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Trane TRG-TRC007-EN User's Manual

1. sound pressure dB ref 20 Pa 2 bre 63 125 250 500 1 000 2 000 4 000 8 000 octave band frequency Hz As mentioned previously the total sound heard by the receiver is the sum of sounds from multiple sources following multiple paths After each path is modeled to determine its contribution to the sound pressure level at the receiver location the paths must be summed to complete the model While separating the individual paths is necessary for modeling a secondary benefit is that the magnitude of the various paths can be compared In this example sound travels from a single source to the receiver along four separate paths supply airborne supply breakout return airborne and transmission through the adjacent wall By modeling these four paths independently you can see that the supply airborne path contributes to the total sound pressure level in the space much more than the other three paths In fact when the sounds due to all four paths are logarithmically summed the total sound heard by the receiver is nearly the same as the sound due to the supply airborne path alone This would indicate that if the sound pressure level in the space is too high the designer should focus first on reducing the sound due to the supply airborne path Reducing the sound due to the return airborne path without addressing the supply airborne path would have no effect on the total sound pressure le
2. File Number E AV FND TRG TRC007 1101 EN Supersedes New Stocking Location La Crosse Since Trane has a policy of continuous product and product data improvement it reserves the right to change design and specifications without notice
3. 0ceeeeeeeeeeeeeeeeeteeeeeees 48 HVAC Equipment Sound Rating c0ccccceeeeeeeees 54 PRO dec ad 63 CR bess ges AAN A 68 Answers errn 70 Glossary 71 iii S TRANE TRG TRC007 EN S TRANE period one Fundamentals of Sound Fundamentals of HVAC Acoustics period one Fundamentals of Sound People have become increasingly conscious of acoustics as a component of a comfortable environment Sound levels both indoor and outdoor can be affected to varying degrees by HVAC equipment and systems The degree to which the HVAC system affects the sound at a particular location depends on the strength of the sound source and the environmental effects on the sound as it travels from that source to the listener TRG TRC007 EN S TRANE period one Fundamentals of Sound What is Sound A Audible emissions resulting from vibration of molecules within an elastic medium A Generated by vibrating surface or movement of a fluid A In buildings it may be airborne or structure borne 4 Noise is unwanted sound What is Sound Sound is the audible emissions resulting from the vibration of molecules within an elastic medium It is generated by either a vibrating surface or the movement of a fluid In the context of building HVAC systems this elastic medium can be either air or the building structure For structurally borne sound to become audible however it must first become airborn
4. ARI Standard 260 A Uses reverberant room method B z 3 2001 A Tests entire air STANDARD for handler not just fan sowo RATINI A All common DUCTED AR configurations and CONDITIONING components included EQUIPMENT A Includes effects of secondary sound sources standard 260 The objective of ARI Standard 260 2001 Sound Rating of Ducted Air Moving and Conditioning Equipment is to deliver sound data that accurately represents the acoustical impact of the air handling equipment after it is installed This standard uses the reverberant room method to measure the sound generated by the entire air handler not just the fan As mentioned a fan performs differently inside an air handler than it does in a stand alone application The air handler casing generally changes the airflow patterns at the fan inlet and discharge openings This effect is the major reason for the difference between fan only sound data and the actual sound produced by the air handler after it is installed To eliminate such inaccuracies ARI 260 requires that the entire air handler be tested in all of the configurations in which the equipment is commonly applied in the field This involves the four test configurations discussed in Figure 64 as well as various combinations of options such as inlet and discharge plenums different types of filters dampers coils and so forth Unlike the other methods ARI 260 requires that secondary s
5. Air Conditioning Clinic Fundamentals of HVAC Acoustics One of the Fundamental Series TRG TRC007 EN BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE TRANE Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 BUSINESS REPLY MAIL FIRST CLASS MAIL PERMIT NO 11 LA CROSSE WI POSTAGE WILL BE PAID BY ADDRESSEE TRANE Attn Applications Engineering 3600 Pammel Creek Road La Crosse WI 54601 9985 NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES NO POSTAGE NECESSARY IF MAILED IN THE UNITED STATES Comment Card We want to ensure that our educational materials meet your everchanging resource development needs Please take a moment to comment on the effectiveness of this Air Conditioning Clinic Fundamentals of HVAC Acoustics Level of detail circle one Too basic Just right Too difficult One of the Fundamental Series Rate this clinic from 1 Needs Improvement to 10 Excellent TRG TRC007 EN Booklet usefulness 1 2 3 4 5 6 7 8 9 10 Slides illustrations 1 2 3 4 5 6 1 8 g 10 Presenter s ability 1 2 3 4 5 6 7 8 9 10 Training environment 1 2 3 4 5 6 7 8 g 10 Other comments About me Type of business Job function Optional name phone address Give the completed card to the presenter or drop it in the mail TRANE Thank you Trane An American Standard Company www t
6. perfectly reflective Some of the sound is reflected by these surfaces but a portion of the sound is absorbed or transmitted An understanding of how sound behaves in a semireverberant field is important when taking sound measurements The characteristics of the sound field change with distance when a small sound source is placed in the center of a room Close to the source in the near field sound measurement is unpredictable Near the wall in the reverberant field the reflected sound begins to add to the sound coming directly from the source The reduction in sound level due to the distance from the source tends to be cancelled out by the addition of the sound reflecting off the wall This results in a near constant sound pressure level near the wall In the semireverberant field sound behaves similarly to how it would in a free field The sound level will decrease as the distance from the source increases but not as much as it would in an ideal free field The construction of the room plays a significant role in determining what portion of the room behaves as a reverberant field and what portion behaves as a semireverberant field Small rooms with hard reflective surfaces behave similarly to reverberant rooms This description often fits a mechanical equipment room that is constructed of concrete and is small with respect to the size of the sound source TRG TRC007 EN 53 amp ranw 54 period four Equipment Sound Rating
