire 2006 11 09 15 51 32 Overlay files AL 1 piree AL 1 BHT pire AL 2 piree AL 2 BHT pir Figure 6 6 3a Woofer on axis green red and axis HT A black B blue Figure 6 6 3b shows the measurements for the tweeter The angle error at 60cm starting at 1 5kHz can be clearly seen 111 Measurement distance c 150cm 55 0 100 200 500 1k 2k 5k 10k 20k Cursor 6131 1 Hz 92 63 dB Frequency Hz Current file BHT 2 AL pire 2006 11 09 16 00 55 Overlay files BHT 1 piree 1 2 BHT 2 AL pir Figure 6 6 3b Tweeter on axis A green red and axis HT black B blue However at when the measurement distance 15 increased to 150cm the error caused by angle of measurement can b
56. speaker builders do not usually have the luxury of access to gymnasia 10m high measurement towers or anechoic chambers and must make do with whatever living or basement rooms are available or possibly a garden parking area outside in summer when the weather is calm with no wind So how do measurements in confined spaces work and how can ARTA help us How do the measuring environments used by professionals differ from normal living rooms To answer these questions two different measurement areas were compared The DUT and measurement conditions were clearly defined http www visaton de vb keyword Ringversuch proficiency test and kept constant in both measurements The sole difference was in the measuring rooms themselves indicated at the bottom of Figure 6 4 2a by reverberation times While the anechoic chamber had RT well below 0 15 seconds the normal room had an average RT of 0 35 seconds 88 The measuring distance was 30cm the DUT 8cm full range Visaton loudspeaker was flush mounted on a small baffle The speaker and measurement microphone were set up at approximately half the height of the room The unsmoothed frequency response is shown in the top row in Figure 6 4 2a The room reflections are Clearly visible in the right hand panel The second shows the traces after 1 24 octave smoothing black line the irregularities are still present The only way to eliminate the reflections is to measure within a defined wind
57. the group delay curve shows considerable variation when File Overlay Edit view Smoothing as g the time axis is set at the desired resolution This be adjusted 7 Magnitude Magn tPhase for by identifying the difference between the acoustic sources 10 0 ph EUR across a wider range than just the transition frequency Unwrap Phase For a parallel measurement this can be done directly Delay for Minimum phase 0 0 iion Phase Estimation is adjusted until the curves coincide within the Excess group delay desired range Figure 6 6 11 right The difference the example Sound pressure units is 0 064msec 2 20cm The frequency and phase responses for 10 0 Setup export to the simulation must cover the delay of each individual driver in the region around the crossover frequency Time Bandwith Requirement Excess group delay m5 averaged 1 24 1 3400 1 3200 1 3000 1 2800 1 2600 TE 4 1 2400 12200 1 2000 1 1800 1 1600 1 1400 200 500 1k 2k 5k 10k 20k 200 500 2 10k 2 Cursor 200 4 Hz 1 443 ms Frequency Hz Cursor 200 4 Hz 1 379 ms Frequency Hz HT amp TT Parallelmessung 8 TT Parsiebnessung GD Matching 4 1 gt Nite Quim oo 2 mb Figure 6 6 11 Excess Group Delay matching parallel measurements Phase Phase and group delay represent two sides of the same coin and phase can also be used for the de
58. 0 0 100 1000 10000 Cursor 265 0 Hz 17 16 dB Frequency Hz Figure 6 8 3 Load and Sum with two individual frequency responses Rather than the summed trace expected you may see something like the above This happens because ARTA always sums the newly loaded impulse with the data in its memory To avoid this it is better to go to New in the File menu and clear the memory Then e Load a file e g for woofer as normal with Open e Load the second file e g tweeter with Load and Sum e The final combined analysis will shown as in Figure 6 8 4 FR Magnitude dB V smoothed 1 24 act 15 20 0 25 0 30 0 35 0 85 0 100 Cursor 279 5 Hz 54 56 dB Frequency Hz Figure 6 8 4 Summation with ARTA previous memory deleted Note that tweeter polarity however is not correct To fix this e Clear the memory with New as above e Load the tweeter file as normal and use Inv to invert the phase 135 Fie Edit View Record Analysis Setup Mode Help Du 1 Fre Fret am E e D RE RES IU 627 85 FFT 32768 Windows LInifarm Delay for phase estimation ms 3 563 Get Zero e Load the woofer file using Load and Sum e The result should resemble Figure 6 8 5 FR Magnitude dB smoothed 1 24 act 20 0 25 0 30 0 35 0 55 0 100 1000 10000 Cursor 279 5 Hz 22 56 dB Frequency Hz Figure 6 8 5 Load and Sum with inverted tweeter 1
59. 0 4 Hr Frequens Curmor 230 4 Hr 3 B deg Frequens Suro 2001 4 Hz 23 212 Frays s Hz Chine else 1 me Dein 1 26230 ms Deby Figure 6 9 5 Target and measured phase with 0 0msec 1 0msec and 1 3639msec delay Loading your own targets If the desired target cannot be mapped with the standard functions you can import your own using Load target response Exported data from all known simulation programs TXT ZMA and are accepted Below is a representation of typical frequency and impedance responses The data are loaded either from LIMP or as or a txt file into an existing frequency response via Overlay and Load impedance overlay 139 gt Smoothed frequency response Untitled File Overlay Edit View Smoothing Set as overlay Set as overlay Below cursor Set as overlay Above cursor Load overlays Save overlays Manage overlays Ctrl M Delete last overlay Delete all overlays Generate karget response Load target response Delete target response Load impedance overlay Delete impedance overlays 55 0 50 0 45 0 10 20 50 100 Cursor 10 0 Hz 57 75 dB e setup Y smoothed 1 8 act 0 0 200 500 Frequency Hz Record Overlay copy Loading of the impedance overlay opens up a secondary Y axis This can be manipulated in Graph Graph Setup X Magnitude dB 95
60. 0 50 100 200 500 1k Zk 10k 20k 20936 3Hz 0 00766 D2 0 00766 55 f Hz t amp mp Tey Figure 5 4 6 Distortion versus frequency at 16W Since release 1 3 voltage or output related distortion measurements have been possible with STEPS Figure 5 4 7 shows voltage dependent harmonic distortion at three different frequencies For more details of this type of measurement see the STEPS Handbook Distortion 96 100 0 0 1 0 01 0 001 0 01 0 1 1 0 10 0 0 THO 24 05 Voltage rm Figure 5 4 7 Voltage dependent harmonic distortion THD of the amplifier at 4 1 Ohms at 100Hz 1kHz 10kHz Amplifier parameters of interest besides power frequency and phase response and harmonic distortion are shown in Figure 5 4 8 62 Input voltage Voltage gain V UA UE Output voltage UA Input impedance RE Output impedance RA Load impedance RL RS lt lt RE RA lt lt RL Figure 5 4 8 Amplifier schematic The input impedance RE is the internal impedance of the input side of the amplifier and can be measured with a resistance RV connected in series with the amplifier input The input voltage drops from UEI to UE2 and with it the output voltage falls from UAI to UA2 The input impedance of the amplifier is characterised as follows Example t amp RV 47kQ UAI 10 502V UA2 3 144V 47kQ 3 144V 10 502V 3 144V 20 082kO Figure 5 4 9 Measurement of input impedance The
61. 0KHz of 1 25dB Level dBr fsmoothed 1 3 och 5 0 100 200 500 zk Sk 10k 20K Cursor 372 6 Hz 0 06 dB Frequency Hz Figure 5 3 4 On axis frequency response correction Export ASCII can then be used to create the compensation file Rename the txt file to mic and import it as described earlier 5 3 2 Calibration below 500 2 with a pressure chamber As noted above Earthworks uses a pressure chamber for microphone calibration below 500Hz Construction and operation of the pressure chamber are described in detail in ARTA Application Note No 5 The largest dimension of the chamber should not exceed 1 6 to 1 8 the wavelength of the upper cut off frequency of 500Hz 1 e 11 5 8 4 cm V 20 38 Liter Figure 5 3 5 Design and application of the measurement pressure chamber The use of the pressure chamber is illustrated in Figure 5 3 5 The test microphone is inserted into the chamber via an adapter that creates a good seal and measurements are made with ARTA STEPS in the frequency range of interest The chamber seals the microphone off from its surroundings and provides a suitable measuring environment but be aware that very high sound pressure levels can be 55 expected if normal voltages are used e g 2 83V is likely to yield 145dB Because of this small excitation signals only c 0 01V should be used to avoid damage to the microphone Figure 5 3 6 shows the frequency response of the MK221 with STEPS when th
62. 1033 rey amp TG Cure 000 Odes DD DOS n Cave Ds Fi LPS pir 20 0 Tu or Pit LPO pir 22 02 24 122443 Siren ms m 350 1576 P rey ocn 13 55 P i 12 06 Cure 0 000 Os Came Os On Ceo Dv Ds 00 Fix BEZD 10 AENM DG zm BE DD 1000 pir BESDDOU 2201 pir Siren v hpass 24 12 06 i x 13 06 zn ma 010 12 06 Caso O00 DoD 00s n Gunmen DoD 00s 000 Fia HEZD pir Fix HPD pir 819 00 24 T2 51 45 Pit 1000 1000Hz Figure 6 2 9 Influence of cutoff frequency on the appearance of the step response In the final example a tweeter connected to a 12dB high pass filter with a corner frequency of 1000Hz was simulated Figure 6 2 10 shows the simulation top and the measured frequency response of the tweeter Note the significant differences between the measured and theoretical curves The individual characteristics of the tweeter the driver setup and the measurement conditions distance room ambient noise etc are all evident the impulse response and ultimately the derived analysis The peculiarities of the phase response are discussed in Section 6 2 2 76 impulse response im v 353 5 10 5 56 Cursor 0 000 n O 000me 07 File pir Impulse res
63. 24 900 FOO 600 z 100 t 10k 100 200 Sk 10k 208 Cursor 70 0 45 9 08 Fragua HT Cursor 4612 Hz c3 985 oH Fraguancs Cut beka Cursnr Cut bekri Curz r FR Wagnitude dB re 20uP at 8v smoothed 1724 FR Wagnitude dB re 8 smoothed 1 24 1000 roo 600 500 apa 300 100 2 5k 1 20 50 110 1k ak BE 10k ah Cursor 200 Hz 42 B7 da HZ Cursor 168 2 Hz 46 84 98 Fraguancs diwri Revi eset Time Een Figure 8 2 1 Cut below cursor function and Time Bandwidth Requirement Figure 8 2 2 shows the measurement of a small full range loudspeaker with two different microphones NTI M2210 T Bone MM 1 The NTI M2210 is a Class I microphone and is used here as a reference for generating a compensation file for the inexpensive MM 1 Figure 8 2 3 shows the effect of the Subtract overlay and Subtract from overlay functions In the arrangement shown here you would use Subtract overlay file as a compensation function for the MM 1 FR Magnitude dB re YOuParml 83w smoothed 1 24 oct 130 0 125 0 120 0 115 0 110 0 105 0 100 0 95 0 90 0 95 0 zl 50 100 200 500 zk 10k 20k Cursor 4506 8 Hz 119 85 dB Frequency Hz t bone black NTI 2210 red Figure 8 2 2 Overlay NTI M2210 measurement T Bone MM 1 181 a Lewel dEr smoothed 1124 oci Lewel d
64. 24 Echo AudioFire 4 Echo Layla 24 Echo Indigo M audio Audiophile 2496 Firewire Solo USB Transit Delta 44 Terratec EWX 24 96 Firewire FW X24 YAMAHA 46 Sound Devices USBPre2 Digigram VxPocket 440 a notebook PCMCIA card TASCAM US 122 USB audio ESI Quatafire 610 Juli U24 USB and Waveterminal Soundblaster X Fi Infrasonic Quartet Soundblaster Live 24 Audigy ZS Extigy USB but only at 48kHz sampling frequency Turtle Beach Pinnacle and Fuji cards ARTA may be used with a slight loss of performance with the following soundcards Soundblaster MP3 USB note don t install SB driver use a Windows XP default driver Soundcards and on board audio with AC97 codecs Further information on soundcards that can be used with ARTA can be found at the homepage http www artalabs hr See also M ller 1 and section 2 1 Amplifier A power amplifier with linear frequency response and power 5 10 watts is adequate The output impedance should be 0 05 Ohms Do not use an amplifier with a bridge amplifier virtual ground this may damage your soundcard Check the manufacturer s specifications before first use if in any doubt An inexpensive solution that meets the above requirements and is small and easily portable is the Thomann t amp PM40C see also section 5 4 Microphone Affordable measurement microphones are available but whatever model is used it must be omnidirectional 4 1 4 with linear frequency response Inexpensi
65. 27 77 uH 1 50 ohms 0 0 0 59 0 66 5172 45 0 13 16 grams 1 016202 kg s 90 0 0 000390 n H 8 79 liters Sd 126 68 cm Avg 18 Hl 7 377537 ETA 0 54 Lp 2 83U 1m 89 65 dB Rms CAS Was Closed Box Method Box volume 5 40 liters Dianeter 12 70 cn 10 20 50 100 200 500 Cursor 113 53 Hz 11 94 Ohm 50 2 deg Frequency Hz TSP If you have not used LIMP it is possible to use manufacturer s data for initial simulation Note that only data from reputable mainstream manufacturers should used to ensure reliable simulations Dateneingabe SPL d B satus rn Treiber Fre 110 ad Frontloaded Konisch Halbreum 90 Abstand 1 m 90 dl 0 70 1 b 127 bl Info Predator 20 50 100 200 500 Frequenz Hz 100 Figure 5 2 2 Simulation of a 6 TMT driver with AJHorn half space 2 83V The above figure shows an example of a simulation in AJHorn for a 16cm woofer with an input voltage of 2 83V The simulated frequency response can serve as an objective comparator for data obtained with our microphone see Section 6 6 The only prerequisite for the procedure is that the sound card should be calibrated see Section 5 1 Note that the SPL of most microphones or capsules used in DIY constructions is limited to approximately 120dB so start with low levels and avoid overdriving the input channels of the soundcard 46 54 Note
66. 36 6 9 Working with targets Targets or objective function are useful for developing crossovers determining baffle effects and confirming simulations Smoothed frequency response Untitled 18 Untitled Arta File Overlay Edit View Smoothing File Overlay Edit View Record Analysis Setup Tools Set as overlay Fre Fra Ses 0 mm Set as overlay Below cursor Set as overlay Above cursor Target responses are found under Imp in Smoothed Frequency Response and as shown on the left in the Load overlays Overlay menu Save overlays Common standard filter functions be generated using Manage overlays Generate target response by using Load target response to load a file generated by third party software txt and frd files are accepted Delete target response erases all displayed targets Delete last overlay Delete all overlays target response Load target response Delete Earget response Standard filter functions The Target Filter Response window Target Filter Response X opens when you click on Generate filter response in the Overlay menu You can select the filter type low Ref band tivity 0 dB 98 high or bandpass Butterworth Bessel Linkwitz Riley the filter order and the transition frequencies Filter type eL Az eet Click O
67. 4 TN Figure 6 9 7 Determination of Thiele Small parameters from the impedance curve of a bass reflex speaker with CALSOD Before left and after right parameter optimization gi Figure 6 9 8 Parameters derived from the calculated frequency response Figure 6 9 8 shows the calculated frequency response for the measured prototype The parameters required for this were calculated from the impedance response Figures 6 6 9 and 6 6 10 show comparisons between measured black and simulated red data for two different enclosures and tuning frequencies 142 FR Magnitude dB 200 B3 smoothed 1 24 act PEN R 90 5 e xL aes SL LA LL LLL LLLLLL A LLLLLL dL LL P45 LE LLLI 20 100 1k Cursor 257 0 Hz 80 38 dB CALSOD VBFIT Figure 6 9 9 Comparison of simulated red and measured black responses for Vb 18L 310 Cursor 1003 9 m Br 85 dB E in 31Ltr vB Figure 6 9 10 Comparison of simulated red and measured black responses for Vb 31L 143 The third set shows a simulation of a long transmission line TL to be verified by measurement The simulation was carried out using AJHorn 5 0 www aj systems de by Armin Jost w
68. 40 0 5000 2 8280 2 0000 6 4 c mmm Pues LL ERE 10 15 20 29 30 35 40 Temperature C 194 Series and parallel driver connection When multiple drivers of the same type are connected series or parallel their parameters may be affected Drivers can be connected together in series Ser and parallel Par electrically they may also be series compound enclosure or in parallel mounted next to each other acoustically Possible combinations and their effects on driver parameters are shown in the following table 2 2 dme Driven iD fs Hz Re Ohm 1 SD cm 1 1 1 1 e r 195
69. 550 Note the differences in level and response The first job is to compensate for the difference in levels 53 FR Magnitude dB re 2 28 3 smoothed 1 3 act 111 0 108 0 Tm 107 0 105 0 103 0 101 0 99 0 a u 95 0 93 0 91 0 100 1000 10000 Cursor 9421 7 Hz 102 01 dB Frequency Hz Figure 5 3 2b Reference microphone MK 221 blue and test microphone MB550 Use Scale Level Scale Magnitude dialog in the Edit menu of the FR window to reduce the level of the MB550 until it 1s superimposed as much as possible over the reference response Enter value in dB to scale magnitude Figure 5 3 3 You may need to experiment a little with this as the best value is not always obvious at first glance aa dB E Smoothed frequency response MB550 A CAL SUM ABC pir Cancel File Bax View Smoothing Overlay Copy Colors 2 83V smoothed 1 3 oct Cut below cursor Cut above cursar Scale level Subtract overly Subtract from overlay Power average overlays Merge overlay below cursor Merge overlay above cursor oy 0 95 0 93 0 91 0 100 1000 Cursor 9421 7 Hz 101 61 dB Figure 5 3 3 Scaling and subtraction We then use Subtract Overlay to account for the difference between the two frequency responses as shown in Figure 5 3 3 54 Figure 5 14 shows the result of this operation There is maximum variance between microphones from 150Hz to 2
70. ARTA programs are nevertheless under constant development Thus you may occasionally come across illustrations or examples in the Handbook that differ slightly from the version of ARTA STEPS and LIMP that you may be running This is unavoidable but in the very large majority of cases will not cause significant problems We ask for your patience and understanding and welcome any comments or suggestions for improvement With the growth of ARTA STEPS and LIMP and in light of comments received the original Handbook which was published as a single document has as of Version 2 4 been split into three with each program having its own dedicated volume Please note also that this Handbook in English 15 a translation of the German original We have tried as far as possible to update and amend all figures and tables where German continues to appear in this translation because it has not been possible to amend figures etc without obscuring detail English translations are given in the figure legend The programs of the ARTA family currently include ARTA STEPS and LIMP as mentioned above The tasks carried out by these programs are as follows Measurement of impulse response transfer functions and real time analyzer Step gt STEPS Transfer functions distortion measurements linearity measurements e LIMP Impedance measurement and determination of Thiele Small parameters Note that some of the metho
71. Audio Audiophile 24 96 Typically these cards each have separate input and output RCA connectors with left white and right red channels Figure 1 3 shows a professional high quality soundcard with firewire port On the front there are two XLR microphone inputs This input is a combo jack the centre of the XLR socket can accept a 6 3mm plug and serves as an instrument input with impedance between 470 kOhms and 1 MOhm Both inputs have a volume control PARSE XASS HEADPHONE d OE TACE PRODUCER PHASE X24 24 Bit 192 kHz Extended Audio System ANALOG DESIGN IN COOPERATION WITH CHANNEL 1 CHANNEL 2 INPUT INSERT MONITOR DIGITAL OUT f FIREWIRE OUT 12VAC Figure 1 3 Professional soundcard system with firewire interface Microphone inputs can be switched to phantom power which gives power supply of 48V to pins 2 and 3 of the XLR microphone connector There is also a master volume control for adjusting output level and input monitor level Finally there is a headphone volume control and a headphone stereo TSR connector On the back panel there are two balanced inputs two balanced outputs SPDIF optical connectors and two firewire connectors Up to now ARTA has been used successfully with a following soundcards RME Fireface 800 RME Fireface 400 RME DIGI96 RME HDSP Duran Audio D Audio EMU 1616m EMU 0404 USB EMU Tracker Echo Gina
72. B per octave Q 1 6 f 1400H2 it is closer to 24dB octave see also simulation in Figure 6 10 4 This illustrates the pitfalls of relying on formulaic assessment of crossovers 147 FR Magnitude dB re uPalz 83v smoothed 1724 oct 20 0 20 50 100 200 500 1 zk 10k 20k Cursor 1236 0 Hz 88 53 dB Frequency Hz HT mit ohne VVveiche Figure 6 10 3 Frequency response with without a 6dB crossover 19 Figure 6 10 4 6dB crossover simulated with a resistive load ___ tweeter 1 6 _ Filter 6dB Filter tweeter The difference is due to the two acoustic frequency responses see Figure 6 10 5 the result of which is except for the peak at about 1 2kHz an apparent filtering effect of 6dB octave 148 5 Level dBr smoothed 1 24 act 0 0 10 0 20 0 30 0 20 50 100 200 500 1 2k 10k 20k Cursor 1236 0 Hz 1 06 dB Frequency Hz 6d Differenz mitfohne Weiche Figure 6 10 5 Variance from target response with 6dB crossover acoustic Differenz mit ohne Weiche Difference with without crossover Suppose now that the signal is not coming from the microphone but as shown in Figure 6 10 1 from the crossover via the probe The electrical filter effect 15 clearly 6dB octave Figure 6 10 6 The peak at 1 2kHz seems to be due to the interaction of the tweeter grey with the capacitor in the crossover
73. Because of the proximity of the microphone to the ground the only interference effects will be at very high frequencies frequencies representing wavelengths that are smaller than the microphone diaphragm Enclosure effects should also be considered As these are essentially vertical effects polar or distortion measurements can be carried out as usual 87 FR Magnitude dB V smoothed 1 24 oct Ground Plane gt AD Halbraum 217 Freifeld 4m 20 50 100 200 500 1k Cursor 239 7 Hz 1 53 dB Frequency Hz Figure 6 3 7 GPM freefield and half space measurements at metre Provided that the same input power is used a 2 metre GPM measurement has the same sensitivity at mid and high frequencies as a 2 or 4 measurement at 1 metre At low frequencies the level is the same as a 2 measurement In between there is a region in which the radiation characteristic of the source changes gradually depending on the size of the baffle and its reflection from 2 to 4 Half space For half space 21 measurements a wall or floor mounting serves as an infinite baffle In unobstructed outdoor areas a hole may be dug while in a building the speaker has to be mounted flush with a floor or wall this potentially involves structural alterations see e g www hobbyhifi de measurement room Measurements in the open are essentially freefield For measurements in rooms see Section 6 4 6 4 In room measurement DIY
74. Distortion Graph Setup Frequency range Hz Magnitude dE Show harmonics level y ET Magn top 20 iw 2nd v 3rd Low freq ERE Magn range 80 View all Thick plot lines w Default Time Bandwidth Update Figure 7 1 4 Graphic setup dialogue Figure 7 1 5 shows a comparison of the Farina method and STEPS in single channel mode at four different levels Test conditions were identical The results are very similar for both distortion traces and levels the user just has to take note of the limitations of the Farina method referred to above Notice that the acoustic levels of the STEPS traces decrease with reducing excitation signal levels This is because ARTA works with reference levels while STEPS identifies the absolute level in single channel mode For this reason single channel measurement is well suited for the determination of absolute sound levels at the microphone and to determine peak sound pressure levels For more information on distortion measurement see the STEPS Handbook Note from version 1 4 export of ASCII and CSV for processing in other programs is supported 156 30 Er 100 205 Frag 20 1 Hz marc 08 19 dB 30 77 dB 20 45 OB Dk 23 dB 16 TT T1de pir 1 48 100 ina Fre 10 1 Hz aen BB dB De 32 78 dB 25 62 dBi D 35 T8 dB fh Lint 3 dB 100 200 Fr g 20 1 Hz hs RB 26 dB 012 3112 dB Drs 30 7T OB 64 3055 06
75. Er smoothed 1124 qa 2n 20 40 55 B0 10 0 0 100 200 5 5 100 0 5 208 Cursor 43505 5 Hz 0 91 dB Fr guency HTI Cursor 1902 4 Hz 0 43 0B HT overlay Sui Trom creer Subtract from Overlay Figure 8 2 3 Effect of Subtract overlay and Subtract from overlay Figure 8 2 4 shows the effect of Power average overlays on measurements taken from a small midrange speaker and a tweeter in 10 degree increments The red curve in each case shows the averaging over all overlays FR dE re EV 1524 acil 1000 FR di re BV 1524 oci 050 950 800 zm NO S en 150 it m 700 70 0 HET 65 0 Im 50 0 iul 800 AH 550 550 500 20 5p 10 200 1 z 20 im 3 1 1 2 k Curar 1561 5 Hz 92 18 BB Hz Cursor 814D 1 Hz 92 18 dB Fraqueresg Hz Pur Oa Poet iraa 0 53 Figure 8 2 4 Power average overlays function This feature has been recommended by Joseph D Appolito 22 23 and Floyd E Toole 24 182 8 3 Scale and Scale Level Below is a small collection of useful formulae Level normalized to dy in the farfield 20 log d dy Nearfield level Pyr adjustment for farfield level Ppr half space 2 1 r 2d 20 log r 2d Nearfield level Pyr adjustment for farfield
76. FR Magnitude dB re 200 2 23 smoothed 1 24 oct Zahm 20 50 100 200 500 1k 2k 10k 20k Cursor 20 8 Hz 55 59 dB Frequency Hz Figure 6 10 6 Amplitude response with 6dB crossover electrical What happens if we add a parallel RLC compensation network m 149 Figure 6 10 7 shows the acoustic response compared with the circuit without the RLC network The interaction of the tweeter resonance and the capacitor has been almost eliminated The response is now first order Figure 6 10 8 FR Magnitude dB re uPalz 83v smoothed 1 24 oct 20 0 20 50 100 200 500 2k 10k 20k Cursor 205 9 Hz 31 79 dB Frequency Hz BdEB mitfohne RLC Glied Figure 6 10 7 Response with without RLC network acoustic Level dBr smoothed 1 24 act 0 0 10 0 20 0 30 0 40 0 50 0 60 0 20 50 100 200 500 Zk 10k 20k Cursor 208 7 Hz 24 73 dB Frequency Hz 6d Differenz mit RLC Glied Figure 6 10 8 Variation from target response with 6dB XO RLC network acoustic Checking again we see that the 1 2kHz peak was significantly reduced but not eliminated The RLC network is not optimal 150 FR Magnitude dB re 200 2 23 smoothed 1 24 oct Zahm 20 50 100 200 500 1k 2k 10k 20k Cursor 20 0 Hz 55 04 dB Frequency Hz BdEB mitfohne RLC Glied Figure 6 10 9 Amplitude response of 6dB XO RLC network electrical What can be achieved by furt
77. K to plot the target function Low pass 02 Butterworth II order P 5 ae ied e The process can be repeated Figure Hiah nass LINES 11 Order High p AG EESTI OPER 6 9 2 and all targets generated mr NR 06 Bessel order remain active until Delete target 07 Butterworth Iv order response is used Note that selective 08 Bessel Iv order ere deletion of individual target curves is 10 Butterworth v order not possible 11 Bessel V order 12 Butterworth VI order Lower 500 Hz 13 Bessel VI order 14 Linkwitz VI order Upper 1000 Hz Figure 6 9 1 Target Filter Response window Cancel Crossover Frequencies 137 FR hagnitude dB 200 2 os smoothed 1 24 act a a LLL a sot I odU A 20 100 10k 20k Cursor 20 1 OMA dB Frequency Hz Target 1000 Hz LP 1st 4rd Order Figure 6 9 2 Target examples filter functions with differing orders Standard filter functions can be useful in the development of crossovers You choose the desired target function and then vary filter components in order to fit the target Figure 6 9 3 FR hagnitude dB 200 2 os smoothed 1 24 act 45 0 200 10k Cursor 200 7 Hz 92 47 dB Frequency Hz Figure 6 9 3 Target and measured frequency response of a crossover 138 In modes and FR2 this can be done a dynamic fashion
78. Magn range 50 Frequency range Hz Magn top Ph top High Freq 1000 Low freg 10 Minimum View all Phase deg 180 Ph range 360 Impedance ohm Maximum 5 Group delay ms ad top so ad range 100 Thick lines Time Bandwidth Update Default B x E Fit Range E Set Smoothing 119 High Fr Low Fr C Eo 2 140 T FR Magnitude dB re uPalz 83v smoothed 1 24 act 50 0 E 45 0 0 0 100 200 500 zk Sk 10k 20k Cursor 100 0 Hz 44 13 dB Frequency Hz Current file Untitled 2010 09 12 22 01 16 FR amp i FR Magnitude dB re 20uPa 2 83V smoothed 1 8 oct zZ ahm 1 20 50 100 200 S00 1k Cursor 10 0 Hz 57 75 dB Frequency Hz Current file Untitled 2010 09 12 21 52 17 FR amp IMP Figure 6 9 6 Representation of typical frequency and impedance response top tweeter below bass reflex enclosure The second set of illustrations deal with verification by measurement of a CALSOD bass reflex simulation In this illustration the speaker and enclosure parameters are determined from impedance measurements on the prototype Withold Waldman presented this method in 1993 at the AES Convention in Munich 19 Figure 6 9 7 shows the impedance curve before and after parameter optimisation with CALSOD dotted line measurement dashes simulation 141
79. Measurement of harmonic distortion with a sine signal Although not fully validated and not free from other types of distortion noise and artefacts the method suggested by Farina 20 may be used for rapid sine determination of frequency response and harmonic distortion The method 15 useful because it is quick to perform but it needs to be carried out in environments with low reverberation and noise 5 21 To determine frequency response and harmonic distortion using this method Impulse response measurement Signal recording Periodic Moise Sweep mus External excitation Recorder Prefered input Left af Dual channel measurement made Sweep generator Sequence length 128 Sampling rate Hz 96000 Time constant 1365 33 ms Invert phase of input channel Oukput volume Number of averages 1 Filter dual channel impulse response Record Log Frequency sweep v Generate voce activation Center peak af impulse response Close after recording L 70 50 30 10 dB R 50 0 40 m dB Abbrechen Figure 7 1 1 ARTA setup for impulse and sine response 1 Enable single point mode in Sweep mode disable dual channel measurement mode by unchecking the box 2 Check Center peak of impulse response 3 Perform the measurement Record The length of the excitation sequence must be at least 64k Th
80. Mic For a standard PC soundcard the is as follows e Windows 1 ate or had 2 Click on menu Options gt Property and select the soundcard chanr playback as shown in Figure 4 3 3 Mute Line In and Mic channels in the Master Volume dialog Figu 4 Set the Master Volume and Wave Out volume to maximum 5 Click on menu Option gt Property and select the soundcard input channel and enable Line In and Mic channels in therecording mixer 6 Choose Line In or Mic Input Normally ARTA uses Line In to which the external microphone amplifier should be connected 7 Set the Line in volume control to a lower position This will be set more precisely el that will be used for output re 4 3 later 25 I Miner Conexant Audio output Mixer Conexant HD Audio input Lautstarke regeln fur Lautstarke regeln Fur Wiedergabe C wiedergabe Aufnahme Aufnahme C Andere Andere Folgende Lautstarkeregler anzeigen Folgende Lautstarkeregler anzeigen Lautstarkeregelung CD Audio Wave Mikrofori Sw Synthesizer Waveausg CO Audio Mikrofon Eingangslautstarke Abbrechen Abbrechen Figure 4 2 Soundcard input output channel dialog in German Options Help Master Volume Wave CD Audio Line In Microphone Balance Balance
81. PS See AES id 1991 27 Manufacturers should follow the recommendations of the AES their data sheets Mainstream manufacturers usually do this as a rule but the published specifications of generic products should be treated with more caution ARTA STEPS and LIMP can of course assist in this respect Dimensions and mounting information 1 6 are usually given The physical information in item 7 is partly dependent on manufacturer s specifications 1t may be possible to gather other data although this may require partial dismantling of the driver The measurement of Thiele Small parameters item 8 12 is carried out with LIMP as described elsewhere Large signal parameters such as Xmax items 185 9 and 14 are dealt with in Section 9 1 or in Application Note 7 Frequency response including off axis is measured with ARTA Chapter 6 Application Note No 6 For notes on the Standard Baffle see the end of this section Figure 9 2 Item 11 can be dealt with using ARTA Farina method Section 7 1 or STEPS For maximum electrical load items 13 and 15 we must usually depend on the manufacturer s data It is difficult to measure a speaker at maximum output for 2 hours without causing a significant noise nuisance Figure 9 1 shows the data sheet of a midrange speaker by Visaton With the exception of point 11 and some parameters which can be calculated from the given data all required information is included Instead of the
82. RTA in SPA mode at the resonance frequency with sine excitation as a function of the amplitude U1 F JPQ fF 3 I Enable Two Sine excitation and choose a frequency range between f2 2 5 fs in a linear representation Rest the cursor on the marked frequencies as shown in Figure 9 1 2 and note their values The second order modulation distortion g POS fi PCR fi Pfa and the third order modulation distortion d 2 2 P fy 100 100 15 Untitled Arta File Overlay Edt View Recorder Generator Setup Mode Heip Uy Iw Fez Sa gt A eee S 249 Gen Two Sine 48000 FFT 65535 l Wnd Kaiser l Avg Linea Reset 32 Spectrum magnitude dB re 1Pa Avg 10 t j Je mM Mv F2 2fs F2 s 2 2 Cursor 2725 Hz 93 20 dB Frequency Hz RMSz 7 3 99 1 MD 2 15 21 948 R 20 9dB Spectrum Analyzer Figure 9 1 2 Determination of second and third order modulation distortion are calculated as indicated above in the formulae Note that the level values read off the chart must be converted to absolute values by Abs 10979 The following table shows an example 189 PindB Pabs fl 43 58 2 3 04 486 0003715 f2 f1 3269 89 46 0 000034 2f1 414 87 95 0 000040 f2 2f1 2833 86 24 0 000049 2 2f1 457 6 103 63 0 000007 From these values the second and
83. T N A LLL eo A LU LU D LO LED LET METTI 100 Cursor 999 4 0 00 dB Current file MPATUOUZ Lin A pir Overlay files LP 12 kHz amp HP 100 Hz Mikrafanvarverstarker MFA 102 100k Frequency Hz 2007 10 20 12 35 37 Figure 3 5 MPA 102 Monacor microphone preamplifier Note that the low pass filter cut off LP overlay file comment is not correct it should read 10 5kHz 21 3 2 Single channel measurement calibration If you want to perform calibrated measurements in single channel mode you must also enter the gain of the power amplifier Power amplifier gain Audio Devices Setup X Sound card Input Device co46 s046 1 In 1 Output Device co46 so46 1 uk WawveFormat C i amp bit z4bit 32bit Extensible Extensible v TO Amplifier InterFace LineIn Sensitivity 3130 83 Line Quk Sensitivity 1498 mvpeak left ch mvpeak left ch 190 Ext left preamp gain 9 1349 channel diff dB 0 05116 0 05116 Ext right preamp gain 0 04808 Power ampliFier qain 1 3 148 Microphone Microphone Used Left Ch Sensitivity 37 9637 37 9637 Save setup Load setup OK OK Figure 3 6 Audio devices setup menu in ARTA and STEPS To measure the power amplifier gain use one of the following two procedures 1 Take a frequency response recording in single channel mode Deter
84. agram courtesy of J Backman 7 83 Radiation from vibration of enclosure surfaces the enclosure Sound propagation across the enclosure surface Edge Transduction diffraction and driver movement Diaphragm vibration Direct radiation from the driver Port radiation Enclosure Port acoustic acoustic modes modes A two way speaker with a crossover frequency of about 2000Hz requires a lower frequency limit of at least 500Hz Figure 6 3 2 If the baffle step right panel is to be accounted for measurement at 200 150Hz would be required depending on the baffle width for sufficient resolution H H gt 4 H e i 4 gt H 5 4 t H 5 4 H i i 4 4 i H H 4 H 4 M i ML ae ocn m v J m H H H H P y at E 4 i s H M Figure 6 3 2 Simulation of two way crossover left woofer with without baffle step right i k 20 200 saa ik 2k The task is complicated further by the contribution of room reflections and standing waves Various methods are described in the literature 8 9 2 10 11 4 for dealing with th
85. aker measurements Impulse response uV V 49 65 37 24 24 83 12 41 0 00 12 41 24 83 37 24 Range up to first room reflection 0 00 38 25 76 87 115 12 153 75 Cursor 25 002 nV 0 000 ms 0 Gate 216 000 ms 10368 Impulsantwort Raum 4 9 X 3 8 x 2 2m Figure 6 5 2 Impulse response of the room ms 102 Click on to open the following window gt Acoustical Energy Decay Dadil pir Edit Automatic 1503382 evaluation Energy decay MET m m 0 00 453 33 944 00 1413 33 1666 00 m Cursor 0 00098 0 000 ms Figure 6 5 3 Acoustical Energy Decay window controls The fields outlined in red are required as follows Filtering Choice of octave band to be evaluated or the entire frequency range Wide T60 Starts the calculation of acoustic parameters Results are shown below the graph Noise tail Consists of two variables the first determines which part of the curve is used for the evaluation while the second is the noise reduction setting Trun The portion selected is not included in the calculation Sub the mean noise level of the tail is subtracted from the curve dB range Sets the Y axis Log Outputs report with the calculated room acoustic parameters Zoom Horizontal zoom or Scroll Moves the trace to the left or right The procedure 15 as follows 1 Select the frequency band to be evaluated under Filtering 2 Specify the porti
86. an be copied using the copy paste operation menu Edit gt Copy Slowly increase the volume of the line in channel using the soundcard mixer until the peak level is close to 3dB FS a Untitled Arta Fle Overlay Edt Yew Recorder Generator Setup Mode Iw Fre gt 8 gt P Smee aS Gen Sine Fs Hz 49000 FFT 15384 Whnd Kaiser5 Avg None Reset Spectrum magnitude dBFS 100 200 500 ik 2k Cursor 205 2 124 32 dB Frequency Hz RMS 33dBFS 0 00081 0 0052 Ready Li 3 3dB 3 308 Spectrum Analyzer 10k 20k Frequency and amplitude values at the cursor position are displayed under the chart with RMS THD and THD The cursor itself 1 shown as a thin line and can be moved using the left mouse button or the arrow keys left or right Note that you can reset the type of averaging the sampling frequency the type of excitation signal and the FFT length during measurements via the control bar 33 Results for three popular soundcards shown below for comparison 5272 M Audio Transit THD N 0 0069 aA FON ey cS WE OWS t A Oe Jg trim sr F Realtek AC97 Audio THD N 0 1845 Intelonboard card THD N 0 08585 7134 Cow 9997 9 4 212
87. bitrary because of the lack of calibration data The red line shows the imported simulation data 0 3 First the uncorrected nearfield level must be adjusted for the 29 97dB calculated above by using Edit Scale Level in the FR window under IMP 60 0 90 100 500 264 Mr 125 2 om Frequercy Hz FR Magnitude dB re 20 2 63V smoothed 1 24 cet The calibration factor 1s determined from the wa 5 remaining difference The example left shows a 4 difference of approximately 36dB aah Peter codon MN sos om rm Cave E Use Scale Level once again to reduce the trace by a further 36dB as shown here FR Magnitude dB re 20uP an 83V 120 tsmonthad 1 24 oct The simulated and measured traces are now superimposed From the second correction step the correction factor for the microphone and its preamplifier is Gain 10 63 0957 see also Section 3 1 1100 100 0 1 issu oum Lift channel diff dB 0 05116 36 93 VENE 0 04808 Power amplifier gain 3 148 Finally enter this value in the Sensitivity field of the Audio Device Settings dialog Remember that any change in the measurement chain e g a change in the gain n Left Ch Sensitivity rmVv Pa 63 0957 setting of the microphone preamplifier will result a change in sensitivity that will require correction Figure 5 2 5 Microphone cali