7. proved that sound levels predicted by the algorithm could vary from actual measured readings by as much as 10 dB in a given octave band ASHRAE removed the fan prediction algorithm from the handbook stating that The sound power generated by a fan performing at a given duty is best obtained from manufacturers test data taken under approved test conditions 1999 ASHRAE Handbook Applications Chapter 46 page 4 Another sometimes used rating method measures actual sound data for the fan by itself then uses acoustical equations to predict the effects of the cabinet coils filters and other components that make up the air handler These prediction algorithms vary from manufacturer to manufacturer and since they are usually proprietary it is difficult to judge their accuracy This prevents designers from effectively comparing data between manufacturers or applying that data in an analysis AMCA Standard 300 1996 Reverberant Room Method for Sound Testing of Fans defines the test methodology for collecting fan only sound power data However it has also been used to test entire air handlers Just because the data was taken in accordance with AMCA 300 does not indicate whether the sound data is for the fan only or for the entire air handler This leaves the designer to determine whether the data reflects the entire air handler or as intended just the fan by itself TRG TRCOO7 EN S TRANE period four Equipment Sound Rating
8. sound as well as its loudness However because most HVAC equipment manufacturers do not have sound power data for the 16 Hz and 31 5 Hz octave bands it is difficult to predict the sound pressure levels in these octave bands Finally both sound power levels and sound pressure levels can be described using either full or one third octave bands Octave band sound power data is commonly used for describing the sound generated by HVAC equipment It can also be used to describe the sound in either indoor or outdoor environments TRG TRCOO7 EN TRG TRCOO07 EN period five Review Review Period Three source T supply airborne Period Three walked through the steps of an acoustical analysis including setting the design target for the indoor or outdoor environment and performing a source path receiver analysis This method of analysis traces sound from the source to the location where we want to predict the sound the receiver How the sound travels between the source and the receiver and everything it encounters as it travels along the way constitutes the path Remember E One piece of equipment may contain several sound sources E Sound may travel from a single source to the receiver along multiple paths m The total sound heard by the receiver is the sum of all the sounds from various sources that travel along several paths Computer software analysis tools are available to aid in performing this type of calculation
9. 2 1 Lp1 6 TRG TRCO07 EN 49 50 TRANE period four Equipment Sound Rating Distance Correction in a Free Field R k Ly 95 dB Ly 83 dB air cooled chiller us o r 120 ft line 36 6 m In practice this equation is commonly used to determine how loud a piece of equipment will be at a given distance For example the manufacturer of an air cooled chiller lists the sound pressure level of the chiller as 95 dB at a distance of 30 ft 9 1 m from the chiller This equation can be used to estimate the sound pressure level at a lot line which is 120 ft 36 6 m from the chiller In this example the sound pressure level at the lot line is 83 dB L 95 dB 20 logo 120 ft 30 ft 83 dB Lp2 95 dB 20 logo 36 6 m 9 1 m 83 dB This equation is only valid for sound pressure It cannot be used to convert sound power L to sound pressure L Also it is only valid in a free field environment TRG TRCOO7 EN S TRANE period four Equipment Sound Rating Near Field near field The near field is an area adjacent to the source where sound does not behave as it would in a free field Most sound sources including all HVAC equipment do not radiate sound in perfectly spherical waves This is due to the irregular shape of the equipment and different magnitudes of sounds radiating from the various surfaces of the equipment These irregularities cause pressure wave interac
10. 8 True NC 65 NC 60 NC 55 NC 50 NC 45 NC 40 NC 35 NC 30 NC 25 NC 20 NC 15 9 Receiver sound correction or room effect 10 Reverberant A weighted sound pressure level is better for describing outdoor sound Noise Criteria rating is better for describing sound in an office or classroom 11 ARI Standard 260 2001 Sound Rating of Ducted Air Moving and Conditioning Equipment TRG TRCOO7 EN TRG TRC007 EN S TRANE Glossary absorbed sound Sound energy that strikes a material and is converted from sound energy to heat energy within the material absorption coefficient The ratio of the sound energy absorbed by the material to the total sound energy incident upon the surface of that material AMCA Air Movement and Control Association International www amca org ARI Air Conditioning amp Refrigeration Institute www ari org ASHRAE American Society of Heating Refrigerating and Air Conditioning Engineers www ashrae org attenuation The reduction in the sound level as it travels along the path from a source to the receiver A weighting A single number used to describe sound It uses weighting factors by octave band to approximate human response to sound in the range where no hearing protection is needed It is most appropriately used for low volume or quiet sound levels and is expressed as dBA broadband sound Sound energy that occurs at many frequencies usually covering the entire audi
11. If we use the quietest perceptible sound as the reference value this ratio would range from 1 to 1 000 000 000 Converting this arithmetic range to a log scale yields a range of 0 to 9 This unitless result is described in terms of bels Multiplying by ten results in the more commonly used broader range of 0 to 90 decibels dB TRG TRC007 EN S TRANE period one Fundamentals of Sound Equation for Sound Power sound power W Ly 10 logs TE When a reference value is established and placed in the denominator of the ratio the dB can be calculated for any value entered into the numerator The reference value used for calculating sound power level is 1 picowatt pW or 10 watts Therefore sound power level L in dB is calculated using the following equation sound power watts Lw 10109 C 10 watts Equation for Sound Pressure sound pressure uPa L 20 log 20 Pa The reference value used for calculating sound pressure level is 20 micropascals Pa or 2 x10 Pa Therefore sound pressure level Lp in dB is calculated using the following equation TRG TRC007 EN 13 S TRANE 14 period one Fundamentals of Sound _ sound pressure ure sound pressure il Lp 20 logs 20 Pa or 10 log 20 Pa Again these reference values can be considered the threshold of hearing The multiplier 20 is used in the sound pressure level equation instead of 10 because sound power is proportional to the square of s
12. Rating HVAC Equipment HVAC Equipment Sound Rating As mentioned earlier because sound pressure is influenced by the surroundings often the best way for an equipment manufacturer to provide sound data is to provide sound power levels Sound power levels for many types of HVAC equipment are determined in an acoustics laboratory usually by the manufacturer Sound power levels are determined by measuring sound pressure levels in a test facility with known acoustical characteristics and adding back any environmental effects attributed to the surroundings Formal written standards qualify such test facilities and methods in order to promote uniformity of data between different manufacturers across the industry This allows for objective comparisons of similar equipment The two most common methods of determining the sound power of HVAC equipment are the reverberant room and free field methods TRG TRCOO7 EN S TRANE period four Equipment Sound Rating Reverberant Room Method E The most common test method for HVAC equipment is the reverberant room method The objective of a reverberant room is to create a uniform or diffuse sound field by reflecting and mixing the sound waves The walls floor and ceiling of the reverberant room are hard in order to cause multiple reflections of sound waves In this environment the sound pressure is essentially the same at all locations within the room Sound pressure levels are me
13. Trane literature order number FND AM 5 Trane Acoustics Program TAP ASHRAE Handbook Fundamentals chapter 7 2001 ASHRAE Handbook Applications chapter 46 1999 A Practical Guide to Noise and Vibration Control ASHRAE 1991 Application of Manufacturer s Sound Data ASHRAE 1998 Algorithms for HVAC Acoustics ASHRAE 1991 Sound and Vibration Design and Analysis National Environmental Balancing Bureau NEBB 1994 Visit the ASHRAE Bookstore at www ashrae org and the NEBB Bookstore at www nebb org For information on additional educational materials available from Trane contact your local Trane sales office request a copy of the Educational Material price list Trane order number EM ADV1 or visit our online bookstore at www trane com bookstore TRG TRCOO07 EN 67 S TRANE 68 Quiz Questions for Period 1 1 What unit of measure is used to describe frequency 2 Define a tone 3 Sound power or pressure is what our ears hear and is influenced by the surroundings Questions for Period 2 4 Which of these two single number descriptors A weighted sound pressure level or Noise Criteria NC rating is better for describing outdoor sound 5 Which of the above mentioned descriptors is better for describing sound in an office space or classroom 6 What is the NC rating of a space with the following sound pressure levels octave center measured band frequency
14. for an interior space either an NC value or an RC value is used To aid HVAC system designers the American Society of Heating Refrigerating and Air Conditioning Engineers ASHRAE recommends target RC ratings for various types of spaces and encourages the use of the RC rating method whenever the space requires a neutral unobtrusive background sound Figure 34 includes an excerpt from the ASHRAE Handbook Applications Table 43 in Chapter 46 of the 1999 edition As mentioned earlier A weighting is also used in many hearing protection safety standards for industrial environments These standards generally take the form of a maximum A weighted sound pressure level at a specified distance from the piece of machinery TRG TRC007 EN S TRANE period three Acoustical Analysis Setting a Design Goal When defining the acoustical design goal for an outdoor environment to meet a local noise ordinance for example the A weighted scale is typically used This generally takes the form of a maximum A weighted sound pressure level at the lot line of the property More sophisticated noise ordinances may specify maximum sound pressure levels for each octave band and possibly a restriction on other characteristics of the sound For example a sound ordinance may define that a tone is present when the sound pressure level in any one third octave band exceeds the arithmetic average of the sound pressure levels in the two neighboring one thi
15. loss of the door is determined by the manufacturer by taking measurements in a special laboratory and expressing the results as sound power It is also expressed in terms of dB reduction TRG TRCO07 EN 43 S TRANE period three Acoustical Analysis Absorption absorbed sound ener incident W ay sound energy SZIN W v reflected W sound energy transmitted sound energy RUN er Absorptive materials work by converting acoustical energy into heat energy The absorbed energy W is the portion of the incident sound energy W that is neither transmitted through the material nor reflected off the material The absorptivity of a material depends on several factors including thickness frequency of the sound and whether there is a reflective surface located behind the absorptive material Materials that are porous such as open cell foam or fibrous such as fiberglass insulation are more absorptive than materials that are smooth and dense such as sheet metal or gypsum board Increasing the thickness of a material and installing a reflective surface behind the material both increase its absorptivity It is also important to note that absorption is dependent on frequency High frequency sound is more easily absorbed than low frequency sound because it has a shorter wavelength and more cycles occur within the thickness of the absorptive material The absorptivity of a material is typically described in
16. sound pressure Hz dB ref 20 Pa 1 63 48 2 125 47 3 250 45 4 500 39 5 1 000 35 6 2 000 27 7 4 000 19 8 8 000 14 TRG TRCO007 EN Quiz sound pressure dB ref 20 Pa 63 125 250 500 1 000 2 000 4 000 8 000 octave band frequency Hz Questions for Period 3 7 True or False Sound can only travel from a source to the receiver along one path 8 True or False One piece of HVAC equipment may contain several sound sources 9 What term is used to describe the reduction in sound that enters a room as it travels to the receiver It is influenced by distance and the absorptive and reflective characteristics of the surfaces and furnishings in the room Questions for Period 4 10 A free or reverberant sound field is characterized by a uniform or diffuse sound field where the sound pressure is equal at all points in the field 11 What recent ARI standard which rates the sound due to ducted air moving and conditioning equipment requires testing of the entire air handler not just the fan TRG TRC007 EN 69 S TRANE 70 Answers 1 Hertz cycles per second 2 A sound at a single frequency A sound at a narrow band of frequencies that is significantly greater than the sound at adjacent frequencies would be similar to a tone Sound pressure O a A O NC 35 see Figure 75 sound pressure dB ref 20 Pa 63 125 250 500 1 000 2 000 4 000 8 000 octave band frequency Hz 7 False
17. was demonstrated with the previous example The total sound heard by the receiver is the sum of all the sounds from various sources that travel along several paths TRG TRC007 EN TRG TRCOO07 EN S TRANE period three Acoustical Analysis Modeling Sound Paths elbow eg gt E PA p Si source f Lg straight duct 4 y LS diffuser tke a receiver lt 7 Sound Path Modeling When all the paths have been identified they can be individually modeled to determine the contribution of each to the total sound heard by the receiver Sound path modeling studies how sound from a source changes on its way to a receiver The pieces that make up the path from source to receiver can be called elements of the path Returning to the air handler example one path that sound travels from the air handling unit source to the person in the office receiver is to follow the conditioned air supplied to the space In addition to the source and receiver the elements of this path include the components of the air distribution system such as straight pieces of duct possibly duct silencers elbows junctions and diffusers The path also includes the acoustical characteristics of the occupied space such as its size floor coverings furnishings and wall construction 37 S TRANE 38 period three Acoustical Analysis 80 Example of Multiple Sound Paths suppl E airbor
18. C 25 ENC 20 ET NC 15 63 125 250 500 1 000 2 000 4 000 8 000 octave band frequency Hz The following steps describe how to calculate an NC rating 1 Plot the octave band sound pressure levels on the NC chart 2 The highest curve crossed by the plotted data determines the NC rating Although the NC curves are popular and easy to use they do have a few shortcomings Specifically they do not account for the tonal nature and relative magnitude of each octave band Figure 27 shows octave band data measured in an open plan office space and plotted on an NC chart The resulting value NC 39 is generally considered to be acceptable for this type of environment Notice that this NC value is set by the 63 Hz octave band and the sound drops off quickly in the higher octave bands In this particular example sound generated by the air handling unit travels through the ductwork breaks out through the duct walls and radiates into the office area To achieve the desired NC level two layers of sheet rock were added to the exterior surface of the duct to block the low frequency sound Unfortunately because high frequency sounds are much more easily attenuated than low frequency sounds the upper octave bands are now over attenuated Although an objective analysis deems the resulting NC 39 sound level acceptable in this type of open plan office space most listeners in the space would probably perceive this u