88. bration using nearfield measurement 48 5 2 3 Tweeter method This following calibration method relies heavily on the reliability of manufacturer data You will need a tweeter and its data sheet Use only products from reputable manufacturers for this method as unreliable data will yield misleading results that are of no use Daten Impedanz 8 Ohm Belastbarkeit Nenn Musik 60 150 Watt Resonanzirequenz fs 900 2 MEUM pe Gleichstromwiderstand Re 4 8 Ohm Luftspalth he 2 0 mm LO t oM PB o o e o 4 Schwingspule D Lange 26 mm 1 5 ium Kraftfaktor F T 3 80 N A Scliwaingspuleninduktivit t 0 05 mH a ee ae Effektive Membranflache 7 60 qcm Bewegte Masse mcl Luftlast 0 29 g Figure 5 2 6 Data sheet for a known tweeter The procedure is as follows 1 Mount the tweeter in a small baffle and measure the impulse response at a distance of approximately 20 40cm Impulse response 2 92 4 73 54 8 37 10 19 Ims Cursor 3 673 uw 6 250 ms 300 Gate 0 817 ms 31 53 Figure 5 2 7 Gated impulse response of the tweeter 2 Correct the measurement level to 1 metre measuring distance To do this we need the actual distance of measurement which can be estimated in two ways Setthe gate move the cursor yellow c 4 line until it registers 300 samples Then M deua pir set the gate marker red line on the first File Edit
89. cles in the measurement area The distance from the source speaker to the next obstacle should be at least five times the measurement distance This ensures that the level of any reflection is reduced by at least 20dB so that less than 1dB contributes to the overall sound pressure Ground plane measurements eliminate floor reflections and if far enough away from other reflecting surfaces will yield a true anechoic response raised by 6dB The speaker should be on the ground and be tilted so that the loudspeaker axis points directly at the measuring microphone The microphone must be located directly on the floor Figure 6 3 6 The angle a is calculated as follows arctan H d H Floor diaphragm center distance d Microphone loudspeaker distance The measuring distance should be in the far field As long as far field conditions are met placing the driver at 2 metres for measurement provides the benefit of yielding a true 1 metre sensitivity due to the natural 6dB boost with this measurement GPM can be used to measure the vertical axis as well by simply laying the speaker on its side and taking measurements at different angles at exactly the same distance from the driver in a semi circle GPM makes the speaker appear to have a baffle twice as wide as it actually does with a different shape because two sources are mirrored along the measurement axis and it is therefore effective for resonance measurements but not for diffraction effects
90. d Problems caused by room size should be dealt with by manipulating the measurement window and increasing data smoothing When exporting the data for simulation the cursor position adopted for phase and frequency response measurements must remain the same for all drivers 115 The cursor should be positioned a few samples before the rising impulse of the driver with the shortest propagation time usually the tweeter T The exact position of the cursor is not crucial although a point near the start of the impulse is preferred to minimise phase wrapping The most 410 426 443 458 475 ms important consideration is that the position KOEK O c Dro a should be the same for both drivers equa cursor poston Figure 6 6 8 Impulse responses tweeter black woofer red Data for the woofer W and tweeter are exported as FRD files No further information is required as we are working with actual measured phase with both drivers in the same reference position relative to the microphone Additional data dealing with relative driver positioning are therefore unnecessary Exceptions to this rule include simulation programs that simulate an infinite measurement distance e g BoxSim In such cases driver positioning data may be entered optionally with no effect on the simulation Note however that acoustic source path length information must not be entered as this will interfere with the simulation results Validation Although thi
91. dance measurements Power amplifier Loudspeaker left in soundcard 1 DO ohm soundcard Loudspeaker Testing and calibration Right out soundcard Right input Lag Soundcard protection NNV R1 Power Amplifier Output Zener Sound Card Input Impedance measurement with a power amplifier See also ARTA measurement box in Chapter 3 Headphone impedance measurement soundcard output Note that headphone outputs on soundcards are not usually designed for connection to low impedance loads Soundcard loopback setup Each output is connected to the corresponding line input This is used for soundcard testing see also Chapter 4 Voltage probe with soundcard input channel overload protection The probe shown provides 20dB attenuation when 8k2 and R2 910 assuming that the soundcard s input impedance is 10kQ Note that this protection circuit is built into the ARTA measurement box Chapter 3 12 2 Quick setup with Understandably you may want to start using ARTA straight away This section is therefore provided to address various issues related to the setting up of the measurement system single channel frequency response measurement and impedance measurement Further and more detailed explanations can be found in the respective chapters dedicated to these subjects Mixer adjustment The most common mistake made during a quick set
92. ds described in these handbooks are suitable for DIY use only We realise that most DIY speaker designers do not have access to professional measuring equipment and facilities The methods described here if followed correctly should therefore give good and reliable results more than sufficient for the home builder 1 First steps with ARTA 1 1 Setup To use the ARTA suite of programs you will need e Operating system Windows 98 ME 2000 XP VISTA Windows 7 Windows 8 e Processor Pentium 400MHz or higher memory 128k e Soundcard full duplex Installation is very simple Copy the files to a directory and unzip them That s it All registry entries are automatically saved at first start up 1 2 Equipment The following is a brief summary of the equipment required accompanied by some basic directions and cross referenced to more detailed information elsewhere OP preamp cee ta Ww USB soundcard NS There are three types of soundcard Soundcard e Standard onboard soundcard found typically on a computer motherboard e Plug in cards for PCI or ISA bus e Soundcards connected via USB or firewire Cards vary according to type of use quality and connectivity For standard connections and cables see section 1 3 Standard soundcards use a stereo cable and 3 5mm jack sockets Figure 1 1 Semi pro high quality soundcards usually have RCA jacks and
93. e Figure 6 2 27 shows phase without this repeated flipping process in ARTA this is referred to as Unwrapped Phase and in this case phase runs continuously Under these conditions pure running phase should yield a straight line on a linear frequency axis Both types of phase representation are equivalent and whichever is used depends on the application So for example the differences between minimum phase green and adjusted real phase red are illustrated more clearly in Figure 6 2 29 than in Figure 6 2 28 Excess phase averaged in 1 24 oct 20 50 100 200 500 1k 2k Sk 10k 20k 20 50 100 200 500 1k 2 Sk 10k 20k Cursor 20 1 Hz 13 6 deg Frequency Hz Cursor 69 4 Hz 314 3 deg Frequency Hz Current file Dirac _Butteryy 3 APSO0 pir em 2012 05 01 13 44 34 Current Dirac _Butteryy 3 APSO0 pir e 2012 05 01 13 40 56 Overlay files Min Excess Reale Overlay files Real Excess om Figure 6 2 28 Phase wrapped Figure 6 2 29 Phase unwrapped Group delay GD is defined as do do The View menu offers the options Group Delay and Excess Group Delay Excess group delay refers to the theoretical pure duration of sound without the contribution of the speaker Figure 6 2 30 shows the group delay red and excess group delay grey for our virtual tweeter A Excess group delay rms averaged in 1124 act Cursor 3019 4 Hz 0 338 ms Frequency Hz Current file Dirac Buttervw 2
94. e measured impulse response should look something like Figure 7 1 2 154 Impulse response mV V Cursor 728 726 1364 956 ms 55518 Figure 7 1 2 Impulse response IR The sections in red show the gates for the linear IR and the IR induced distortion for the second third and fourth harmonics 4 Place the cursor 250 samples before the peak and 5 hit Shift F12 on the keyboard ARTA automatically generates a frequency and distortion trace Figure 7 1 3 E Frequency Response and Distortions Untitled128 pir Edit View Smoothing Top FR Magnitude dB V V smoothed 1 6 oct 4 Fit eL LLL ILLI Ree aS ACI ks EA TE 126 TT 1 il 50 3100 200 S00 2k 5k 10k 20k Low Fr Freg 1374 6 Hz Magn 6 85 dB Frequency Hz 4 D2 35 83 dB D3 45 57 dB 4 84 81 dB Eas pps Figure 7 1 3 Frequency Response and Distortion window The top curve shows the frequency response and the lower traces are the second third and fourth harmonic distortion curves 155 Manipulation of the graph is comparable to that of in the Smoothed Frequency Response window The complete setup menu is obtained via View Setup or by right clicking on the graph This opens the dialogue box Magnitude Distortion Graph setup as shown in Figure 7 1 4 Magnitude
95. e applications priority Restore Defaults Figure 4 7 Setting the native bit resolution and sample rate in Vista Note that many drivers are unstable in Windows 7 in which case an ASIO driver should be used if available 4 1 3 ASIO driver setup ASIO drivers are decoupled from the operating system They have their own control panel for native resolution and memory buffer size adjustment The buffer is used for the transfer of sampled data from the driver to the user program The ASIO control panel is opened by clicking Control Panel in the ARTA Audio Device Setup dialog Figure 4 8 Audio Devices Setup x Sound card Soundcard driver sto i3 dx Input channels 112 Control Panel Output channels 112 Playback WOM Format 16bit 24bit 32 bit Float Figure 4 8 ASIO audio devices setup 30 Asio Latency Preferences 17 6 ms OK Cancel Buffer 1536 Samples Buffer Size 20 ms 32 bit Bit Depth Per Application Preferences CANCEL Figure 4 9 Adjusting resolution and buffer size in ASIO For music applications the buffer size 15 usually set as small as possible while retaining stability in order to yield the lowest input output latency system introduced delay In ARTA latency is not the main problem because it is encountered in software anyway but the choice of buffer with size exceeding 2048 samples or smaller than 256 samples is
96. e can be seen wrapping as a result of inclusion of the 0 0 time elapsing between the propagation of sound from the XCESS PNAS eer AT des tweeter and its arrival at the microphone The distance between the cursor and marker 34 33cm corresponds to a Sound it E time of 0 998msec and is referred to as the gate Figure 10 0 Setup 6 2 15 left Time Bandwith Requirement _ FR Magntuce dB Vy smoothed 1 24 Phase Impulse response mV V Zoom 4 1 A 501 33 R an A 76 376 00 250 56 400 125 33 000 20 42633 250 58 0 998 ms xx 376 00 EL 501 33 400 4 ___ _ 5 1001 100 200 500 X 560 5 94 630 6 54 7 01 ms Cursor 101 2 Mz 53 81 dB 137 8 deg Frequency OR Cursor 0 000 nV 5 80Sens 256 Gate 0 998ms 44 Current Diac Putenw 3 pir 2012 05 00 223240 File Butterw 3 HPB00 pir 2012 05 03 2243 02 Figure 6 2 15 Phase response including flight time from source cursor to microphone wrapped phase We can move the cursor towards the peak of the impulse response in the Impulse Response window or use Delay for Phase Estimation under the Edit menu to reduce or eliminate pre delay This will results 1n phase responses similar to those shown in Figures 6 2 16 to 6 2 19 79 FR Magnitude dB iemaathed 1724 oct Phase i FR Magnitude dB iemaathed 1 24 oct Phase 7 0 0 10 0 20 0
97. e data will have only very limited utility for simulations involving other distances and heights Option B is more flexible in this respect For simulation the acoustic path difference in addition to frequency and phase response data is needed The relative acoustic path difference should be determined using parallel measurements and averaging as this gives results with the least error The FRD files for export should be as free as possible from running phase which leads to excessive phase wrapping so use either minimum phase or match the cursor position to determine the difference between acoustic centres 121 Despite great care being taken during measurements and data processing characteristics of the loudspeaker size driver separation location on baffle driver type etc and the distance used for measurement can still be associated with errors to a greater or lesser degree Before running a simulation it is always worth validating measurement data against the conditions to be simulated 6 7 Scaling and splicing of near and farfield measurements For further simulation analysis full response information amplitude and phase 15 required Near and farfield measurements have to be combined to achieve this The process is illustrated in the following two examples 1 a 2L closed box with a Visaton FRS8 full range driver 11 an 8L bass reflex enclosure with a 5 driver 6 7 1 Closed box Take a nearfield impulse response m
98. e from 10Hz to 20kHz and power output of 6 10W is sufficient For distortion and power compression measurements an output of 100W into 8 Ohms is acceptable For measurements with ARTA we use the test setup in Figure 5 4 1 This ensures that the input channel of the sound card is not overloaded and is protected from excessive voltages by diodes 58 20 1 Rx R2 Rx R1I ZintR1 Example Zin input impedance of soundcard 10 kOhm RH 10dB 510 1047 20dB 510 30dB 510 15k Figure 5 4 1 Voltage divider for amplifier measurement As an example the Thomann t amp PM40C was selected The manufacturer s specifications are as follows Technical Specifications Output Power into 8 Ohms 36W rms into 4 Ohms 50W rms Frequency Response 10Hz 20 Khz 1dB Voltage Gain 26 dB Input Impedance active balanced 20 kOhm THD N 0 03 Slew Rate 19 V us Signal to Noise Ratio 92 dB Power Consumption 75 VA Dimensions WxHxD 155 x 166 x 55 5 Weight 1 8 kg Figure 5 4 2 shows the harmonic distortion of the t amp into 4 1 Ohms black and 8 2 Ohms red in response to output voltage The t amp delivers about 34W into 4 Ohms and 23 2W into 8 Ohms without distortion 10V RMS into 4 1 Ohms 24W 15 safe for measurement purposes 59 981 fee ae ohm 1 ME _ Lama J N L 01 H P 0 01 n DR
99. e from 12cm As discussed earlier a measurement distance 0 11 x dimension of the sound source should yield an error of IdB The largest dimension of the speaker in the above example FRS 8 in a 2 0L closed box is approximately 26cm Thus as 0 11 x 26 2 86cm we should be able to keep the margin of error below 1dB with measuring distances below about 3cm What about high frequencies Figure 6 4 12 shows windowed frequency responses at various measurement distances 97 Fr response magnitude dB fsmoothed 1 3 act xem BN TEES EDS L E NEL 24 CLER DEP LE dL ELI Cursor 161 1 Hz 8 45 dB Frequency Hz Figure 6 4 12 Farfield to nearfield transitions We see that the traces are no longer truly parallel and that the expected 6dB increase in SPL per halving of distance starts to be lost from the 24cm to 12cm transition This shows the measurement gradually moving into the nearfield see also Klippel AN 4 12 What happens if we increase the measurement distance still further Measurements were taken in a gym with dimensions 27m x 15m x 5 5m at a measurement height of 2 80m and measuring distances from 1 35m to 3 79m The reverberation time was also determined Figure 6 4 13 shows the results the mean reverberation time was approximately 3 seconds 4 00 2 5 RT60 3 50 1H 3 00 2 0
100. e is positioned on the tweeter axis left panel rather than mid way between the two drivers right If the measurement and simulation coordinates are identical option A works perfectly for crossover development However a sufficiently large measurement distance should be used if the room used creates reflection problems it is nevertheless better to use as wide a window as possible while attempting to iron out irregularities with additional smoothing Note however that applicability of any results obtained in this way to other measuring distances or microphone listening levels is very limited as even small changes in simulation coordinates can lead to significant deviations from reality Option B This involves the use of pure phase data for each driver with no information relating to the propagation time from each source Acoustic source information must therefore be entered separately Acoustic source determination with ARTA The most basic speaker design approaches assume that each acoustic source is in the same plane This unfortunately is rarely the case There is currently no universally accepted method to address this problem what we are therefore looking for is a workable solution that allows us to simulate speakers with reasonable accuracy As a rule the acoustic source is determined directly or indirectly from the measured impulse response However there are also some particular procedures that rely on a combination
101. e pressure chamber is used The figure illustrates how the reference and measurement curves can be used for calibration FR magnitude dB re 2UuPal os 150 0 145 0 EI DITE 140 0 135 0 130 0 125 0 120 0 115 0 10 Cursor 1015 8 123 22 dB Figure 5 3 6 Reference red and MB550 black frequency responses The microphones will usually have differing sensitivities and an initial level adjustment will therefore be required The easiest way to do this is to choose a reference frequency and use the cursor to read off the respective sensitivities The difference is then compensated for by using the Scale function If the measurement is made using ARTA the correction can be made via Edit Subtract Overlay as described above If STEPS 15 used for its superior reproducibility some manipulation of data in Excel and a suitable simulation program e g CALSOD will be needed Figure 5 3 7 shows the trace obtained with STEPS for the MB550 microphone from 5 to 500Hz Using this compensation curve together with the results from the previous section see Figure 5 3 4 you can obtain a compensation file for the full frequency range of about 5Hz to 20kHz as shown in Figure 5 3 1 56 550 Abweichung von der Referenz dB Referenzmikrofon Mikrotech Gefell MK 221 1 10 100 1000 Frequenz in Hz Figure 5 3 7 MB550 deviations from reference frequency response Results with other microphones a
102. e response How do we repeat the measurement What does this tell us Each measurement should be properly planned and documented The aim and purpose should be defined it should be clear what the main parameters are and any special conditions should be stored and documented provides us with the tools to apply this level of documentation and traceability of measurements but they can only help us if we remember to use them Try to retain save each measurement in its original format PIR LIM HSW because this will be the source data upon which other evaluations are based If you evaluate the data during the measurement session copy the results immediately into a suitable word processing file and add any comments straight away 8 1 Graphical representations in ARTA Although ARTA does not have a direct print function there are a number of other ways to output graphics 8 1 1 Outputting and formatting charts One of the easiest ways to output a chart is via a screen dump Print Screen This can be copied into Word Powerpoint etc Smoothed frequency response Speaker 1 pir File Overlay Edit View Smoothing 2n FR Madnitude dB Viv tsmoothed 1 6 act Smoothing 146 High Fr 30 0 Low Fr 20 50 100 al rl Cursor 20 1 Hz 11 59 dB Frequency Hz Mag Eh Gd Record Overlay Figure 8 1 1 Screen dump of frequency response window Alternatively to get just
103. e tolerated as deviations from the on axis reference only start at around 10kHz 1 5 to 2 octaves above the normal transition frequencies Figure 6 6 4 estimates the error attached to measurements taken at various distances and angles 112 4 00 9 N 2 00 1 50 1 00 Microphone distance D m 0 50 0 00 Figure 6 6 4 Measuring distance as a function of angle and microphone position Example What should the minimum measuring distance D be if a is not to be greater than 10 when d 21cm between the two driver centres If we look at the intersect corresponding to d 0 21m in the 10 line the minimum working distance D can be seen to be approximately 1 18 We can see therefore that increasing measuring distance minimizes angular errors This however conflicts with the requirements for freefield measurements Delay As well as angles and distances time can also affect measurements Microphone distance LS Dyr y hic hyr D Drr y hrr D Path difference AD Drr Dyr Delay At AD 344 m s Figure 6 6 5 Phase shift caused by delay The following table shows in red measurement conditions corresponding to the examples from Figures 6 6 3a and 6 6 3b At 60cm there is a path difference AD of 1 85cm which corresponds to a time difference At of 0 054msec At a measuring distance of 150cm the path difference is reduced to 0 75cm and the time differ