19. Decibel measured value dB 10 log reference value Sounds encompass a wide range of volumes or levels The loudest sound the human ear can hear without damage due to prolonged exposure is about 1 000 000 000 times greater than the quietest perceptible sound A range of this magnitude makes using an arithmetic scale cumbersome so a logarithmic scale is used instead The measurement of sound level is expressed in terms of decibels dB a dimensionless quantity A decibel is a calculated value based on the ratio of two quantities It is defined as ten times the logarithm to the base ten log of the measured quantity divided by the reference quantity The reference quantity must be specified to prevent confusion regarding the magnitude of the ratio dB 1010949 aes ad reference value TRG TRC007 EN 11 S TRANE 12 period one Fundamentals of Sound Logarithmic Scale ratio log 10 x log 1 0 0 10 1 10 100 2 20 1 000 3 30 10 000 4 40 100 000 5 50 1 000 000 6 60 10 000 000 7 70 100 000 000 8 80 1 000 000 000 9 90 A logarithm is the exponent power of the base In this case the base is ten For example the log of 10 or 10 equals 1 the log of 100 or 102 equals 2 and the log 0 of 1 000 000 000 or 10 equals 9 As mentioned earlier the loudest sound the human ear can hear without damage due to prolonged exposure is about 1 000 000 000 times greater than the quietest perceptible sound
20. akes it difficult to predict sound pressure levels in these octave bands TRG TRC007 EN TRG TRCO07 EN S TRANE period two Sound Perception and Rating Methods Phon and Sone loudness level phons 120 110 00 sound pressure dB ref 20 uPa 20 50 100 200 500 1 000 2 000 5 000 10 000 frequency Hz The phon is another descriptor used to indicate loudness with a single number The loudness of a sound expressed in phons is equal to the sound pressure level of a standard sound at 1 000 Hz that is considered equally loud For example a sound pressure level of 40 dB at 1 000 Hz is considered to have a loudness of 40 phons Any sound that falls on this same loudness curve at any frequency would also be described as having a loudness equal to 40 phons While the phon scale is logarithmic the sone is the linear equivalent to the phon In principle the sone scale is linear when compared to the response of the human ear For example two sones is twice as loud as one sone and half as loud as four sones While the phon and sone scales are not widely used some HVAC equipment primarily non ducted fans and power ventilators is still rated in sones AMCA Standard 301 Methods for Calculating Fan Sound Ratings from Laboratory Test Data provides a method for calculating the sone rating from octave band data Use caution when comparing equipment based on sones Multiple methods exist for calculating a sone rating and they pro
21. analysts from the calculation intensive equations Also computer programs make it easier to perform tradeoff or what if analyses Examples may include determining the effects of using a duct silencer changing the construction of the equipment room wall adding absorptive materials to a ceiling or placing a barrier wall between an outdoor sound source and the property lot line TRG TRC007 EN S TRANE period three Acoustical Analysis Attenuation and Regeneration heat pump return source duct return air grille diffuser Terms Used in Sound Path Modeling This section introduces several terms that are fairly specific to the science of acoustics Attenuation refers to the reduction in sound level as sound travels along the path from a source to a receiver It is typically used to refer to the reduction of sound as it travels through a duct system Straight ducts elbows junctions and silencers are examples of elements that attenuate sound Regenerated sound results from components of the duct system that create turbulence in the air stream Turbulence is caused by an abrupt change in airflow direction or velocity with a corresponding static pressure loss Regenerated sound increases with air velocity or when the air is forced to make sharp turns Elbows junctions diffusers silencers and dampers are all examples of elements that regenerate sound Notice that some elements can both attenuate and rege
22. asured in the reverberant room and used to calculate sound power levels for the piece of equipment The reverberant room method is commonly used to determine the sound power of fans air handlers compressors in room air conditioners terminal equipment such as fan coils and VAV boxes and diffusers TRG TRC007 EN 55 amp Trane period four Equipment Sound Rating Free Field Method The free field method is commonly used for HVAC equipment that is too large to be tested in a reverberant room This includes water chillers cooling towers and the outdoor sound from packaged rooftop air conditioners and air cooled condensers This type of equipment is placed on a hard surface in an anechoic or sound absorbing room or on a large parking lot outdoors This approximates the characteristics of sound in a free field above a reflecting plane That is the sound pressure waves travel evenly in a hemispherical pattern away from the equipment Sound power levels are determined by measuring sound pressure levels on an imaginary hemispherical surface surrounding the equipment 56 TRG TRC007 EN TRG TRCO07 EN S TRANE period four Equipment Sound Rating Industry Standards SAA AIR CONDITIONING amp REFRIGERATION INSTITUTE Formal standards are written by industry organizations to promote uniformity of data between different manufacturers Air Conditioning amp Refrigeration Institute ARI the Air Movement and Cont
23. based on frequency but is also dependent on sound pressure level and composition Pitch is not measured but is described with terms like bass tenor and soprano TRG TRCOO7 EN 3 TRANE period one Fundamentals of Sound Wavelength 4 one cycle __ speed of sound wavelength frequency The wavelength of the sound is the linear measurement of one complete cycle The wavelength and frequency of a sound are related by using the following equation speed of sound wavelength frequency The speed of sound transmission is a physical property of the medium For air the speed varies slightly with temperature change Because the temperature range encountered in the study of HVAC acoustics is relatively small the speed of sound can be considered a constant 1 127 ft s 344 m s For example sound traveling through the air at a frequency of 200 Hz has a wavelength of 5 6 ft 1 7 m 1 127 ft s wavelength 200Hz 5 6 ft _ 344 m s _ wavelength 00H 1 7m TRG TRC007 EN S TRANE period one Fundamentals of Sound Broadband Sound amplitude time The wave form shown in Figure 5 represents sound occurring at a single frequency This is called a pure tone A pure sinusoidal wave form however is very rare in HVAC acoustics Typically sounds are of a broadband nature meaning that the sound is composed of several frequencies and amplitudes all generated at the same time F
24. become airborne sound power The acoustical energy emitted by the sound source It is not affected by the environment sound pressure An audible pressure disturbance in the atmosphere that can be measured directly Its magnitude is influenced not only by the strength of the source but also by the surroundings and the distance from the source to the listener Sound pressure is what our ears hear and what sound meters measure source path receiver model A systematic approach to analyzing the sound in a space It traces sound from the source to the location where we want to predict the sound the receiver How the sound travels between the source and 73 S TRANE 74 Glossary the receiver and everything it encounters as it travels along the way constitutes the path tone A sound in a single frequency A sound in a narrow band of frequencies that is significantly greater than the sound at adjacent frequencies would be similar to a tone transmitted sound The sound that travels though a barrier transmission loss TL A term used to measure the effect of a barrier on reducing the amount of transmitted sound It is the ratio of sound power on the receiver side of a barrier to the sound power on the source side TRG TRC007 EN TRANE Trane An American Standard Company www trane com For more information contact your local district office or e mail us at comfort trane com O E Literature Order Number TRG TRC007 EN
25. ble range center frequency Single frequency used to identify an octave band It is calculated by taking the square root of the product of the lowest and highest frequencies in the octave band decibel dB A dimensionless ratio of two quantities that is used to describe both sound power and sound pressure It is defined as ten times the logarithm to the base ten logi9 of the measured quantity divided by the reference quantity dynamic insertion loss The sound insertion loss of a duct silencer with air flowing through it free field A homogeneous isotropic medium free from boundaries An example of a free field over a reflecting plane would be a large open area void of obstructions like a parking lot or meadow free field method A common method for testing HVAC equipment that is too large to be tested in a reverberant room such as water chillers and cooling towers The equipment is placed on a hard surface on a large parking lot outdoors to approximate the sound conditions in a free field above a reflecting plane The sound pressure waves travel evenly in a hemispherical pattern away from the equipment Sound power levels are determined by measuring sound pressure levels on an imaginary hemispherical surface surrounding the equipment frequency The number of cycles or oscillations per second of a wave in periodic motion Expressed in hertz 71 S TRANE 72 Glossary hertz Hz The unit of measure for frequency One her
26. ce but also by the surroundings and the distance from the source to the listener Sound pressure is what our ears hear and what sound meters measure 63 amp Trane 64 period five Review Review Period Two 4 A weighting Outdoor environments and hearing protection 4 Noise criteria NC Indoor environments 4 Room criteria RC Indoor environments sound pressure dB ref 20 Pa Also describes sound E EN character a 31 5 63 125 250 500 1 000 2 000 4 000 a Octave band data octave band frequency Hz Period Two discussed how the human ear perceives sound As a selective sensory organ the human ear is more sensitive to high frequencies than to low frequencies The sensitivity of the human ear at a particular frequency also changes with loudness Many single number rating systems have been developed over the years and each has its advantages and drawbacks This clinic focused on some of the more commonly used rating systems including A weighting Noise Criteria NC and Room Criteria RC A weighting is typically used to describe the sound in outdoor environments and for determining whether or not hearing protection is required in certain industrial environments Noise Criteria and Room Criteria are used to describe the sound in indoor environments The RC method has the added feature of describing the character or quality of the
27. d data cannot be derived from any of the single number ratings TRG TRC007 EN S TRANE period three Acoustical Analysis Fundamentals of HVAC Acoustics period three Acoustical Analysis The primary acoustical design goal for an HVAC system is to achieve a background noise level that is quiet enough so that it does not interfere with the activity requirements of the space and is not obtrusive in sound quality What is considered acceptable varies dramatically with the intended use of the space Obviously a factory has less stringent acoustical requirements than a church while an office has a different set of requirements altogether Therefore the acoustical design goal depends on the required use of the space TRG TRCO07 EN 29 S TRANE 30 period three Acoustical Analysis Setting a Design Goal room type RC N criteria hotels motels guest rooms 25 to 35 banquet rooms 25 to 35 libraries 30 to 40 Office buildings open plan offices 30 to 40 public lobbies 40 to 45 performing arts theaters 25 max practice rooms 35 max schools small classrooms 40 max large classrooms 35 max Setting a Design Goal The first step of an acoustical design is to quantify the goal Period Two introduced several single number descriptors that designers commonly use to define the acoustical design goal for a space Each descriptor has its advantages its and drawbacks In general when defining the acoustical design goal
28. d the absorptivity and reflectivity of the surfaces in the room For example in a completely hard room with concrete walls and floors and no furnishings the room effect is very small Conversely in a soft room with carpeted floors and wall coverings the room effect can be quite substantial Receiver sound correction will nearly always result in a reduction in sound level in each octave band Sound spreading refers to the reduction of sound energy as a listener moves away from the sound source It is a factor in room acoustics and typically is the primary factor in outdoor sound calculations 46 TRG TRC007 EN TRANE period four Equipment Sound Rating Fundamentals of HVAC Acoustics period four Equipment Sound Rating As explained in Period One sound pressure can be directly measured however sound power cannot Because sound pressure is influenced by the surroundings the most accurate sound data that can be provided for a piece of equipment is sound power Sound power levels are determined by measuring sound pressure levels in an environment with known acoustical characteristics and adding back any effects attributed to the surroundings TRG TRC007 EN 47 amp Trane 48 period four Equipment Sound Rating Free Field source Fields of Measurement To measure sound pressure correctly it is important to understand the behavior of sound in various environments or fields In theory a
29. e Noise is different than sound Sound is always present but is not always obtrusive Noise is defined as unwanted sound Generally people object to sound when it interferes with speech concentration or sleep TRG TRC007 EN S TRANE period one Fundamentals of Sound Sound Wave and Frequency 4 one cycle gt cycles seconds frequency Hz amplitude time Airborne sound is transmitted away from a vibrating body through the transfer of energy from one air molecule to the next The vibrating body alternately compresses and rarefies expands the air molecules The pressure fluctuations that result from the displacement of these air molecules take the form of a harmonic or sine wave The amplitude of the wave depicts pressure The higher the amplitude the louder the sound This transfer of energy takes time Each complete sequence of motion compression and rarefaction constitutes a cycle and the time required to complete one cycle is the cycle period The frequency of the periodic motion is the number of cycles that occur in a second The unit of measure for frequency is the hertz Hz One hertz is equal to one cycle per second cycles f Hz Mis ali seconds The terms pitch and frequency are often incorrectly used interchangeably Frequency is an objective quantity that is independent of sound pressure level Pitch however is a subjective quantity that is primarily