104. easurement Impulse response i v 0 00 2 12 427 6 40 9 54 Is Cursor 3 835 mV 6 146 ms 295 Figure 6 7 1 Nearfield impulse response Place the cursor yellow line at the beginning of the impulse response to obtain the correct phase relationship Take care however as placement of the cursor too close to the impulse peak will result in loss of information It is better to place the cursor further back and later apply delay correction if necessary With the cursor left mouse button about I msec before the first pulse place the marker right mouse button precisely on the pulse maximum and click Get next to Delay for phase estimation on the top menu bar Delay for phase estimation ms 0 688 Zero Smoothing Overlay FR Magnitude To get the frequency response click on Under View tick Magn Phase Phase Group delay Minimum phase Unwarp Phase 122 The frequency response and phase will be shown in the chart thus generated A driver of diameter approximately 6 4cm will have a usable nearfield response up to about 900Hz Figure 6 7 2 FR Magnitude dB smoothed 1 6 act Phase 50 Sk 10k Cursor 995 0 Hz 14 26 dB 7 5 deg Frequency Hz Figure 6 7 2 Nearfield frequency response Usable range is marked by the cursor Correct the nearfield frequency response to the farfield measurement distance ARTA provides two options for doing this 1 In the Smoothed frequency response c
105. ecify the number of measurements to be made by ARTA after which the mean of the measurements is calculated automatically Doubling the number of measurements increases the S N ratio by 1 Vn 3dB although other factors such as jitter limit the extent to which this can be achieved The effectiveness of averaging is illustrated in Figure 6 1 4 where measurement results with 2 to 32 averages are shown for noise floor levels 69 6 2 ARTA excitation signals ARTA has a wide range of integral excitation signals and can also be used with external excitation as follows Impulse response measurement Signal recording Impulse response Periodic noise PN pink white speech Periodic Noise sweep MLS External excitation Sweep sine linear log MLS External excitation Trigger Ua Fez Fri Spa FR2 SPA Random pink white PN pink Fs Hz 48000 Periodic white pink speech External white noise J y ep jnitude db The difference between periodic and pink random noise is explained in Figure 6 2 1 speech Untitled Arta Signal Time Record available from File Overlay Edit View Record Analysis Setup Tools Mo release 1 6 2 Signal generator with wide range of F Ine F Impulse response Time record ona 5 E END D BRENNEN continuous signals sine square multi eet iz tone etc and pulse and burst signals The selection o
106. ection 3 1 specifications for the 102 microphone preamplifier Manufacturer and model Thomann t bone MIMI Superlux ECM999 Behringer ECM 8000 Monacor ECM 40 DBX RTA M Beverdynanuc MMi SennheiserKE4 211 2 10 125 Panasonic WM 61A 6 mo Sensitivity Maximum Max SPL Dynamic mV Pa SPL dB dB 3 range THD dB L 8 L 3 9 I 84 90 119 00 123 96 154 00 Nd NENNEN 35 00 39 00 49 00 Earthworks M30 8 150 142 18 639006 NTI M2210 20 145 O 120 1 098 00 Microtech MK221 amp MV203 146 15355006 112 295 00 459 00 More information on measurement microphones can be found in Section 1 2 and in the STEPS Handbook 44 Figure 5 2 1 Measurement microphones from left to right Haun MB550 t Bone MMI NTI M2210 Audix 1 5 2 2 Nearfield method If no calibrator is available and the sensitivity of the microphone preamplifier is unknown the following method can be used to provide an approximate level calibration After Thiele Small parameters of a low or midrange driver have been calculated and VAS determined by installing the driver in a sealed enclosure of known volume the resulting data can be used in a simulation program to calculate the half space frequency response 45 i WMagnituderohmes Impedance Phase 1 78 27 Hz 6 22 ohms dc 653 87 uH 220 7
107. ee below 83 Dodil pir Arta File Overlay Edit View Record Analysis Setup Tools Mode Help F Twp Fr Audio devices Calibrate devices FFT 64 Window Uniform Del gt gt gt FR compensation Analvsis parameters Impulse response Use 64 bit FFT CSV Format T d Figure 6 5 6 shows the statistical analysis of three measurement positions with Excel The red bars show the standard deviation spread of the measurements 0 6 0 5 0 4 0 3 RT60 sec 0 2 0 1 E 125 250 500 1000 2000 4000 8000 Frequency Hz 0 0 Figure 6 5 6 Statistical analysis of individual results 6 5 1 Automatic evaluation of reverberation time As of Version 1 5 ARTA provides for automatic evaluation of room acoustic parameters according to ISO 3382 This facility is in the Acoustical Energy Decay menu under Automatic ISO 3382 Evaluation Five options are available as shown below 105 Acoustical Energy Decay NHI pir Edit Automatic 15033892 evaluation Graphical presentation For 1 1 ackave bands Table presentation For 111 octave bands Graphical presentation Far 113 ackave bands Table presentation For 113 octave bands Setup Select the desired menu item to carry out the evaluation The 1 1 octave graphic should resemble the trace in Figure 6 5 7 Room Acoustical Parameters NHI pir Edit Ove
108. ell under 0 2 sec would be needed to give a freefield measurement at a distance of 1m How does this information help us It allows us to estimate the kinds of measurements we can make and their likely quality in a given room Reference plane baffle Nearfield microphone position Farfield microphone mm am adba am am RT 0 01 0 1 ee Acoustic RT 0 2 centre TT 0 4 RT 0 8 RT 1 6 3 2 6 4 Nearfield meas Hallrad e Position the microphone as close as possible to the centre of the speaker cone e Measuring distance lt 0 11m x source dimensions error lt 1dB e Upper frequency limits for nearfield measurements are illustrated in Figure 6 4 7 Bear in mind when making nearfield measurements that care must be taken not to drive the microphone into distortion and that there is an upper frequency limit to this type of measurement dependent on the size of the source but generally up to approximately 300Hz Figure 6 4 7 10000 gt 1000 Q D 100 10 1 10 100 Largest dimension of source cm Figure 6 4 7 Upper frequency limits for nearfield measurements 94 90 0 45 0 40 0 35 0 30 0 25 0 20 0 15 0 10 0 5 0 0 0 Level correction dB 0 1 1 0 10 0 100 0 Measurement distance cm Figure 6 4 8 Estimat
109. emissionen elektroakustischer Beschallungstechnik The measurement of sound levels and the equipment required for this purpose is defined in IEC 61672 1 2002 As of version 1 4 a virtual SPL meter has been included with ARTA Figure 7 2 1 Microphone input input amplifier clipping indicator A C Z Lin C filter Weighting filter Squaring Linear integration Exponential integration Peak detector fast slow impulse Square root Logarithm dB Figure 7 2 1 Block diagram of the SPL meter The microphone signal enters through the input amplifier via the clipping indicator that indicates the status of the input amplifier and the A D converter of the sound card From there the signal goes to the weighting filter A C or Z see IEC 61627 1 or Figure 7 2 2 where Zis unweighted or linear Weighting is used for RMS level measurement whereas the C filter is used for peak level measurements At the next stage the signal is squared and then goes to the integrator or the peak detector The square root and logarithm of the signal are then taken and the final display shown as the sound level in dB 158 Attenuation dB e 40 A 50 60 7 70 20 80 315 1250 5000 20000 Frequency Hz Figure 7 2 2 Weighting filters A 7 The sound level meter in ARTA 15 activated by Tools meter This will open a window as shown in Figure 7 2 3 159 SPL meter noname2 spl X
110. ence becomes 0 022ms 113 2000 m 4 000 0016 ms OWE ms ms The time difference 15 equivalent to a delay which corresponds to a phase shift that increases with frequency dPhi Delay m Frequency Hz Speed of sound m s 360 So for the commonly used transition frequency of 3000Hz a path difference of 1 847cm will correspond to a phase shift of dPhi 0 01847 m 3000 Hz 344 m s 360 relative to the tweeter The simulation in Figure 6 6 6 gives an idea of the effect that this 1 847cm path difference can have under the conditions stated on an idealized loudspeaker with a Linkwitz Riley second order crossover The acoustic source Figure 6 6 6 Impact of time difference left 2 without right 2 with delay ji i B 3 i i i i LI 1 1 i LI 1 i i il L LI zi te ha thes 130 So far we have assumed that the acoustic source is a simple driver mounted on a baffle Unfortunately reality is a little more complicated than this After excitation by a signal the speaker cone driven by the voice coil produces sound This membrane deflection does not correspond purely to piston like radiation however as distortion and resonances are present in the membrane The processes that take place between the voice coil and the individual sections of the membrane and that lead to the propagation of the sound take a certain amount of time to elapse the duration of this
111. ency response from measured or imported individual frequency responses 133 G 96cm pir Arta Edit View Record Analysis Setup Tools Mode Help e malit ex uem ues et em n Open gt UU metra Ctrl 5 ay for phase estimation ms 0 000 Get Zero Save Info Marker Export 58 Dal _ Import Offset tf S 1 96cm pir Gain 2 NF pir 3 00 1 221 v Zoom 2 an Scroll 4 gt 4 C icalsod MB5S50 4 CAL pir Exit Cursor 3 412m 8 979 ms 431 Gate 4 167 ms 200 Loads pir and sum with existing L 28 2dB R 19 6dB Impulse Response Figure 6 8 2 The ARTA File menu There are two options e Export ASCII data and combine frequency responses in a simulation program e Sum directly using Load and Sum in ARTA and Sum loads a previously saved PIR file to the current signal record This means that it is possible to combine ARTA data in the time domain The details are exactly as described in the ARTA manual e First measure or load the PIR file e g tweeter e Load a previously saved PIR file with Load and Sum e g woofer Analyse overall response using The result will be the sum of the individual frequency responses Figure 6 8 3 134 FR Magnitude dB V smoothed 1 24 oct 0 15 20 0 25 0 30 0 35 0 5
112. ent Setup Measurement contig Reference channel Right Reference Resistor Before measuring adjust the output level in the Generator Setup menu so that the input channels are not clipping Generator Setup Generator Y Sinefreq Hz 1000 Output level 12dB Pink cut off 20 Input level monitor 70 20 Calibrate the system using the Calibrate Input Channels menu Connect the output of the signal generator Line Out to the left and right channel inputs of the soundcard perform the calibration Calibrate and exit by clicking on Calibrate Input Channels Generate Calibrate Status Seq length 788 Connect left and right input Mot calibrated channel to signal generator Sampling rate output Humber of averages Output volume dB 12dB d 1 Generate Calibrate Uncalibrake 70 Input Level Monitor Cancel You can read more about impedance measurement and Thiele Small parameters in the LIMP Handbook 16 3 The ARTA measurement box The ARTA measurement box is recommended to simplify measurements with ARTA STEPS and LIMP It is designed for impedance and two channel frequency response measurements eliminates the need for cumbersome test leads See Figures 3 1 to 3 3 and ARTA Application Note and the ARTA Hardware amp Tools Manual 2 Note that use of the measurement box with the optiona
113. erworth Fc 900 2 Bessel V order Crossover frequencies Butterworth VI order 14 Bessel VI order 15 Linkwitz VI order High4Low pass Lower 900 pz FR Magnitude dB re 2 uParz 83v smoother 105 0 Upper 30 mg 100 0 SS ae ox 95 0 07 0 mE 90 0 ee eh 85 0 80 0 E es 75 0 gt EEE es 70 0 eeN PEE P4 55 0 100 Cursor 257 1 Hz 75 02 dB Figure 5 2 8 Measured frequency response and target 4 Calculate the correction factor From the frequency response we can set the cursor to frequencies that are at least one octave above resonance and read off the corresponding level values 50 Adjusted amplification 4 2121 3 5237 3 544 3 5810 3 7627 Thus the average correction value is 3 7247 standard deviation 0 2884 Note that this method is influenced by the effect of the baffle on which the tweeter is mounted Inclusion of baffle effects can be simulated with software such as The Edge see Figure 5 2 9 10 0 LLLI psu 0 0 c 5 0 10 0 Figure 5 2 9 Influence of a simulated red trace 25cm x 25cm baffle at 30cm Ideally you should use a baffle that has a minimal effect on the frequency range used
114. ese issues freefield measurement measurement in an anechoic chamber ground plane measurement half space and windowed measurements and nearfield measurement Freefield measurements 84 As suggested by its name the first and oldest method is to take measurements out in the open speaker and microphone are situated so that there is no interference from any reflecting surfaces most notably the ground To achieve this a crane or tower is required and the equipment must be lifted into the measurement position as shown in Figure 6 3 3 9 Sound reflected from the ground takes 2H d 344 seconds to reach the microphone The left panel shows the effect of ground reflection at heights of 1 2 4 and 10 metres Clearly where a reverberant floor worst case is present the height of the tower should be around 10 metres for reasonably trouble free measurements 80 1 Rot Direkts chall 4 Blau Bodenreflexion T2 io af ul La w MIC 2 754 T10 tata i j t tn p E 5 e a 70 H 65 10 100 1000 Frequenz Hz Figure 6 3 3 Freefield measurement simulation ground clearance left and measurement setup 9 right Direktschall direct sound Bodenreflexion ground reflection A freefield setup has the advantage of creating theoretically ideal measurement conditions but is of course weather dependent Precipitation
115. evels the voltage divider must be adjusted accordingly For example if your amplifier has a rated output of 56W at 8 Ohms and you want to take advantage of its full power you must make the following modifications to the ARTA measurement box k S Volt RMS VPMA X ZSpeaker 1V V56W 8 Ohms 0 0472 When R2 910 and Zy 10K is calculated as R21 Zm k R21 Zm 834 1 0 0472 834 1 16837 Ohms Note the sensitivity of the soundcard is specified in the calibration menu under mVPEAK The adjustment calculation for the measurement box requires that 0 707 19 Matching the measurement box the microphone preamplifier Line in left For the calculation of the gain of the left channel input Ext left preamp gain Figure 3 4 you will need the details of your mic preamp In the example shown here values for the Monacor 102 MPA are shown Figure 3 5 VicPreAmp 10 20dB see below output impedance of the mic preamp Zour 100 R5 719 Zin 10000 Left channel V MicPre Amp Zin Zout 5 Zin 10 10000 10819 9 243 The value of R5 is calculated as R5 R1 R2 Zour 819 100 719 This relationship results from both input channels having the same source impedance Audio Devices Setup Sound card Input Device Crystal WOM Audio Output Device Crustal WOM Audio wW aveFarmat f 1B bit 24 bit 32 bit Extensible 120 Amplifier Interface Lineln Sensitivit
116. f the best excitation signal depends on the quality of the sound card and the measurement environment Dual channel measurements should be used with high quality sound cards with flat frequency response and tight tolerances for the sensitivities of the input channels Single channel measurements can be used with lower quality sound cards see also Chapter 4 Ivo Mateljan suggests the following guidelines for selecting appropriate excitation signals e Periodic noise PN gives the best results in environments with high ambient noise levels Averaging improves the S N ratio and the effects of random and stationary noise and nonlinear distortion are minimised e In quiet surroundings a high crest factor makes the sine sweep ideal for high power speaker testing In this case averaging does not always improve the S N ratio and it is therefore better to increase the duration of the sweep Periodic noise generator For periodic noise pink ARTA can protect the DUT against low frequency high energy signal 5 length eek components by including a high pass filter Pink Sampling rate Hz 48000 cutoff Time constant ms Noise spectrum Pink The effect of the Pink cutoff is shown in Figure 6 2 1 By increasing the cutoff frequency we cut Output volume 0 the lowest frequencies off at progressively lower levels ARTA automatically compensates for the Pink cutoff Hz 50 frequency level cap Generate
117. f the curve to the right of the cursor as an overlay Load Overlays loads an overlay file Save Overlays saves a file as an overlay Manage Overlays enables FR Overlay Manager for editing Delete last deletes the last overlay Delete all deletes all overlays Generate target response generate targets for standard crossovers Load target response loads txt format files as targets Delete target response deletes standard crossover target Load impedance overlay loads impedance files txt zma imp for simultaneous representation of frequency and Impedance transitions Delete impedance overlay deletes the impedance overlay Further processing of overlays is possible in the Overlay Manager screen Figure 8 1 5 This is opened with the command Overlay Manage overlays 175 XO RLE 2 xia amp dB RLC amp dB elektr Color Cancel Add above crs Replace sel Add below crs Delete all Check al Ok Figure 8 1 5 FR Overlay Manager menu i Some commands Add Add above crs crs below Add Delete all are already familiar the rest are as follows e Replace sel replaces the selected curve with the current one Sel Delete deletes all selected overlays e Color Changes the colour of the highlighted overlay on the Overlay Colors menu Use the mouse to click on the overlays themselves as follows e Single click select the desired item e Sin
118. ft channel is used as the measurement channel The ARTA measurement box 1s designed to be suitable for most users If you need to adapt it for your own special requirements some calculations will be required This section shows you how to use the standard measurement box settings how to adjust them if necessary and how to calculate the Ext Preamp Gain values to be used in Audio Devices Setup Matching the measurement box to the power amplifier line in right The resistors R1 R2 form together with the input impedance ZIN of the soundcard a voltage divider k defined by k R2 ZN R2 ZN where R2 ZN 2 R2 The maximum voltage that the power amplifier can send to the line in right channel of the soundcard 15 Vmax S Volt RMS where 5 input sensitivity of the soundcard see also Section 5 1 The maximum power that can be used in the measurement is Pmax S Volt RMS k Zspeaker With the values chosen for the measurement box for R1 8k2 R2 910 and assumed standard values for Zi 10k and input sensitivity of the soundcard 1V we can calculate the gain in the right input channel Ext right preamp gain see Figure 3 4 Right channel R2IZg R1 R2lZgy 910110k 8k2 9 10110k 0 0923 when PMAX 29W or 14 5W for speakers with nominal impedance 4 or 8 Ohms respectively If your amplifier cannot deliver these power levels or you wish to take measurements using higher power l
119. gen mainmenu 35 verschiedenes mainmenu 70 199 tsp checken einfach gemacht ARTA Support Internet cited 2014 Sep 21 Available from http www artalabs hr support htm Khenkin A How Earthworks measures microphones Internet Earthworks Technical Articles cited 2014 Sep 21 Available from http www earthworksaudio com support technical articles Melon M Langrenne C Rousseau D Herzog P Comparison of Four Subwoofer Measurement Techniques J Audio Eng Soc 2007 55 1077 91 Mateljan I Ugrinovic The Comparison of Room Impulse Response Measuring Systems Proceedings of the First Congress of Alps Adria Acoustics Association and Third Congress of Slovenian Acoustical Society Portoro Slovenija 2003 Struck CJ SF Simulated Free Field Measurements J Audio Eng Soc 1994 42 467 82 Havelock DI Kuwano S Vorlander M editors Handbook of signal processing in acoustics New York NY Springer 2008 2 p D Appolito JA Testing loudspeakers Peterborough N H Audio Amateur Press Distribution agents Old Colony Sound Lab 1998 Sanfilipo M Subwoofer Measurement Tactics A Brief Topical Overview amp Method Comparison Audioholics Internet Audioholics Online A V Magazine 2007 cited 2014 Sep 21 Available from http www audioholics com loudspeaker design subwoofer measurements Messbedingungen bei Visaton Visaton Diskussionsforum Internet Visaton der Lautsprecherspezialist 2006 cited 2014 Se
120. gle click on the check box makes overlay visible or invisible e Double click enables editing of the overlay name Check all activates all the overlays Note that the space available below each chart is quite limited so keep file names as short as possible To assist with this you can select the corresponding item in the FR Overlay Manager and overwrite text Figure 8 1 6 100 200 400 ek 5 Cursor 20 0 Hz 55 18 dB Frequency Hz Current file D BdB elektrisch mit ALC 2 pires 2010 10 26 18 14 00 Crerlay files elektrisch mit RLE 2 pir HM HT elektrisch mit FLC pir x Gd elektrisch Mustertext Mustertext Mustertext Mushertest Mustertext Mustertest Mustertext Mustertext Mustertext Largest 1024 Voller Text 50 100 200 500 dk 2 Sk 10k 2 50 100 200 dk 5k 10k 20k Cursor 20 0 Hz 25 13 dB Frequency Hz Cursor 20 0 Hz 55 18 dB Frequency Hz Curen Te Mo ea elektrisch mit RLE 2011 10 28 181 Current file elekirisch mit RLE 2 pir 2010 10 26 18 1 Overly tiles XO Bde elektrisch mit HLC 2 pir Crerlay fies XO RLC 2 XO dA RL 608 eiektrisch mit RUC pr elektr nc erate d Iviistertext wiustertext Mustertext dustertext Mustertext Mustertext Smallest 400 Voller Text m Smallest 400 Reduzierter Text Figure 8 1 6 Adjusting a capti
121. hart click on Edit then Scale level 1 FR Magnitude dB re 20uPa V smoothed 1 3 oct Fit Range Set Smoothing ws vj 20 50 10k 20k Cursor 20 1 Hz 80 82 dB Frequency Hz Mag Ph Gd Record Overlay BW Copy Enter the dB level difference to align the traces 123 2 Apply a scale factor in the ARTA main menu click Edit then Scale amplitude The correction factor 2 1 is calculated as FF 20 log a 2d where a speaker radius d measurement Enter number or arithmetic expression to scale PIR distance Thus if a 3 18cm and d 48cm the correction 3 18 2 48 factor FF 29 6dB Apply the baffle step correction This 15 a special feature of ARTA that is described in more detail FN Magnitude Fre 2D The baffle step correction is found in the Edit menu in the smoothed frequency response Enter the shape of the enclosure and its dimensions and click OK A chart similar to Figure 6 7 3 will be generated Note that the curve is stored as an overlay 124 FR Magnitude dB vi smoothed 1724 act 10 0 20 0 30 0 40 0 50 0 60 0 0 0 20 50 100 200 500 2k 10k 20k Cursor 20 2 Hz 56 53 dB Frequency Hz VISA TON FRS 8 Mahteld mit Battle Step Korrektur Figure 6 7 3 Nearfield frequency response with baffle correction black Load or measure the farfield frequency response Ope
122. hat T60 15 not shown by the automatic evaluation Figure 6 5 10 Room Acoustical Parameters NH1 pir dB dE 050 5 Save ASCII Save cs Figure 6 5 10 Tabulated results 107 108 6 6 Setup for loudspeaker acoustic measurements Together with some background knowledge and technical ability the combination of ARTA measurements and listening tests is sufficient for the development of a loudspeaker The development process can be further assisted however by the use of simulation software e g BoxSim CALSOD which can help to reduce the need for testing time and materials Simulation results can be very close to reality but they are heavily dependent on the frequency response and impedance data required by the programs The following discussion gives some suggestions for use of simulation software Simulation programs We will examine two commonly used pieces of software BoxSim e CALSOD BoxSim allows for the free positioning of individual drivers on the baffle X amp Y axes as well as input of the acoustic source Z axis As the virtual microphone is positioned at an infinite distance the individual drivers cannot be misaligned Because of this configuration the speaker must be measured on axis and the FRD and ZMA files imported into BoxSim CALSOD is more flexible in this respect It allows for free positioning of the driver on the baffle as well as the microphone In this way any
123. he nearfield measurement of a speaker cone to determine SPL For this to be carried out effectively the measurement system must be set up so that the soundcard input can cope with levels as high as 130dB The following parameters are known Maximum input voltage of the soundcard Uy max 0 988V RMS see definition below microphone gain Gpgg 20dB 10 sensitivity Smc 11mV 944 at 1kHz At 130dB 36dB difference from 94dB the output voltage of the microphone is 10 63 1 11 694mV RMS This is amplified further by the microphone by a factor of 10 Gy Un max Uout sensor 0 9988 10 0 694 0 1439 16 84dB A voltage divider giving 16 17dB of attenuation is therefore required Sensor Zin R2 Zin R2 1 G Rx R1 Rx 2 R1 Rx G Rx 3 Soundcard Zin If the input impedance of the sound card Zm 10kOhm and the value of R2 1 kOhm R1 calculated by 1 and 3 is as follows Rx 10000 1000 10000 1000 909 09 Ohm R1 Rx G Rx 909 09 0 1439 909 09 5408 42 Ohm gt 5 6 kOhm And 909 09 56004 909 09 0 1397 17 01 Step by step management of the measurement system 15 described detail in the following sections 5 1 Soundcard calibration The soundcard and microphone calibration dialogue is found under Setup Calibrate devices The following shows the preset default values 40 Soundcard and Microphone Calibration X
124. her optimisation of the RLC network Figure 6 10 10 shows the electrical filter response black virtually superimposed on the 6dB target The amplitude peak red at the resonance frequency has been eliminated FR Magnitude dB re 200 2 23 smoothed 1 24 oct zZ ohm 20 50 100 200 500 1k 2k 10k 20k Cursor 20 0 Hz 55 69 dB Frequency Hz mit Figure 6 10 10 Amplitude response with 6dB XO optimised RLC network electrical Figures 6 10 11 acoustic and 6 10 12 electrical show the optimisation of the RLC network Trace Figures R ohm L mH C uF 6 10 11 amp 6 10 12 151 Blue 8 2 1 17 27 0 Green 8 2 1 17 17 0 Red 8 2 1 17 13 3 FR Magnitude dB 200 smoothed 1 24 act Z ahm s00 1k Zk 10k 20k Cursor 2524 5 Hz 4 99 dB Frequency Hz HT BdEB Gualitat dez RLC Gliedez Figure 6 10 11 Amplitude and impedance response of optimised RLC network acoustic FR Magnitude dB re 20uP alz 23 smoothed 1 24 act Zahm s00 1k Zk 10k 20k Cursor 4568 0 Hz 1 44 dB Frequency Hz BdEB Gualitat dez RLC Gliedes Figure 6 10 12 Amplitude and impedance response of optimised RLC network electrical This example shows how electrical crossover measurement can yield additional useful information It is therefore worth having the probe illustrated in Figure 6 10 1 in your measurement kit 152 153 7 Special measurements and examples 7 1
125. igure 6 2 7 Impulse response left step response middle and frequency response right of a 1000Hz low pass filter The first example Figure 6 2 7 shows 12dB low pass filter with a cutoff frequency of 1000Hz Note the changes in impulse and step response in comparison to Figure 6 2 6 impuha resp ngss mua Shep responsa i UMS Magndude dev empcihed dori Phase 183 135 208 oui AT 5 11 13 na 601 ido zw 0000 nOn Cu ODM hm 0 Ginter 20 1 Hz 100 9 deg Hz Fi BPH O0 1 000 9 EL HZ 108 12 24 181224 Fie EP100 1000 55RHT pir 2008 00 24 131314 Current O 1000 S6k pi 2108 12 23 1841430 Figure 6 2 8 Impulse response left step response middle and frequency response right of a band pass filter with 100Hz 1000Hz crossover frequencies The second example in Figure 6 2 8 shows a 12dB band pass with 100Hz and 1000Hz cutoffs See the changes in impulse and step response compared with the Dirac impulse Note also the different timescales To illustrate the effect of different cutoff frequencies on the appearance of the step response the next illustration shows 12dB low pass band pass and high pass filters Note particularly the band pass filter as all speakers exhibit this behaviour 75 Siren reece res vno Siep res vv Siren v yass 12d B da 51 Tid 1019 4 33
126. import a file with another data format File TIM MLSSA file ASCII MLSSA File Ext ASCII File Ext Load and Sum Sum impulse responses see 8 1 Edit view Smoothing Colors and grid style Br background color CErl B Use thick pen below cursar Cut above cursor Scale level LF box diffraction Subtract overlay Subtract From overlay Power average with overlays Merge overlay below cursor Merge overlay above cursor Delay For phase estimation Cut below above cursor deletes the part of the current response to the left right of the cursor Scale Level scales the frequency response to the desired level Scale Magnitude Enter value dB to scale magnitude 12 52 dB Cancel Subtract overlay Subtracts the overlay from the current frequency response Subtract from overlay Subtracts the current frequency response from the overlay Power average overlays Averages all existing overlays Merge overlay below above cursor Links the current overlay to the left or right of the cursor to the current trace The upper half of Figure 8 2 1 shows the Cut below above cursor operation In this case the left portion of the curve below cursor was cut The lower half shows the effect of Cut below cursor on the Time Bandwidth Requirement 180 n FR Magnitude dB re 20uP ac 62 smoothed 1 24 imm FR Magnitude dB Bv srmanglbed 11
127. increases with decreasing sampling frequency At SKHz and 16kHz the lowest modes can be readily characterised with respect to location and decay duration As of version 1 6 2 ARTA provides a downsampling function which allows for the production of PIR files of any resolution at reduced sampling rates for the analysis of low frequency room modes A sampling rate of 4kHz to 8kHz should deliver good results 164 185800 alleine richtig gepolt pir Arta File Overlay Edit View Record Analysis Setup Tool Cy A 1 Copy Chrl c FFT 32k Colors and grid style Use thick pen Set Marker Delete Marker Current sampling rate Hz 5000 nan Invert Mew sampling rate Hz 8000 Rotate at cursar Antialiasing Factor 0 5 0 957 0 9 Scale amplitude Resample to lower Frequency Cancel Scale acoustic model response aan Load the desired PIR file and then go to the impulse response view the Edit menu then select Resample to lower frequency Now you can use the new sampling rate and the anti aliasing factor cut off frequency of the anti aliasing filter see Figure 7 3 4a Factors in the range 0 5 to 0 95 give good results but the default value of 0 9 is recommended After downsampling the frequency response is cut Signal spectrum off above fsampling 2 4KHZ 2 see Figure 7 3 46 nd right middle panel attenuation 0 dB 3 Frequency
128. interval between excitation and sound production depends on the 114 dimensions of the membrane and the properties of the membrane materials It is not difficult to see that this process will be frequency and location dependent Furthermore it is also evident that unlike in the point source model a speaker diaphragm in the real world consists of sections that are not all at the same distance from the microphone One of the most common suggestions for solving this problem 15 to treat the voice coil as the acoustic source but this is inaccurate The question of how to treat the speaker as an acoustic source is extremely complex and has been investigated repeatedly in numerous publications ranging from software user manuals to scientific theses 7 02009677 2260 990940000977 099 280999 256 S hd 4499 7237 7 4 il __ saei 7777777 stg aet oT 5 _ A 47 I m mC 2 O N LL UC US EC m os OP ASEO Vows D Figure 6 6 7 Phase shift due to differing acoustic path lengths
129. ion of level correction Pegelkorrectur for nearfield measurements Messabstand measurement distance Figure 6 4 8 allows us to estimate whether a microphone is likely to be overdriven in the nearfield If the DUT has for example a specified output of 86dB W m and an effective membrane diameter of we can assume that the output will be approximately 86dB 32dB 118dB at a power of IW at a distance of This puts us within the SPL range of conventional electret measurement microphones Note I In Figure 6 4 6 The difference between the acoustic center and the reference plane baffle of the speaker is illustrated It is clear from this that the selected reference point and the actual sound origin do not coincide and this is evident from an analysis of impulse responses The difference between the two which may be several inches and can be measured with a ruler affects sound transit times The degree of resolution of this method is dependent on the sampling rate of the sound card 7 2mm 48kHz 3 58 9 96kHz Note 2 Use of the largest dimension of the source diagonal measurement across the speaker cabinet usually results in a measuring distance that is not feasible in a normal domestic situation As a compromise perform the calculation using three times the diameter of the largest driver or for high frequency measurements use at least six times the distance to the nearest edge of the cabinet Farfield measurement
130. ith the data then being exported Figure 6 9 11 105 43 81 ha 2 y _ A RR 20 5 100 200 ek Frequenz Hz Figure 6 9 11 Simulation of a Im TL in AJHorn 5 0 Nearfield measurements for the speaker membrane and the end of the TL were calculated using the combined volume flow method in Section 6 7 2 144 R 100 0 A 90 0 80 0 70 0 60 0 50 0 LE LLL Sees m 1 024 m Wk 0 40 0 cm Tr Pos 0 m o d 0 20 50 100 200 500 1k 2k Cursor 1802 5 Hz 85 14 dB Frequency Hz 1m TL Figure 6 9 12 Imported target function red and measurement yellow Figure 6 9 12 shows good agreement between the simulation and measurement Figure 6 9 13 shows the effect of damping of the end of the TL FR Magnitude dB 200 2 83 smoothed 1 24 oct R 100 0 1 90 0 ttt 80 0 l1 t 70 0 r71 1 F E 50 0 r7 T7 a 8 1 ost Mil ETT 50 0 I tI 1 t 1 1 024 m wkp 7 m at fo bi b 190 180 B2 500 30 0 10 20 50 100 200 500 1k 2k Cursor 145 0 Hz 85 32 dB Frequency Hz 1m TL 1 Matte Ende Figure 6 9 13 Effect of TL damping 145 Comparison of the measurement simulation gives an idea of the impact
131. l resistor separating the power amplifier and the soundcard eliminates the risk of ground loops between the soundcard output and input Figure 3 2 Interior conventional wired version left PCB version right 17 Impedance measurement 2 Ur Response measurement Aref 27 ohm tity Impedance lt measurement X Impedance calibration uo R2 F4 910 gt 910 2 Impedance SW1 h J3 10 pa Wf Response id 04 N Zener 41 Note 1 the power amplifier and the soundcard can be separated using an optional 1 kOhm resistor Note 2 warning do not use a bridged amplifier with virtual ground Safety note the inputs of the soundcard are protected by Zener diodes The power amplifier is protected as specified by the manufacturer Do not exceed the manufacturer s stated nominal impedance Figure 3 3 Schematic of the ARTA measurement box Note The measurement box is not necessary for single channel measurements When such measurements are performed however the microphone input should be calibrated 18 3 1 Two channel calibrated measurements with the ARTA measurement box For calibrated frequency response measurements with ARTA and STEPS in dual channel mode you should enter gain values for both input channels External preamp gain Note that the default programming defines the right input channel of the soundcard as the reference channel while the le
132. lation results BoxSim with matched minimum phase We suggested earlier that BoxSim would cope better with measured data taken on each driver axis To test this Figure 6 6 18 shows results for a simulation based on parallel measurements The two traces match perfectly Positional data for each driver on the baffle were not entered as this would influence the results in terms of diffraction effects only FR amp MinPhase Match path difference lA 2 2cm n 2 0 0000 0 0000 0 0000 QUUM i L NES 0 0000 0 0000 0 0224 Parallel 1a amp 2a Phase GO 0 0000 0 0000 Figure 6 6 18 Simulation results BoxSim parallel measurements target minphase with acoustic path difference Summary The realism of a simulation depends on the quality of measured data which must accurately reflect the speaker setup and measurement conditions Neither nearfield measurements nor conditions allowing the effects of the room to predominate are ideal for this The simplest way to carry out measurements and process the data is to make sure that the measurement and simulation coordinates are identical option A Measurements for woofers and tweeters are performed from a single reference point and the distance from the source to the microphone should not be excessively short Frequency and phase responses for export should be processed with the cursor in the same position for all drivers The only drawback is that th
133. lections thus at each point in space incident sound 15 equally likely to be picked up from any direction in that space The local sound energy density is equal at all points in the mixing field if the microphone is far away enough from the sound source and from all reflective surfaces This is referred to as the diffuse sound field At a certain distance from the source freefield conditions are lost and the sound wave enters the reverberant field The boundary between the free and reverberant fields where the contributions of the two fields are equal can be calculated T 222 riticalldistance 222 wo 2 2 Figure 6 4 4 Sound fields and radius of reverberant field 92 Reverberation radius Ry 0 057 y V RTso where V room volume m and RT60 reverberation time seconds If the distance from the sound source 15 less than this critical distance the sound field is defined as the free field of the source space 10 0 e 001 8 La LT 0 1 0 z RT 0 2 RT 0 4 RT 0 8 E 1 6 3 2 z RT 6 4 Q 10 100 1000 Room volume in m Figure 6 4 5 Reverberation radius study For example in a room with volume 50m 5m x 4m x 2 5m and reverberation time 0 4 sec the reverberation radius is about 0 64m Figure 6 4 5 In the same room a reverberation time of w
134. level Ppr full space 4 1 r Ad 20 log r 4d Level adjustment of bass reflex port Pp to membrane in the nearfield Sp Sp 20 log 96 96 measuring distance dy reference distance usually 1m reference Sp area of bass reflex port Sp area of speaker membrane membrane radius level of nearfield P level of farfield 183 8 4 Keyboard shortcuts ARTA provides a number of keyboard shortcuts for those who prefer them to the mouse for certain operations Up and Down Change the gain shown on the screen Moves the graph to the left Ctrl B Changes the background colour colour black amp white 2 x ALT R Repeats a measurement with the same setting ALT M Displays the magnitude window frequency response ALT P Displays the phase window phase transition ALT G Displays the group delay window Shift F12 Farina sweep evaluation see Section 7 1 184 9 Recommended speaker specifications Loudspeaker measurement is not a new science and it is therefore not surprising that its underlying principles are well established Two of the major standards are e AES2 1984 12003 AES Recommended Practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement 25 60268 5 Sound system equipment Part 5 Loudspeakers 26 The following is the list of requirements according to AES2 for low and high frequency drivers Low Frequency
135. lp Co Fre Be LE ee RES ES 32768 window Uniform Delay for phase estimation m 10 000 Get Zero Inv Impulse response my Marker Sa Del Offset Nu ain Scroll 4 k Cursor 186 795 0 000 ms 07 Ready L 5 7dB R 5 7dB Impulse Response FR Click on the frequency response icon in the toolbar to see the frequency response of your soundcard Smoothed frequency response Untitled File Edit View Smoothing Overlay FR Magnitude dB smoothed 1 24 act 10000 Frequency If your sound card is of good quality you should see a straight line However note the resolution of your measurement chart You can change the settings of the chart by clicking to automatically find the upper limit of the Y axis or you can manually search using the two arrows on the left next to 36 the Fit button The measurement range be adjusted the same way by using the two arrow buttons to the left of Range If you click on Set the following menu appears Magnitude dB Phase deg Group delay ms Magn top Ph top 180 Gd top 50 Magnrange 50 Ph range 360 Gd range 100 Frequency range Hz Impedance ohm Thick lines F High freq 20000 Maximum 50 Time Bandwidth Low freq 20 Minimum 0 Update Default View all this menu you can adjust all graphic parameters