30. equency sounds The contours are flatter at higher decibels gt 90 dB indicating a more uniform response to loud sounds across this range of frequencies As you can see the human ear does not respond in a linear manner to pressure and frequency TRG TRC007 EN TRG TRCOO07 EN S TRANE period two Sound Perception and Rating Methods Response to Tones Additionally tones evoke a particularly strong response Recall that a tone is a sound that occurs at a single frequency Chalk squeaking on a blackboard for example produces a tone that is extremely irritating to many people Single Number Rating Methods 4 A B and C weighting 4 Noise criteria NC curves 4 Room criteria RC curves A Sones A Phons Single Number Rating Methods The human ear interprets sound in terms of loudness and pitch while electronic sound measuring equipment interprets sound in terms of pressure and frequency As a result considerable research has been done in an attempt to equate sound pressure and frequency to sound levels as they are perceived by the human ear The goal has been to develop a system of single number descriptors to express both the intensity and quality of a sound With such a system sound targets can be established for different environments These targets aid building designers in specifying appropriate acoustical requirements that can be substantiated through measurement For example a designer can speci
31. es from the boundary lines drawn in Step Four Use the following criteria to choose the appropriate letter descriptor that characterizes the subjective quality of the noise m Neutral N The sound level in each of the octave bands between 31 5 Hz and 500 Hz is below line D and the sound level in each of the octave bands between 1 000 Hz and 4 000 Hz is below line E Rumble R The sound level in any octave band between 31 5 Hz and 500 Hz is above line D m Hiss H The sound level in any octave band between 1 000 Hz and 4 000 Hz is above line E TRG TRC007 EN TRG TRCO07 EN S TRANE period two Sound Perception and Rating Methods MH Perceptible vibration RV The sound level in the octave bands between 16 Hz and 63 Hz falls in the shaded regions A and B These regions indicate sound pressure levels at which walls and ceilings can vibrate perceptibly rattling cabinet doors pictures ceiling fixtures and other furnishings in contact with them Region A High probability that noise induced vibration levels in lightweight wall and ceiling constructions will be felt Anticipate audible rattles in light fixtures doors windows and so on Region B Noise induced vibration levels in lightweight wall and ceiling constructions may be felt Slight possibility of rattles in light fixtures doors windows and so on The RC rating for the sound is the numerical SIL value calculated in Step Two and the letter descriptor d
32. es should be avoided TRG TRCOO7 EN TRG TRCO07 EN S TRANE period two Sound Perception and Rating Methods Noise Criteria NC Curves sound pressure dB ref 20 Pa approximate threshold for hearing continuous noise Fn E Nc 15 63 125 250 500 1 000 2 000 4 000 8 000 octave band frequency Hz Noise criteria NC curves are probably the most common single number descriptor used to rate sound pressure levels in indoor environments Like the equal loudness contours on which they are based the loudness along each NC curve is about the same Each NC curve slopes downward to reflect the increasing sensitivity of the ear to higher frequencies It should also be noted that NC charts do not include the 16 Hz and 31 5 Hz octave bands Although HVAC equipment manufacturers typically do not provide data in these bands because it is very difficult to obtain reliably these octave bands do effect the acoustical comfort of the occupied space Nevertheless these octave bands can be measured in a space that is already built and may provide useful diagnostic information 21 S TRANE 22 period two Sound Perception and Rating Methods Noise Criteria NC Curves E NC 65 E NC 60 E NC 55 ENC 50 ENC 45 E NC 40 ENC 35 sound pressure dB ref 20 uPa E NC 30 E N
33. etermined in Step Five 25 S TRANE 26 period two Sound Perception and Rating Methods Room Criteria RC Curves sound pressure dB ref 20 uPa TE 315 63 125 250 500 1 000 2 000 4 000 octave band frequency Hz If we plot the acoustical data for our example office space on the RC chart we find that it results in a rating of RC 31 R The SIL is 31 and the sound pressure levels in the 63 Hz and 125 Hz octave bands are above line D indicating a rumble characteristic of the sound This time our objective and subjective analyses lead to the same conclusion Although the space is quiet enough the background noise is perceived as having a rumble A sound spectrum that falls in the RC neutral category would be judged as excellent by most observers It is this conformity of analysis results that makes the RC rating method a better tool than the other single number descriptors for specifying acoustical requirements indoors Despite the advantages of the RC rating system it is less widely used than other single number descriptors Finally accurate determination of sound power levels for the 16 Hz and 31 5 Hz octave bands requires a very large reverberant room Most HVAC equipment manufacturers do not provide sound data in these two octave bands due the cost of constructing such a large test room and the difficulty in qualifying it This m
34. focus on sounds in the frequencies between 45 and 11 200 Hz Despite this reduced range measuring a sound at each frequency would result in 11 156 data points For some types of analyses it is advantageous to measure and display the sound at each frequency over the entire range of frequencies being studied This is called a full spectrum analysis and is displayed like the example shown in Figure 7 To make the amount of data more manageable this range of frequencies is typically divided into smaller ranges called octave bands Each octave band is defined such that the highest frequency in the band is two times the lowest frequency The octave band is identified by its center frequency which is calculated by taking the square root of the product of the lowest and highest frequencies in the band center frequency lowest frequency x highest frequency The result is that this frequency range 45 to 11 200 Hz is separated into eight octave bands with center frequencies of 63 125 250 500 1 000 2 000 4 000 and 8 000 Hz For example sounds that occur at the frequencies between 90 Hz and 180 Hz are grouped together in the 125 Hz octave band TRG TRC007 EN S TRANE period one Fundamentals of Sound Octave Bands logarithmic sums amplitude 63 125 250 500 1 000 2 000 4 000 8 000 frequency Hz Octave bands compress the range of frequencies between the upper and lower ends of the band into a single value Sound meas