136. mation of the other parameters derived from the room boundary measurements as above 1s shown in the following table The measurement conditions stipulate a measurement time window of 8 6 to 12 8 msec 99 The left hand panels in Figure 6 4 14 show measurements with 1 24 octave gray and 1 3 octave smoothing blue without windowing gating The red lines show windowed measurements 1 room effects eliminated The right hand panels show quite clearly the increasing influence of the measurement space as the measuring distance increases The transition from freefield to reverberant field is easy to see reverberation radius of approximately 1 40m Unfortunately no measurements were made at smaller distances to demonstrate this further 6 5 Determination of the reverberation time characterization of the measurement space As previously noted the measurement space significantly influences results Echo and reverberation effects make it difficult to characterize the speaker as it would behave in isolation ISO 3382 lists reverberation time RT60 as one of the most important room acoustic parameters A very short reverberation time is desirable and for domestic rooms an RT60 of approximately 0 4 seconds is recommended 13 ARTA supports the determination of RT60 as per ISO 3382 The following boundary conditions are required by the standard e The microphone should be positioned at least 1m from any reflective surfaces and not to
137. measurement or listening position can be simulated at least in principle Measurement environment Speakers have to meet the requirements of the listener in the listening environment It would therefore be logical to measure and develop the speaker under these conditions but as we have seen those same conditions cause problems in measurement Ultimately acoustic measurements taken in a room provide results that are actually the sum of the loudspeaker and the room itself We can eliminate the contribution of the room and simulate freefield conditions by gating but may compromise data in terms of low frequency limits and resolution Figure 6 6 1 109 10 0 500 1 20m 8 0 400 N M Tu xu o 60 300 E Q ym d 9 40 200 a 2 0 100 d 0 0 0 0 0 5 1 T 2 2 5 3 Measurement distance d m Figure 6 6 1 Window length and lower cut off frequency as a function of measuring distance for a room of height 2 40m h H 2 Test setup alignment errors Figure 6 6 2 shows a measurement listening arrangement Horizontal angle O arctan hi hgur D O arctan h4 hrr D Figure 6 6 2 Geometry of a typical measurement listening arrangement For a real measurement however we would not usually position the microphone off axis relative to both drivers The reasons for this are explained as follows Figure 6 6 3a shows two different distance measurement positions for a two way
138. mine and record the sound level with the cursor at IkHz 2 Repeat in dual channel mode Determine and record the sound level at 1kHz as before 3 Determine the difference between the two measurements and calculate the power amplifier gain as Power amplifier gain 10 difference in level 1kHz 20 Note that this approach requires a circuit as shown in Section 1 4 above or the ARTA Measurement Box e g After running FR recordings the following values were obtained at 1kHz Figure 3 7 single channel 106 21dB dual channel 96 25dB Thus the difference is 9 96dB and the power amplifier gain 1097920 3 148 After entering this value in the Power amplifier gain field Figure 3 6 single and dual channel measurements taken within the limits of error of the soundcard and amplifier should match Note that this procedure must be repeated every time the gain volume of the power amplifier 1s changed 22 FR Magnitude dB re 83 smoothed 1 3 act Power Amplifier Gain 10 2 20 r50 100 200 500 1 2k ok 10k 20 Cursor 999 4 Hz 96 25 dB Frequency Hz Single vs QualChannel Figure 3 7 Power amplifier gain single vs dual channel Alternatively you can use the following slightly more accurate procedure in dual channel mode FR2 1 Connect the left channel input with the selected Line OUT R output channel of the soundcard amp 2 Connect the right input channel via a voltage b divide
139. mponents with a power of P 10 log 1 16384 42dB below RMS level Crest factors of approximately 10 11dB for white noise and 6 9dB for MLS should also be taken into consideration n b Crest factor the ratio between peak RMS value of an alternating quantity CF UsVpgys The excitation signal will therefore be roughly 50dB below the full scale level between 48dB and 53dB depending on the signal used This leaves a dynamic range of D excitation level noise dB For the M Audio Transit D 50 120 70dB Realtek D 50 80 30dB Thus we see that soundcards with a noise floor of 80dB are of no use in measurements using noise excitation Such cards may be used however for measurements using sine excitation see STEPS 39 5 Calibration of the measurement chain While it is possible to carry out measurements without calibration reliable results cannot be obtained if the individual components of the measurement system are not well matched Soundcard DUT preamp Soundcard Line OUT 1500 2 Line IN i U our U iN 5 amp Suc Gere Sues The measurement chain should therefore be analysed at the level of each component to ensure that the different parts of the setup are suited to each other and that amplifier outputs and devices such as voltage dividers are arranged such that the system is not over or underdriven As an example consider t
140. n the impulse response file and set the gate with the yellow and red markers Impulse response ims 3 04 5 56 8 08 10 60 ms Cursor 1 993 mv 7 500 ms 360 Gate 5 146 ms 247 Figure 6 7 4 Farfield impulse response at 48cm with gating Note the proximity of the floor and ceiling reflections which demonstrates the accurate positioning of the speaker at very close to half the room height The gate length is shown under the impulse trace 5 146msec corresponds to a sound travel distance of 1 77m which is in agreement with the theoretical prediction based on the example given in the last chapter 125 FR A preliminary look at the resulting frequency response trace LL shows that the level adjustment has worked well Figure 6 7 5 FR Magnitude dB smoothed 1 6 act 20 50 100 200 500 1k 2k Sk 10k 20k Cursor 1124 2 Hz 15 43 dB Frequency Hz Fernfeld 48cm und Mahfeld mit Battle Step Korrektur Figure 6 7 5 Near and farfield raw frequency response Now we can determine the transition frequency or the point where the near and farfield traces are to be spliced together In the above example we would splice at around 240Hz Place the cursor yellow line over the desired transition frequency and go to the drop down menu under Edit Dofthved Click on Merge overlay below cursor The nearfield response that has been defined as an overlay is added to the 21 er V left
141. nel and Audio Device Setup When accessed the Vista Win7 control panel has four tabs Figure 4 5 Playback and Recording must both be adjusted 27 amp Sound Select playback device below to modify its settings Speakers GO46 Audio Device Ready Digital Output GO46 Audio Device Ready Speakers SoundMAX Integrated Digital HD Audio Default Device SPDIF Interface SoundMAX Integrated Digital HD Audio Ready Configure Set Default Figure 4 5 Vista sound control panel 1 Select the playback channel do not use the measurement channel as a default audio channel 2 Click on Properties to open the sound properties dialog for that channel 3 Click on the Levels tab to open the output mixer Figure 4 6 Mute the Line In and Mic channels if they are shown 4 Click on the Advanced tab to set the channel resolution and a sample rate Figure 4 7 5 Repeat 1 to 4 above for the recording channel choose the same sampling rate as for the playback channel 28 Speakers Properties Figure 4 6 Playback channel properties output levels 29 J Speakers Properties General Custom Levels System Effects Advanced Default Format Select the sample rate and bit depth to be used when running in shared mode 16 bit 48000 Hz DVD Quality Exclusive Mode Allow applications to take exclusive control of this device Give exclusive mod
142. nnels chooses the soundcard input stereo channels An ASIO driver can have many channels Output Device chooses the soundcard output stereo channels the input and output channels of a single soundcard are normally selected mandatory when in ASIO driver mode Control panel button if a WDM driver is chosen the Windows 2000 XP or Sound control panel in Vista Win7 is opened If an ASIO driver is chosen this opens the ASIO control panel Wave format in Windows 2000 XP select Windows wave format 16 bit 24 bit 32 bit or Float float IEEE floating point single precision 32 bit format Use 24 bit or 32 bit modes when using a high quality soundcard Note that many cheaper soundcards are claimed to be 24 bit but their true bit resolution is often less than 16 bits Select Float for Windows Vista Windows 7 Wave format has no effect when in ASIO mode as the bit resolution has to be setup in the ASIO control panel 24 Amplifier Interface section LineIn sensitivity specifies the sensitivity of the line input i e the peak voltage in mV that corresponds to the full excitation of the line input LineOut sensitivity the sensitivity of the left line output i e the peak voltage in mV that corresponds to the full excitation of the line output Ext preamp gain If you connect a preamplifier or voltage probe to the line inputs you should enter the gain of the preamplifier or probe attenuation in the edit box
143. noise 71 FR Magnitude gt 20 50 00 200 500 1k 2k 10k 20k Cursor 20 5 Hz 43 79 dB FrequencvyrHz muttitone Figure 6 2 3 Multisine As of version 1 6 2 ARTA comes with an additional generator with continuous signals sine square multisine etc pulses e g Dirac and sine bursts of various types See Siegfried Linkwitz s website for information on application of the sine burst http www linkwitzlab com Triggered burst measurements of tweeters Figure 6 2 4 shows the Signal Generation and Recording menu The choice of waveform is made by clicking the checkboxes Continuous Pulse or Sine burst Once this is done the signal can be given more specific characteristics e g type frequency and the frequency transients adjusted using the Repetition drop down Thus 16 384 might prove to be a high repetition rate while 262 144 might contain only one repetition per recording it depends on the number of samples Length and sampling rate chosen in the Signal recording field With the checkbox Invert output signal the output signal can be inverted Trigger on right channel can be used to control recording of two channel measurements via the output of the sound card 72 Signal Generation and Recording X Trigger Trigger channel Left Trigger slope Plus Trigger value Full
144. nse right 6 2 1 Impulse responses theory and practice Depending on the DUT but especially with subwoofers and our knowledge of signal theory the first impulse response seen on the monitor can come as something of a surprise A brief overview with examples from theory and practice therefore follows 74 impuha rispan is Sten response v n db yy wihed 1 24 Phage I Jin Bad 11 34 10 48 2111 575 4258 na 50 dk Cursor 1 000 Caner DOO 007 Daur SEALS HE dag Fi Dirac 15 pi 2108 07 20 DR 22 Fie Drac 15 pi 2007 10 24 141205 eee epi AA Wats EM Figure 6 2 6 Step response middle and frequency response right of a Dirac impulse left To describe the theory a Dirac pulse was evaluated as a WAV file in with reference to target filters low pass band pass and high pass Using this method the generated impulse and step and frequency responses should match the curves predicted by filter theory as long as the bandwidth limit does not cause problems impulsa mu Shep pon ss i empoihgd 154 Phase 534 8 76 oro nas 0 00 rv Of Cu DO oS 05 Cu 250124 Hz 16 6 der 145 7 iden 2108 17 24 18238 Fie LA GOD pi TW 0 04 13 30 45 Curent pl 2108 12 24 15 S002 F
145. o close to the source speaker The minimum distance from the source can be calculated as follows Admin 2 E m where V room volume m c speed of sound m sec T estimated reverberation time sec e The sound source should have a radiation pattern that is as spherical as possible A particularly suitable source is illustrated here e The microphone should be omnidirectional e The excitation signal level should be 45dB above the noise floor Under normal domestic conditions a level gt 90dB is required e The excitation signal should be as energetic as possible sine sweep is recommended improve the SNR further set the number of averages in the Impulse response measurement menu to 4 e The duration of excitation of the room should be significantly longer than the estimated reverberation time The reverberation time can be estimated with the following equation RT60 0 163 V A where V room volume in m A equivalent sound absorption area in m a S e 2 surface sound absorption coefficient S surface area in 100 The above table shows absorption coefficients for common absorbing materials at several frequencies Use the 125Hz values for estimating the required excitation time m 2 2 2 m m 2 2 2 m Piece Example A room has dimensions 4 9m x 3 8m x 2 2m and a volume of 40 96 and is furnished as follows 18 61 carpet 58m of concre
146. of measurement and simulation Both single point and parallel measurements may be used left Both methods are in principle equivalent but the single point measurement has the advantage of reducing the influence of microphone positioning The difference in delay times between the drivers 1s accounted for by determining the relative Pythagorean geometry of the drivers Parallel measurement allows for direct measurement of the acoustic sources and should allow for improved scalability of measurements in terms of distance changes during the simulation as both drivers are measured on axis The parallel method is recommended as overall it is less prone to errors than the single point method Note that the method described here employs an averaging process for determining the acoustic source as it averages out uncertainties introduced by the measurement procedure and by the resolution of the system Note also that we are not treating each driver as an absolute acoustic source but are using one driver usually the tweeter as a reference Group delay This method applies the Excess Group Delay function to the planned crossover region The measurement window should be sized so as to be as free as possible from reflections The first step is 117 to determine the excess group delay for both the tweeter and the woofer The first trace generated should be saved by using the overlay function Figure 6 6 11 left CE iam Note that
147. of the damping action on the AJHorn variables and B2 Fourth is the verification of a baffle effect simulation using The Edge www tolvan com edge Note that The Edge is able to export simulated data 5 0 0 0 5 0 10 0 200 500 Zk ok 10k 20 Cursor 201 9 Hz 70 50 dB Frequency Hz Figure 6 9 14 Battle effect Edge simulation red and measurement black As an extra Figure 6 9 15 shows the measured data blue corrected using the Edge simulation green The red curve thus represents the sound pressure without baffle effects which roughly corresponds to the measurement on a standard baffle Note that these results will apply only for the same measurement position 5 0 4 0 3 0 2 0 1 0 SPL in dB 5 0 100 1000 10000 Frequenz in Hz Figure 6 9 15 Correction of influence of the baffle for a specific measurement position n b Korrektur correction Korrektur EDGE in dB 146 6 10 Electrical measurements on crossovers with Electrical measurements in addition to acoustic measurements are useful in crossover development Note however that this section does not deal with crossover design As mentioned previously caution is advised when making electrical measurements Use a multimeter to measure the voltages at the crossover and protect the soundcard with a voltage divider Chapter 5 Figure 6 10 1 shows the electrical test setup the probe with its voltage protection is shown
148. on Voller Text full text Reduzierter Text reduced text Arta As of version 1 4 overlays are available in the impulse response window File Overlay Edit view F See the relevant menu located at the top of the main menu bar left Cy Set as overlay Delete overlay FFT The daughter menu items are very similar to those in the Smoothed Frequency Response window there are just fewer of them Load as overlay 176 Overlay Info E x The menu item Overlay Info left contains technical data pertaining to the loaded overlay Overlay information Number of samples 65536 Sampling 6000 Hz Mic sensikivibv 6 75 Figure 8 1 7 shows the impulse responses of a woofer TMT blue current measurement and a tweeter HT red overlay The chart shows to good effect the time offset between the two drivers Impulse response ius zoom 8 1 4 05 4 18 4 30 4 44 4 56 ms Curas 182nv 6 404u 4 208ms 404 Gate 0 052ms 1 79 44 5 Overlay im PIR Made Figure 8 1 7 Overlays in the time domain 1 78r C1 477 uv 0 Dorm n Cover ar i PIS Jupe IO 4 20 M 2i Cors ey E 4 Qu 20D udo Cas Ue s Tempo Amos im FIN ede Figure 8 1 8 Overlays in the time domain left highest magnification right bold lines 177 178 8 2 Editing measurement data and data files ARTA provides functions for documentation
149. on of the curve to be evaluated with Noise tail The aim should be to optimize the trace by choosing the percentage and method of compensating for the falling curve The quality of the adjustment is shown as a correlation coefficient r directly below the graph A correlation coefficient of r 1 is optimal 103 Acoustical Energy Decay Dodil pir Edit Automatic 1503382 evaluation Filtering 7 Energy decay dB 0 00 117 33 236 00 352 33 472 00 Cursor 6 612 dB 30 667 ms Diff 30 618 dB 172 000 ms Wideband 60 0 337 s r 0 999 EDT 0 259 s 80 17 1096 50 11 4198 D50 93 3 Ts 16 2 Figure 6 5 4 Analysis with cursor and marker Room Acoustical Parameters Dodil pir x 0 330 1 159 0 989 0 321 1 016 0 981 1 170 0 994 0 662 0 281 5 51 15 88 0 84 11 07 54 79 92 76 57 727 22 574 18 194 15 14 662 Save ASCII Save csv Figure 6 5 5 Output of results 3 Specify the area to be evaluated by positioning the cursor yellow and the marker red The evaluation is started by pressing the T60 button 4 Repeat steps 1 3 for all frequency bands 104 5 Generate the output for the room acoustic parameters by pressing the Log button The results can be displayed as a screenshot or as a CSV file that can be imported directly into Excel to facilitate statistical analysis Ensure that in setup under CSV format that the comma 15 selected s
150. oom length II Resonance range room modes lt 2000 VV RT where V room volume RT reverberation time Random range diffuse or reverberant field gt 2000 where V room volume RT reverberation time Despite these problems freefield measurements can be simulated in normal rooms 6 by splicing near and farfield measurements Figure 6 4 3 Near and farfield refers to the distance from the sound source while free or direct and diffuse field depends on the environment in which the source is placed Free and diffuse fields are independent of the type of source and are influenced by the acoustic properties of the surrounding space If sound is able to radiate from the source without encountering any obstacles or scattering these conditions are referred to as freefield 91 Nearfield Farfield distance from source mu Level dB gt Reverberant field Distance from source gt Figure 6 4 3 Definitions of sound fields Freefield direct sound only with no reflections nearfield measuring distance lt wavelength farfield radiated wavelength gt dimensions of source sound pressure decreases by 6dB for every doubling of distance When a sound source is located a normal room sound waves are reflected from room surfaces or furnishings The overall radiated sound is a mix of multiple ref