35. free field is a homogeneous isotropic medium that is free from boundaries In practice an example of a free field over a reflecting plane would be a large open area void of obstructions like a parking lot or meadow An ideal sound source that is one that radiates sound equally in all directions placed in a free field generates sound pressure waves in a spherical pattern At equal distances from the source the sound pressure is same in all directions As the sound waves travel farther away from the source the area of the sphere increases Doubling of the distance from the source spreads the sound over four times as much surface area TRG TRCOO07 EN TRANE period four Equipment Sound Rating Distance Correction in a Free Field r Loe L 20 log 1 This type of relationship between distance and surface area provides the following simple mathematical model for estimating how sound will change as the distance from the source increases Loe Lp1 20 logs r2 r4 where Lp2 sound pressure level at distance r Lp1 sound pressure level at distance r r4 distance from the source where L was measured r2 distance from the source to where the sound pressure L is desired Using this expression it can be shown that doubling the distance from the sound source results in a 6 dB reduction in the sound pressure level This is a handy fact to know when making estimates of outdoor sound levels Lp2 Lp1 20 109
36. fy that the background sound level in the theater 17 S TRANE 18 period two Sound Perception and Rating Methods shall be X where X is a single number descriptor conveying the desired quality of sound The most frequently used single number descriptors are the A weighting network noise criteria NC and room criteria RC All three share a common problem however they unavoidably lose valuable information about the character or quality of sound Each of these descriptors is based on octave band sound data which as noted earlier may already mask tones Further the process of converting from eight octave bands to a single number overlooks even more sound data Despite this shortcoming the single number descriptors summarized in this clinic are valuable tools for defining sound levels in a space and are widely used to specify the acoustical requirement of a space A B C Weighting 20 frequency responses for sound meter weighting characteristics relative response dB 30 40 20 50 100 200 500 1 000 2 000 5 000 10 000 frequency Hz One simple method for combining octave band sound data into a single number descriptor is A B or C weighting The weighting curves shown in Figure 23 compensate for the varying sensitivity of the human ear to different frequencies A weighting which is most appropriately used for low volume or quiet sound levels best approximates human response to sound i
37. g an NC value but it is still fairly simple The RC value is based on sound pressure data from the eight octave bands between 31 5 Hz and 4 000 Hz Note that these are different than the octave bands included on the NC chart TRG TRCO07 EN 23 S TRANE 24 period two Sound Perception and Rating Methods Room Criteria RC Curves sound pressure dB ref 20 uPa EB 4 315 63 125 250 500 1 000 2 000 4 000 octave band frequency Hz The following steps describe how to determine an RC rating 1 2 Plot the octave band sound pressure levels on the RC chart Determine the SIL by calculating the arithmetic average of the sound pressure levels in the 500 Hz 1 000 Hz and 2 000 Hz octave bands In this example the arithmetic average of 38 dB 31 dB and 24 dB is 31 dB Draw a line C with a slope of 5 dB per octave that passes through the calculated SIL at the 1 000 Hz octave band This is the reference line for evaluating the character of the sound spectrum Between 31 5 Hz and 500 Hz draw a line D that is 5 dB above the reference line C Between 1 000 Hz and 4 000 Hz draw a second line E that is 3 dB above the reference line C These two boundary lines D and E represent the maximum permitted deviation to receive a neutral N rating Judge the character of the sound quality by observing how the sound spectrum deviat
38. gests this modeling method traces sound from the source to the location where we want to predict the sound the receiver How the sound travels between the source and the receiver and everything it encounters as it travels along the way constitutes the path In the example shown in Figure 37 the source is the fan in the mechanical room The receiver is the person working in the adjacent office space The supply duct provides one of the paths for sound to travel from the source to the receiver Using such an analysis the designer can determine the effect of the paths on the sound emanating from the source and can specify the maximum allowable equipment sound power that will not exceed the sound pressure target for the space 33 S TRANE period three Acoustical Analysis Typical Sound Paths A Airborne Sound that travels through supply ductwork return ductwork or an open plenum Can travel with or against the direction of airflow A Breakout Sound that breaks out through the walls of the supply or return ductwork A Transmission Sound that travels through walls floors or ceilings The work and art of an acoustical analysis is in identifying and quantifying the various paths that sound travels from the source to the receiver There are primarily three different types of sound paths E Airborne This is a path where sound travels with or against the direction of airflow In a HVAC system sound travels al
39. igure 6 represents the components of broadband sound Broadband Sound and Tones tone amplitude logarithmic scale frequency Alternatively plotting the amplitude vertical axis of each sound wave at each frequency horizontal axis results in a graphic of the broadband sound that looks like this As you can see from this example the sound energy is greater at some frequencies than at others TRG TRCOO07 EN S TRANE period one Fundamentals of Sound Again a pure tone has a single frequency If a sound in a narrow band of frequencies is significantly greater than the sound at adjacent frequencies it would be similar to a tone Tones that stand out enough from the background sound can be objectionable Many of the sounds generated by HVAC equipment and systems include both broadband and tonal characteristics Octave Bands octave center frequency band frequency Hz range Hz 1 63 45 to 90 2 125 90 to 180 3 250 180 to 355 4 500 355 to 710 5 1 000 710 to 1 400 6 2 000 1 400 to 2 800 7 4 000 2 800 to 5 600 8 8 000 5 600 to 11 200 Octave Bands Because sound occurs over a range of frequencies it is considerably more difficult to measure than temperature or pressure The sound must be measured at each frequency in order to understand how it will be perceived in a particular environment The human ear can perceive sounds at frequencies ranging from 20 to 16 000 Hz whereas HVAC system designers generally
40. intensive analysis 65 amp Trane 66 period five Review Review Period Four E Period Four introduced two common methods used by HVAC equipment manufacturers to provide accurate sound data Because sound pressure is influenced by the surroundings the most useful sound data that can be provided for most pieces of equipment is sound power Sound power levels are determined by measuring sound pressure levels in an environment with known acoustical characteristics and adding back any effects attributed to the surroundings The most common method uses a special acoustical testing facility called a reverberant room The availability of accurate tested sound data for HVAC equipment is vitally important to any acoustical analysis ARI Standard 260 2001 is one example of an industry standard for rating the sound level of equipment This standard addresses a wide range of air handling equipment with a consistent test method It assures accurate verifiable sound data and focuses on the entire air handler in all of its common installation arrangements As with other existing sound standards ARI 260 enables designers to make equitable product comparisons and to more accurately predict sound levels for occupied spaces TRG TRC007 EN period five Review S TRANE An American Standard Company For more information refer to the following references Acoustics in Air Conditioning Applications Engineering Manual