151. ormers power supplies power cables etc e If possible disconnect the computer from the mains if you have a laptop use the battery 6 1 2 The signal to noise ratio of the measurement system The S N ratio is important and should be determined before each measurement session as meaningful frequency and phase measurements can be obtained only if the useful signal level is greater than the noise level Measure sound levels with and without speakers DUT and compare levels Figure 6 1 2 The noise level in the region of interest should be at least 20dB below the signal the greater the separation between the two the better 66 hagnitude dB os smoothed 1 24 act Im 20 50 100 200 500 1k Zk 5k 10k 20k Cursor 20 0 Hz 77 96 dB Frequency Hz Current file Untitled 2010 09 05 19 43 48 Overlay files eM Figure 6 1 2 Determination of signal levels and S N ratio If the separation 15 insufficient there are several options Reduce the noise level or change the room or measurement environment Increase the level of the excitation signal Do not use excitation signals with low energy content e g MLS Use averaging see Section 6 1 3 Phase transition is very sensitive to an unfavourable S N ratio especially when measuring speakers that do not cover the entire frequency range Generally the phase frequency response can only be reliably calculated with a sufficiently large S N ratio
152. osoft GS Wavetable SW Synth Balance 4 J Lautstarke Folgende Laut ececles e 7 Serene Lautstarke fo CO Piyer 7 Mision Lawn Nur Standardgerate verwenden Basically the screen shots show that the line in recording mixers should be enabled the recording volume should be set almost to minimum the output mixer line in should be disabled and the mixer output volume should be set almost to maximum Note that the playback and recording levels should be set similarly in Windows Vista 7 8 the mixers for these operating systems are easier to access and adjust than the XP mixer Loopback testing We are now almost ready for the first measurement with ARTA Connect the inputs and outputs of your soundcard as shown in the loopback measurement diagram in Section 1 4 The types of cable required RCA TRS etc will depend on the soundcard Refer to Section 4 2 onwards for information on matching input and output levels and for ascertaining the quality of your soundcard Having carried out the loopback measurement for setting the mixer and testing your soundcard you will probably want to measure the frequency response of your speakers For this you will need a measurement microphone If your soundcard can provide a supply voltage to power the microphone you can work with a simple DIY electret check the specification of the soundcard to ascertain whether this will be possible It is very
153. ow of time impulse response Room reflection ursor Marker Cursor 5 96 7 31 8 67 10 02 11 38 ms Cursor 152 588 uv 6 933 320 Gate 5 042 ms 173 43 cmi Figure 6 4 1 Elimination of room reflections by windowing The room 15 also visible in the waterfall plot The irregular frequency response between 200 and 2000Hz is caused by the slower decay of the room energy 89 BPN Ev F9 May dede to uo ta E E E A w e t gt E 1 Ow LE Fo c fw t rerom 3908 13 45 Frequency response without smoothing F8 May dde ve SOLO F9 Mayrdede EO VH cet E 2 wi E th E Demy Period based waterfall burst decay Caium 774 Cerny Cama 7 74 Decay b b 125 290 500 1000 2000 4000 8000 125 250 59 1000 2000 4000 8000 Frequenz Hz Frequenz Hz Reverberation time of measurement oom Figure 6 4 2a Comparison of two measuring rooms anechoic chamber left normal room right FR Magnitude dB re 200 2 83V 60 0 20 50 100 200 500 1k 2k 5k 10k 20k Cursor 20 5 Hz 59 98 dB Frequency Hz Figure 6 4 2b Characterization of the measurement environment length L 4 95m width W 3 85m height H 2 25m reverberation time RT 0 385 I Pressurisation range f c 2L where c 344 m s L r
154. p 21 Available from http www visaton de vb showthread php t 12754 Gander MR Ground Plane Acoustic Measurement of Loudspeaker Systems J Audio Eng Soc 1982 30 10 723 31 Klippel GmbH Measurement of Peak Displacement Xmax performance based method Internet 2012 cited 2014 Jul 26 Available from http www klippel de nc know how application notes html sword_list 5BO 5D an4 Fasold W Veres E Schallschutz und Raumakustik in der Praxis Planungsbeispiele und konstruktive L sungen Berlin Huss Medien Verl f r Bauwesen 2003 Vanderkooy J Applications of the Acoustic Centre 122ns AES Convention 2007 192 15 16 17 18 19 20 2 22 25 24 29 26 21 28 Fuhs 5 H ldrich Thomberger Validierung des Entfernungsgesetzes und Korrektur der Gruppenlaufzeit und des akustischen Zentrums des Lautsprechers im Adrienne Verfahrens DAGA 2006 32 Deutsche Jahrestagung f r Akustik Braunschweig Germany 20006 Ahlersmeyer T Akustisch optimale Materialien f r Lautsprechergehause Internet Picosound de cited 2014 Sep 21 Available from http urlm de www picosound de Mateljan I Weber H Doric A Detection of Audible Resonances Proceedings of the Third congress of Alps Adria Acoustics Association Graz Austria 2007 Freeman J Techniques to enhance op amp signal integrity in low level sensor applications Internet Planet Analog Articles 2008 cited 2014 Se
155. p 21 Available from http www planetanalog com document asp doc_id 527763 Waldman W Nonlinear Least Squares Estimation of Thiele Small Parameters from Impedance Measurements AES Convention Munich Germany 1993 Farina A Simultaneous Measurement of Impulse Response and Distortion with a Swept Sine Technique 108th AES Convention 2000 M ller S Massarini P Transfer Function Measurement with Sweeps J Audio Eng Soc 2001 49 443 71 D Appolito JA Testing Loudspeakers Which Measurements Matter Part 1 AudioXpress 2009 D Appolito JA Testing Loudspeakers Which Measurements Matter Part 2 AudioXpress 2009 Toole FE Sound reproduction loudspeakers and rooms Amsterdam Boston Elsevier 2008 550 p AES Recommended Practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement Audio Engineering Society 1984 Report No AES2 1984 12003 IEC Sound system equipment Part 5 Loudspeakers Geneva Switzerland International Electrotechnical Commission 2003 Report No IEC 60268 5 AES Information Document for Acoustics Plane wave tubes Design and practice Audio Engineering Society 2012 Report No AES 1lid 2012 Sound system equipment Electroacoustical transducers Measurement of large signal parameters International Electrotechnical Commission 2010 Jan Report No IEC 62458 ed1 0 193 12 Formulae and figures Speed of sound m sec 0 3535 1 41
156. ponse 425 433 Cursor 18 462 niv 0 000 0 File 12 deg pir 257 2008 02 27 14 41 44 1000 wagnitude dB cemonmthed 1724 och Phase 10 p cot At 450 20 25 0 sery n r p gen LA 20 su 100 200 500 16 Jk EK 10k Cursor 20 1 Hz 53 84 dB 172 8 dag Frequency Hz Current fla H1000 pir 0008 02 21 08 48 06 180 0 90 0 0 0 30 0 1 FR Wagnitude dB re 20uP3 2 23V emoanthad 950 H als d S gt HE 900 E 700 a 1 E is Hot 20 50 100 20 500 ik 2k TOK 206 Cursor 20 0 Hz 55 26 dB 128 3 den Frequency ez Curent fla HcF12 degsD pr 2006 00 20 14 37 55 sso Figure 6 2 10 Impulse and frequency responses of simulated and real tweeters It would be desirable in addition to describe the behaviour of a bandpass filter with a tweeter but the bandwidth limitations of the simulation software 22kHz and measurement system 24kHz allow for only a partial frequency response trace Figure 6 2 11 FR Magnitude dB ferioothed 1 24 net Phase i 00 an I IT E 150 NN HIP TER ah s Er 40 0 30 0 20 50 100 20 50 1 zk 5k 10k 20k Cursor 201 Hz 63 84 dB 173 9 deg Frequency Hz Current fila HP4 O00 pir 2008 02 21 OE 48 08 100 15 0 20 1
157. r with the output of the power amplifier i Power 3 Enter the absolute value of the voltage divider G as Ext right preamp gain see Figure 3 6 SOUND 4 Set the signal generator to Periodic Noise To CARD protect the soundcard reduce the output level to about 10dB 5 Measure in FR2 mode and note the amplitude at This measured value corresponds to the gain of the power amplifier in dB The power amplifier gain value to be entered in Audio Devices Setup i 1 oR level 1kHz 20 23 4 Soundcard setup and testing 4 1 Soundcard setup Before you start measuring you must set up your soundcard and hardware To do this go Setup and Audio Devices Setup or click the toolbar icon The dialog box shown in Figure 4 1 will open Audio Devices Setup Soundcard Soundcard driver WDM Windows multimedia driver Control Panel Input channels Conexant HD Audio input on Output channels Conexant HD Audio output 24 bit Amplifier Interface mcum ow Dee Ext left preamp gain a L R channel diff dB 0 Ext right preamp gain Power amplifier gain NEN Left Ch Sensitivity mV Pa 5 Save setup Figure 4 1 Audio devices setup menu The dialog has the following controls Soundcard section Soundcard driver chooses the type of soundcard driver WDM windows multimedia driver or an installed ASIO driver Input cha
158. re summarised in Figure 5 3 8 They show that significant irregularities can be expected at frequencies below 100 with different DIY microphones Even with high quality microphone capsules e g 211 KE 4 there 1s no guarantee that there will be no significant deviations from specifications me eL TT m M LLL Figure 5 3 8 Results with various microphones MB550 red 211 KE 4 No 1 light blue 211 KE 4 No 2 Nr2K Blue MCE 2000 orange Panasonic WM60 Figure 5 3 9 illustrates that there are in addition other factors to be taken into consideration These harmonic distortion traces clearly show the reason for the extra cost associated with professional quality microphones 57 Microphones small pressure chamber 300Hz THD in 221 0 550 4 211 2000 D 110 115 120 125 130 135 140 145 SPL inside box dB Figure 5 3 9 Comparison of harmonic distortion of microphones at 300Hz 5 4 Testing the amplifier The amplifier is an essential part of the measurement chain You might use your own power amplifier or alternatively a kit or custom built amplifier designed for this purpose Regardless you should know the basic characteristics of the device used If the amplifier is to be used for frequency response and impedance measurements a device with linear frequency respons
159. rlay Parameter Reverberation time 1505 T3 Table 4 Top Fit Range Set um Cyverlays m 53 125 250 500 1 Zk Ak Bk Cursor 250 0 Hz 0 36 Frequency band Hz Figure 6 5 7 Graphical analysis of octave bands Parameter The chart can be edited with the usual functions and results saved as overlays The Parameters field can be used to view acoustic parameters graphically see picture left Axes can be scaled using Set See Figure 6 5 8 for the available options The Update button can be used to show a preview 106 Acoustic Parameters Graph Setup T 0 EDT axis seconds Range o s fo C50 axis dB Range 50 0 Ts axis tmilisecands Range 100 0 Bottom Bottom Bottom Stepped graph Default Update Cancel Figure 6 5 8 Setup menu for charting acoustic parameters The Stepped Graph checkbox can be used to control the type of graphic representation If this is checked the results are shown as bands or steps Figure 6 5 9 Reverberstion tene Ts S Revarberalion teme Ts gt D gt Orso 630148 Fr xuenc y band HZ COwt 3092 1141 Freguency band Hz Ht ox 2 5 Poe Figure 6 5 9 Graphic representation in third octave bands line left stepped right As with the manual method results can be output in tabular format Note however t
160. s e Measuring distance d gt 3 x largest dimension of the source e The lower frequency limit fy depends on the maximum time window gate see below Basically when making farfield measurements we have to make sure that both the source and microphone are placed as far as possible from reflecting surfaces In normal rooms the limiting dimension 15 usually the ceiling height of about 2 50m 95 Path of the floor or ceiling reflection D noorceiling 2 2 h m h Difference between direct sound and reflected sound Delta Dnoor ceiting d m Travel time difference T Delta c s where c 344 m s Lower frequency limit fy 1 T Hz Figure 6 4 9 Measurement setup You should analyze the measurement space in this way Figure 6 4 9 in order to identify more easily room reflections in the impulse response Figure 6 4 10 shows an example Berechnet L Lange 4 90 m W Breite 3 90 m Hohe 2 20 12 1 57 m L1 2 80 m W 2 23 m wo 1 67 D 0 53 m ht 137 I iw h 0 83 m Tnirekt 1 5407 ms 053 m wi 4 8 1128 ms 2 9 5 0656 ms 174 thinks 9 8308 ms 3 38 m Rechts 13 0563 ms 449 m thw Rockwand 17 0790 ms 6 13 tr Frontwand 10 6666 ms 367 m Impulse response mv Gemessen A 1 583 ms 0 54 m 12 thecke 5 313 ms 183m 1 6 104 ms 279m tiu 9 689 ms 330 rrontwand 10 983 ms 3 78 Im th tRechts 15 104 m
161. s 45 I 18 063 ms B21m 5 05 8 48 11 04 13 52 16 04 8 805 uW 6271 301 Gale 11 354 ms 290 58 cma imis Figure 6 4 10 Measurement room analysis The dimensions in the upper panel allow us to pinpoint the main reflection positions in the impulse response This may be helpful if the characteristics of the room do not allow for pronounced reflections that are easy to see in the impulse chart 96 second example refers to Figure 6 4 11 With ceiling height 2 20m measurement distance D 0 53m and measurement height hl 1 37m the acoustic path length Dapor ceiting 18 Dnoorceiling 2 X 0 53 0 5 1 37 2 79m The resulting difference between direct and reflected sound is 2 79 0 53m 2 26m This corresponds to a time difference of T 2 26 344 0 0065697 sec 6 5697 msec Thus the lowest usable frequency is fy 1 0 0065697 152 2Hz The following table shows other measuring distances for a measurement height equivalent to half the room height 158 5 160 7 165 1 174 3 The following figures show how the low frequency response changes with increasing measurement distance Fr response magnitude dB viv smoothed 1 24 act 30 0 Cursor 20 1 Hz 39 67 dB Frequency Hz Figure 6 4 11 Nearfield to farfield transitions for measuring distances of 3 6 12 24 48 and 96cm Room influences start to become apparent at with overt interferenc
162. s html has details of several tried and trusted kits and PCBs may also be available ARTA Measuring Box The ARTA Measuring Box is not absolutely necessary but can make measurements a lot easier see Chapter 3 and ARTA Application Note API Both wired and PCB solutions are available http www mini cooper clubman de html hifi_ projects html Cables Several cables are required all of which should be of good quality Poor connections inadequate shielding etc can interfere with measurements The following will be required e Microphone cable XLR TRS RCA depending on microphone and preamplifier see also Figure 1 6 e Soundcard cable Measuring Box e Amplifier cable Measuring Box e Speaker cable 1 5 2 5mm Keep all cables as short as possible Other useful equipment e Loopback cable to calibrate the soundcard see chapter 4 e Voltage divider for level adjustment see chapter 5 e Y cable for semi dual channel measurement see chapter 2 e Block connectors and alligator clips for temporary connections Multimeter DMM A good multimeter is essential for the calibration of measuring equipment and 15 also generally useful If you do not have one already you should ideally select a true RMS meter A wide range is available with many suitable devices on sale for under 100 If you already have a DMM or are considering a cheaper device that is not categorised as described above you sho
163. s method should work with any simulation program the measuring equipment and data should ideally be validated before the actual simulation For example measured data for the tweeter and woofer should be summed without the crossover using the simulation program and the results compared with measured data using both drivers in parallel Figure 6 6 9 shows an example using two programs BoxSim and CALSOD The measured data are in perfect agreement with the simulation 204 2 83V ismooctwd 1 24 och tt At 500 1 A 10 ah weer 100042 85 99 IHI Mercure muon 400m ove SO CALSOD Simulation dotted line measured BoxSim Simulation red measured black solid line Figure 6 6 9 Validation of simulated data In order to demonstrate what happens when measurement and simulation conditions do not match Figure 6 6 10 shows results for a measurement taken at 40cm compared with a simulated distance of 80cm in CALSOD The relative phase relationships of the woofer and tweeter have been affected by the change in the simulated distance 116 saan 1141 114 200 Sk Lok H i r v7 J 7 te Figure 6 6 10 Simulation at 80cm compared with measurement at 40cm The difference between the simulated and measured data is more apparent when the microphon
164. s of a cubic spline Note however that at least one value in every three should be measured with these values distributed as evenly as possible over the correction area 51 2 Activate the compensation curve in Use FR Compensation You can see in the ARTA main menu if the microphone compensation file is active If FR Compensation 1s ticked the file is in use Click on Compensation again to disable the compensation file You can also use the toolbar icon to control and enable disable the compensation file Avg Line gt Frequency response compensation 0 000 Rangel de 10 show interpolater i S values es 5 0 Hz 1 5 dB Frequency Hz Programme arta Software MBSSO GES MIC Load Use Frequency response compensation OK Figure 5 3 1 Frequency response compensation window Use of the above procedure assumes that you know your microphone s frequency response There are several ways to obtain this e Use the manufacturer s specification e Have the microphone calibrated professionally e Carry out the calibration yourself if you have access to the necessary equipment Substitution method gt 200 2 Pressure chamber method F lt 200Hz 5 3 1 Calibration using a reference quality microphone gt 200Hz If you can obtain a high quality measurement microphone e g see Figure 5 3 2a you can use it to calibrate your own A good description of the procedure
165. s the time axis Graph window e Show Selection of the data display mode Enables either the graphics mode or the currently selected SPL value in large font 161 Update updates graphics after entering new parameters Default sets default values The main menu includes the following commands File SPL statistics and user info Date 03 31 2008 Time 23 57 55 Total Recording Time 08 00 00 LAeqT 46 85 dB LAIeq 0 00 dB LAE 91 45 dB LAFmin 27 56 dB LASmin 21 40 dB LAImin 28 06 dB LCpk max 102 40 SEL LA Fmax 74 44 LASmax 71 58 LAImax 739 27 dB Percent exceeding levels Ln dBA Fast glow L1 57 6 51 17 LS 54 8 54 9 1 10 51 1 51 2 1 50 31 1 31 6 190 28 1 28 3 1 95 28 0 28 2 199 27 8 28 0 User supplied additional information Save SPL history file save the SPL data as a spl file Open SPL history file Load from spl files Position near my house Figure 7 2 5 SPL statistics Export exports data in text format Imp 58 2 52 4 33 6 29 2 28 8 20 4 Peak 78 4 72 9 68 7 52 4 46 2 45 4 44 4 ASCII 1s logged Exports Leq SPL and Lpeak in second increments ASCII 100ms logged Exports SPL Fast in 100msec increments CSV 15 logged Exports Leq SPL Lpeak in second increments in CSV format CSV 100ms logged Exports SPL Fast in 100msec increments in CSV format File statistics and user info SPL statistics and user entered information
166. scale 0 001 Continuous Generator C Continuous sine 1000 00 Set Transient Sener ator Pulse Width C Brera z Freg Hz 1000 Sine periods 5 Level dB FS o H 22942 Invert output signal Signal recording Input channel Right m Length samples 32k hd Sampling rate Hz 44100 Time constant 742 04 ms Number of averages 1 Invert Phase of input channel Predelay samples Time waiting trigger 5 z Close after recording Wait For trigger Generate Link Record Default Trigger on right channel Figure 6 2 4 Signal Generation and Recording menu The Link checkbox between the Generate and Record buttons automates the trigger process by linking the two processes The Signal recording and fields are largely self explanatory Figure 6 2 5 shows a collection of signals from the Transient Generator Excitation signals are shown on the left while the right half shows the tweeter response at 3kHz recorded with a high quality microphone 73 Triangle Window 208 L Cur 4 ELO ew DOHA KO Cuiegr 18 Sab Tha regie Gauss Window T sd LI 201 4 d DEG E 5 Pulse right 10 Figure 6 2 5 Burst and pulse excitation left respo
167. t channel of your soundcard 41 Soundcard full scale output mV 1 Connect electronic voltmeter or scope on left output channel setrange to 2V 2 _Generate sinus 40012 Output level 3dB E 3 Enter voltmeter scope value 676 4 rms 4 Estmate Estimated Current 1132 1132 my L 70 50 R 80 0 1 Connect an electronic voltmeter to the left line output channel Any meter that measures accurately at 400Hz or an oscilloscope is suitable The chart to the right shows how measurements from a quality DMM vary with frequency 2 Press the button Generate sinus 400Hz Right Output card input Abweichung 3 Enter the voltmeter readout edit box in mV TT dn rms Frequenz in Hz 4 Press the button Estimate Max Output mV Soundcard full scale input mV 1 Connect sine generator with known output voltage on cham 2 Enter voltage peak or rms 3 Estimate Max Input mV Estimated Left 2945 69 Right 2980 92 Diff 0 1032 Accept 30 40 Current 2945 69 2980 92 mV 0 10323 dB 5 The estimated value will be shown in the box Estimated 6 If you are satisfied with the measurement press the button Accept and the estimated value will become the current value of the LineOut Sensitivity This will also be entered as a value for the input channel calibration 5 1
168. te stone 10m shelving 1 0 glazing 3 6m doors two upholstered chairs 18 6 0 026 58 0 02 10 0 45 1 0 20 3 6 0 10 2 0 38 7 46m and RT60 0 163 40 96 7 46 0 89 seconds at 125Hz The length of the excitation signal should therefore significantly exceed 0 89 seconds Figure 6 5 1 shows the setting of the excitation signal duration in ARTA outlined in red where Excitation signal duration Sequence Length Sampling Rate Sequence lengths of 16k 32k 64k and 128k give sampling excitation durations at 48kHz of 0 33s 0 66s 1 33s and 2 66s which should suffice for normal living rooms A longer excitation time can be achieved by reducing the sampling rate if this is desired Note To obtain absorption coefficients of materials by in situ measurement see ARTA Application Note No amp 2 101 Impulse response measurement Periodic Noise Sweep MLS Penodic generator 1 Recorder Sequence length Edk Frefered input channel Left Sampling rate Hz 48000 Dual channel measurement mode Time constant 1355 33 ms Invert Phase af input channel spectrum Pink Output volume dB 12 Frequency domain 2Ch averaging Pink cutoff Hz 50 Generate Figure 6 5 1 Setting the excitation signal time The impulse response of the room is shown in Figure 6 5 2 The position of the first room reflection 1s marked as this would normally be used for loudspe
169. termination of an acoustic source One method relies on the approximation of the measured phase to the minimum phase Minimum Phase is checked under the View menu and minimum phase is then stored as an overlay Minimum is then unchecked which reveals the normal phase response Now by selecting Delay for Phase Estimation the phase can be approximated to the minimum phase by means of added delay This method should always give good results when applied around the crossover frequency n 4 n Dhnacs d 471 set p 3 s mmm Ee Cotes deter l or phisie lun 200 00 th i b Os 2M Cursor 20D 4 Hz 34 7 dag Fresuenty HE Woofer delay 1 2429msec Tweeter delay 1 1799msec Figure 6 6 12 Acoustic source determination by minimum phase alignment 118 speed up the process an initial reference value for the delay be obtained by setting a window from the 300th sample to the peak of the impulse response This can then be adjusted by fine tuning Frequency and phase responses for the simulation are then exported with the delay determined for each driver The difference between the two acoustic sources can be deduced from the difference between the two delay values the example shown above the difference 15 between 1 2429 and 1 1799 0 063msec or 2 17cm We see that the two methods described give slightly different results but this should have no
170. that we assume that we have no data on the microphone or its Sound card a preamplifier so we must use input Device 190496046 19 In 1 arbitrary values for now in the Output Device Go46 Go46 1 MC Out Audio Devices Setup WaveFormat 24be I7 Set the left preamp gain and the microphone sensitivity to 1 Amplifier Interface Lineln Sensitivity 3130 63 lLineOut Sensitivity mVpeak leftch 13130 83 mypeak left ch dies Ext left preamp gain 1 L R channel diff dB 0 05116 Ext right preamp gain 0 04808 Power amplifier gain 3 148 Microphone v Microphone Used On left Ch Sensitivity 1 Figure 5 2 3 Audio Devices Setup Save setup Load setup Cancel A two channel nearfield measurement is taken and the level adjusted to a measuring distance of 1 metre Ppp Prp PNF 20 log a 2d d measuring distance a driver membrane radius Pyr 20 log 12 7cm 2 2 100cm Prp PNF 29 97dB The measured nearfield level Pyr must therefore be corrected by 29 97dB to obtain the farfield level at 1 metre Figure 5 2 4 Procedure for estimating microphone sensitivity by nearfield measurement 47 FF Magnitude 2042 367 83V smoothed 1 24 ocd if ene i The image left shows the uncorrected nearfield level black line recorded by the microphone which 15 ar
171. the chart on its own use Ctrl C or Edit and in ARTA In the main UN window the copy function is also displayed as an icon The command opens the window below 173 Copy to Clipboard with Extended Information Enter text chat will be drawn on Ehe bottom of Ehe graph copy Hier kann ein beliebiger Text eingegeben werden Choose bitmap size smal Cancel Add Filename and date Save text Figure 8 1 2 Copy menu window This has four functions 1 A field for text entry this appears in the output directly below the chart 2 You can Add filename and date this outputs this information below the chart 3 You can Save text this can then be recalled and modified later 4 Choose bitmap size determines the size of the chart The options with defined size have a fixed width height ratio of 3 2 Click to copy the chart to the clipboard Cancel aborts the operation A sample output illustrating these options 16 shown in Figure 8 1 3 Note that the text 1s limited to 128 characters FR Magnitude dB v smoothed 1 6 act 20 50 100 200 500 1 Zk 5k 10k 20k Cursor 20 1 Hz 11 59 dB Frequency Hz Current file Speaker 1 pir 2006 10 04 23 02 09 Jetzt die nackte Grafik im Fenster im Format amall mit Fileinfo Figure 8 1 3 Sample chart output from ARTA 8 1 2 Working with overlays Overlays are temporary traces that can be displayed or hidden They facilitate
172. third order modulation distortion values 42 d3 1 98 and 1 49 respectively 6 Find the smallest value of U in the range between UsrAgr and Ugnp where either the harmonic distortion dt or the second or third order modulation distortion d2 or d3 equals 10 7 Determine the deflection Xmax corresponding to amplitude 010 Breitbandsystem mit Unterhangschwingspule I th Auslenkung mm Figure 9 1 3 Determination of linear displacement according to 12 Figure 9 1 3 shows the result In this example THD first reaches the 10 threshold as shown and thereby corresponds to Xmax 3 4mm see green arrows Note As of version 1 4 this procedure has been automated A detailed description can be found in ARTA Application Note No 7 2 190 191 10 ARTA Application Notes I MI IV V VI VII ARTA Measuring Box AP2 RLC Measurement With LIMP AP3 Why 64 Bit Processing Loudspeaker Freefield Response AP5 ARTA Chamber for the Lower End Microphone Calibration AP6 Directivity Measurements Estimation of Linear Displacement With STEPS AP8 Repetitive measurements with script language AutoIT 11 References l 10 11 12 13 14 M ller M TSP checken einfach gemacht Internet Technische Eigenschaften von Soundkarten im PC 2005 cited 2014 Sep 20 Available from https hifi selbstbau de grundla
173. uipment minus microphone connection and stand Avoid poor quality cables and connections Attention to detail in this respect will save much time and effort in the long run Note a well thought out and constructed system consisting of high quality equipment clearly marked connections and an ARTA measuring box will enable you to minimise the risk of errors and damage This is particularly important where the system has not been used for an extended period and familiarity with it has consequently diminished 6 1 1 Test leads Attention should be paid to the test leads as we are using small analogue voltages Signal quality will suffer as a result of noise if transmission over large distances is attempted To avoid earthing and interference problems the following guidelines 4 should be followed e Make cables as short as possible especially when using high impedance sources e If possible use double shielded cables e If necessary take an additional earth lead and place the shielding on one side only 65 Differentia Jource Siuelded twisted pair 4 17 4 source Miteigdeg twisted Single ended amplifie e Avoid ground loops Ensure that earth potentials are the same between the source and the measuring instrument soundcard Measure between earths beforehand with a meter both AC and DC e Do not place the signal cable near any interference source transf
174. uld carry out the following test before using it for calibration and measurements a Connect your multimeter to the left line output of the soundcard and set the 9 measuring range to 2 volts b Open the signal generator in STEPS Setup Measurements or Abweichung c Measure the output voltage of the soundcard at different frequencies from di 20Hz to 1000Hz and record the values T 100 1000 F in Hz Figure 1 5 Multimeter comparison 10000 Plot the values measured at each frequency either absolute or relative Figure 1 5 shows results for good average meter for a True RMS device Up to around 1000Hz variation with frequency is within 2 3 Thus the device is suitable for calibration of ARTA using preset values 500Hz see also section 5 1 1 1 3 Pin assignment for cables and connectors Unbalanced Groun d Shi elg Ground 5Shield STEREO JACK XLR Sleeve earth GROUND SHIELD Pin 1 earth GROUND SHIELD Tip Pin 2 Ring Pin 3 Figure 1 6 Cable pin assignment For a range of ready made cables see Cable Guy at the Thomann homepage www thomann de alle om Lange wahien 1 5 m Cinchstecker XLR Stecker male 3 pol 10 1 4 Measurement setup The measurement setups presented here are Single channel measurement Semi dual channel
175. umes that at frequencies well below the tuning frequency the port response approximately matches the speaker level see Figure 6 7 15 Note that at these very low frequencies it can be difficult to achieve smooth frequency responses however 130 0 120 0 125 0 125 0 120 0 120 0 115 0 115 0 110 0 110 0 105 0 105 0 100 0 100 0 950 95 0 1 900 sia 85 0 85 Il 10 20 50 100 200 10 20 50 100 200 Figure 6 7 16 Displacement method implementation in ARTA The port response must be reduced as in Figure 6 7 16 left blue arrow until it matches the lowest portion of the driver response see Figure 6 7 16 right In the example the required adjustment is approximately 6 5dB Thus the port level must be corrected by 100972 via Pir scaling The rest of the procedure is as described earlier 132 Pir Scaling Enter number or arithmetic expression to scale PIR 10 6 5 20 6 8 Load and Sum The Load and Sum function receives only cursory attention in the manual and is hard to find In ARTA the overlay function is able cache any number of individual frequency responses as already been described Figure 6 8 1 FR Magnitude dB V smoothed 1 24 act 15 0 20 0 25 0 30 0 35 0 50 0 100 Cursor 3471 1 Hz 24 25 dB Frequency Hz Figure 6 8 1 Preparation of 1 to n frequency responses in ARTA What if we wish to create a summed frequ
176. unbalanced connectors Figure 1 2 Professional soundcards have 6 3mm stereo jacks for balanced connection 6 3mm mono jacks for unbalanced connections and XLR connectors for balanced microphone inputs Figure 1 3 Standard stereo soundcards have three channels 1 2 3 while 5 1 surround sound systems have three more ports 4 5 6 on the motherboard One of the outputs is designed for use with headphones with 32 Ohm nominal impedance For soundcard testing a loopback connection from line in blue to line out green is made using a stereo cable with 3 5mm jack plugs The line in input impedance of most PC soundcards is between 10 and 20 kOhms 1 Line in AUX input stereo blue 2 Line out headphones front speaker stereo green 3 Mic In microphone input mono pink 4 Out centre and subwoofer orange 5 Out rear speakers stereo black 6 Out side speakers stereo grey Figure 1 1 Audio connections on a PC motherboard for a 5 1 surround sound system Laptops and notebooks usually carry only a stereo headphone output and a mono microphone input This configuration 15 severely limiting for measurement purposes because the mono input channel does not permit dual channel use or impedance measurements RCA RCA 9 pin connector inputs outputs to breakout box Figure 1 2 PCI card with RCA connectors e g M Audio Audiophile 24 96 Examples of plug in cards include the Basic Terratec 24 96 or the M
177. up is the overdriving of the soundcard To avoid this ensure that your audio devices and sounds are adjusted appropriately access via Control Panel Windows Mixer depending on your operating system The following shows the setup for Windows XP n b example in German Systemsteuerung Datei Bearbeiten Ansicht Favoriten Extras Zurlick 27 5 Ordner Adresse gt Systemsteuerung zd Wechseln zu ower ys Graphics ve V Systemsteuerung gt Zur Kategorieansicht wechseln D s gt M Audio Transit USB Maus Metzwerkinstallatio Netzwerkverbindun Siehe auch 4 3 2 Windows Update Ordneroptionen Regions und Scanner und Kameras Hilfe und Support Sprachoptionen ro Schriftarten Sicherheitscenter Software Sounds und Audiogerate Sprachein ausgabe Symantec LiveUpdate Taskleiste und Startmen ay 4i Telefon und Tragbare Verwaltung Modemoptionen Mediengerate Windows Firewall Andert das Soundschema des Computers oder konfiguriert die Einstellungen fiir die Lautsprecher und 13 Eigenschaften von Sounds und Audioger te fl Summe 3 Lautst rke Sounds Audio Stimme Balance 9 J Lautstarke Soundwiedergabe Standardgerat Crystal WDM Audio Folgende ancesgen 7 Summe Da Soundaufnahme Standardgerat 4 Ciystal WDM Audio MIDI Musikwiedergabe Uu Standardgerat lt Micr
178. v gemonhed 74 Burs Decay x Sp 100 50 50 9 v cquemos CHE Cursor 202 Hr 21 2070 Fraquency 3 Frequency Hz MOF iG BF 15 202 Fie dgi szDoB W z1 16 21 10 NOF1GHIDickEpand 4 pi 2005 04 21 15 34 rez PDF T 2 WORTH Dike pared 4 pr 100 200 500 ik k d k XX B 100 2m Kleber Sperrholz Figure 7 3 7 Decay behaviour of different material combinations 16 Weichfaser softboard Kleber glue Sperrholz plywood Fliese tiles 169 Loudspeaker Membrane resonances are of particular interest in loudspeaker drivers because impedance measurements are very sensitive to them Magnitudefahms Impedance Phase 90 0 23 2 45 0 21 4 0 0 18 5 45 0 17 8 90 0 16 0 Avast 14 2 12 4 10 6 8 5 7 0 20 50 100 200 500 Zk 5k 10k 20K Cursor 5 0 Hz 7 83 Ohm 15 0 deg FrequencytHz 139 free air Figure 7 3 8 Impedance trace of a tweeter Figure 7 3 8 shows the impedance profile of a classic tweeter the KEF B139 The membrane of this driver has resonance problems between 700Hz and 2kHz Figure 7 3 8 shows measurements taken with different sensors microphone blue accelerometer red and laser black Both the microphone and the accelerometer are suitable for the detection of diaphragm resonances 170 FE hMagnitude dB 200 B3 smoothed 1 24 act
179. ve options such as the Behringer ECM8000 with frequency response AR compensation are suitable for loudspeaker s T b P WM design ere i B When the microphone is to be used at higher L output levels or to measure distortion a more AM expensive model may be required 77 Medium priced recommendations 6150 6300 NS 1 d include the Beyerdynamic and the Audix 1 see also section 5 2 1 and the STEPS Handbook Figure 1 4 Polar radiation of the Audix TM1 DIY microphones based on the Panasonic WM61A electret capsule provide yet another option See the ARTA Hardware and Tools manual http www artalabs hr support htm for more on this including notes on construction Microphone preamplifier Depending on the microphone and or soundcard different extras are required If you have chosen a soundcard with integrated preamplifier and 48V phantom power you are ready to go If you have a standalone soundcard you will need a separate preamplifier ideally with phantom power The Monacor MPA 102 is recommended because it is currently the only affordable model with stepped and therefore reproducible gain control see Figure 3 5 If you decide to go down the DIY route you can use either the soundcard s microphone input see also section 1 4 or a preamplifier kit off the internet Ralf Grafe s website http www mini cooper clubman de html hifi project
180. wind and other noise make measurements very difficult and this method is therefore only applicable in benign climates with perfectly calm weather Nevertheless anybody with a reasonably peaceful garden might attempt freefield measurements Even if the tower is only three or four feet high use of windowing might make measurements all the way down to a cut off of 40 50Hz possible see Section 6 4 Anechoic chamber If freefield measurements are to be carried out without interference from the weather and background noise at any time an anechoic chamber is needed Figure 6 3 4 The walls of an anechoic chamber are lined with a sound absorbing material usually glass or mineral wool in order to achieve the fullest possible sound absorption across the entire frequency range of interest The lining is also often arranged in wedge shapes as shown in Figure 6 3 4 10 An anechoic chamber can be full or half space In a full room all boundaries are lined with absorbent material and the room 1s accessed via a recessed or tensioned wire mesh floor For half space the room has a normal floor and 1s thus accessible with no restrictions The best anechoic chambers are rooms within rooms decoupled from the rest of the building by mounting on springs This type of construction minimises the transmission of sound transmitted through the building itself and through the air and allows for an environment virtually free of ambient noise 85 By
181. wing analysis is based on a 96kHz Dirac pulse with a 3rd order Butterworth highpass filter with a corner frequency of 800Hz Figure 6 2 13 Impulse responsa Vv B 0 18 0 02 0 31 6 35 B 73 1 12 7 51 7 BB rms Cursor rv 9 Ems CU Figure 6 2 13 Impulse response 3rd order highpass 78 FR Magnitude dB Viv smoothed 1724 act TT Phase averaged in 1 24 oct 10 20 50 100 200 S00 dk Sk 1 k 20k 10 20 50 100 200 S500 dk 2k Sk 10k 20k Cursor 10 1 Hz 81 58 dB Frequency Hz Cursor 10 1 Hz 2 3 deg Frequency Hz BLIT HP 3rd Order BUT HP 3rd Order Figure 6 2 14 Frequency and phase 3rd order highpass The highpass response tweeter model shown in black is very close to the theoretical or ideal response but shows significant deviation in terms of both amplitude and phase below 100Hz because of the computational limitations of the software Note that phase is apparently more sensitive in this respect than amplitude this is used to advantage in procedures described in Chapter 5 GEMENS As shown on the left the View menu in the Smoothed File Overlay Edit View Smoothing Frequency Response window shows the available options v Magnitude in ARTA for depicting and manipulating phase and group FR Mac Magn Phase delay 10 0 Phase Group delay Figure 6 2 15 right panel shows the frequency and phase response Magn Phase of the modelled tweeter The Unwrap Phase i phas
182. y LineUut Sensitivity mv peak left ch 1376 m peak left ch Ext left preamp gain 3 243 channel diff dB 0 0156328 Ext right preamp gain 0 0923 Power amplifier qain 1 Microphone iw Microphone Used On Left Ch Sensitivity mv 9 5766 Save setup Load setup Cancel o Figure 3 4 Audio devices setup menu for and STEPS Note the ARTA calibration menu specifies the expected gain gain as an absolute and not a dB value It is calculated as Gain IO dB level 20 20 uy ma 5 4 PHANTOM 1 HPIO0Hz V MicPreAmp 10 x dB 20 20 dB 10 40 dB 100 60 dB 1000 FR Magnitude dB re 83 smoothed 1 3 act a Bil Ni Frequency range 20 20000 Hz Gain MIC INPUT 20 7098 switchable STEREO LINE 0dB Input sensitivity for 1V atthe output 0 3 150 mV switchable Input impedance MIC INPUT 2 2kQ STEREO LINE 10kQ Phantom power 24V Outputs PREAMP OUT 1VA2V 1000 STEREO LINE 1V 6 V max 100 Q High pass filter 100 Hz 3dB 12dB oct Low pass filler 12kHz 3dB 12dB oct SIN ratio gt 66dB 80dB WH EELEE TEE lt 0 01 Power supply 15V via supplied AC AC adaptor 230 V 50 Hz or four 9V transistor batteries A

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