41. it does in a stand alone application The air handler casing generally changes the airflow patterns at the fan inlet and discharge openings which can change the sound power for a given flow and static pressure condition Additionally an air handler may have only one source of sound or it may have several For example a ducted packaged rooftop air conditioner has multiple sources It contains a supply fan refrigeration compressors air cooled condenser fans and possibly an exhaust or return fan Finally sound may leave the air handler in multiple ways In the case of the indoor air handler sound travels along with the conditioned air into the supply duct system It also travels back out the return air inlet against the direction of airflow Finally sound is also radiated by the casing of the air handler into the equipment room In order to properly design the HVAC system the designer needs to know the sound power from all of these paths 58 TRG TRC007 EN TRG TRC007 EN S TRANE period four Equipment Sound Rating Air Handler Test Configurations ducted ducted discharge inlet unducted inlet plus casing radiated casing radiated q EA In order to isolate these different paths the air handler must be tested using a number of different configurations For example to determine the portion of the sound that is discharged with the air into the supply ductwork the air handler is installed outside of the reverberant room
42. ld by reflecting and mixing the sound waves The walls floor and ceiling of the reverberant room are hard in order to cause multiple reflections of sound waves In this environment the sound pressure is essentially the same at all locations in the room Sound pressure levels are measured in the reverberant room and used to calculate sound power levels for the piece of equipment room criteria RC A single number used to describe sound in a room It uses a series of curves and reference lines for plotting sound pressure by octave band and determining the RC value and a descriptor of the sound quality i e hiss rumble room effect See receiver room correction semireverberant field A sound field that is somewhere between a free field and a reverberant field The walls and ceiling of a room prevent the sound from behaving in a free field manner however these surfaces are not perfectly reflective Some of the sound is reflected by these surfaces but a portion is absorbed sone A unit of measure using a linear scale used to describe the loudness of a sound A sone is the linear equivalent to a phon sound Audible emissions resulting from the vibration of molecules within an elastic medium It is generated by either a vibrating surface or the movement of a fluid In the context of building HVAC systems this elastic medium can be either air or the building structure For structurally borne sound to become audible however it must first
43. n the range where no hearing protection is needed B weighting is used for medium volume sound levels C weighting is used for high volume or loud sound levels where the response of the ear is relatively flat TRG TRC007 EN S TRANE period two Sound Perception and Rating Methods A Weighting Example octave center actual sound A weighting A weighted band frequency Hz pressure dB factor dB sound pressure dB 1 63 63 26 37 2 125 52 16 36 3 250 45 9 36 4 500 38 3 35 5 1 000 31 0 31 6 2 000 24 1 25 7 4 000 16 1 17 8 8 000 10 0 10 42 dBA The following steps describe how to calculate an A weighted value 1 Starting with the actual sound pressure levels for the eight octave bands add or subtract the decibel values represented by the A weighting curve shown in Figure 23 These weighting factors are also listed in the table in Figure 24 Subtract 26 dB from the 63 Hz sound pressure level 16 dB from the 125 Hz level 9 dB from the 250 Hz level and 3 dB from the 500 Hz level Then add 1 dB each to the sound pressure levels in the 2 000 Hz and 4 000 Hz octave bands 2 Logarithmically sum all eight octave bands together to arrive at an overall A weighted sound pressure level This value is then expressed using the units of dBA For the sound pressure data in this example the A weighted sound pressure level is 42 dBA Most sound meters can automatically calculate and display the A
44. nalysis As mentioned air handling equipment is typically rated in terms of sound power level per octave band Typically this data is given for the octave bands from 63 Hz through 8 000 Hz The availability of accurate tested sound data for HVAC equipment is vitally important to any acoustical analysis TRG TRC007 EN TRG TRCOO07 EN period five Review Fundamentals of HVAC Acoustics period five Review We will now review the main concepts that were covered in this clinic on the fundamentals of HVAC acoustics Review Period One A Sound power Acoustical energy emitted by source Unaffected by the environment Correlates to bulb wattage A Sound pressure Pressure disturbance in atmosphere Affected by strength of source surroundings and distance from source Correlates to brightness in a particular location Period One explained some of the basic concepts of sound Sound is the audible emissions resulting from the vibration of molecules within an elastic medium It is generated at many different frequencies at the same time Noise is defined as unwanted or obtrusive sound Sound power and sound pressure are both terms that are used when describing sound Sound power is the acoustical energy emitted by the sound source and is not affected by the environment Sound pressure is a disturbance in the atmosphere and can be measured directly Its intensity is influenced not only by the strength of the sour
45. nbalanced spectrum as having an annoying rumble TRG TRC007 EN S TRANE period two Sound Perception and Rating Methods Room Criteria RC Curves ERC 50 ERC 45 ERC40 ERC 35 sound pressure dB ref 20 Pa T 4 ERC 30 BE ES 4 ERC 25 315 63 125 250 50 1000 2 000 4 000 octave band frequency Hz approximate threshold for hearing continuous noise Room criteria RC curves are similar to NC curves in that they are used to provide a rating for sound pressure levels in indoor environments The major difference is that RC curves give an additional indication of sound character As discussed in the previous example sound spectrums can be unbalanced in ways that result in poor acoustical quality Too much low frequency sound results in a rumble and too much high frequency sound produces a hiss RC curves provide a means of identifying these imbalances An RC rating consists of two descriptors The first descriptor is a number representing the speech interference level SIL of the sound The second descriptor is a letter denoting the character of the sound as a subjective observer might describe it N identifies a neutral or balanced spectrum E R indicates a rumble E H represents a hiss E RV denotes perceptible vibration Calculating an RC value from octave band sound pressure data is not quite as easy as determinin
46. nerate sound For example as air makes a 90 degree turn in a rectangular duct elbow some of the sound is reflected back upstream attenuating the airborne sound downstream of the elbow At the same time however the turbulence created by the air turning the sharp corner causes some regenerated sound TRG TRCOO07 EN 41 S TRANE 42 period three Acoustical Analysis Sound Transmission absorbed sound energy Wi incident sound energy W transmitted sound energy P W reflected sound energy r FAO The total sound energy that strikes a surface W is either reflected W absorbed by the material W or transmitted through the material W A material provides a barrier to the incident sound energy W when it reduces the amount of sound energy that is transmitted through the material W There are a number of factors that affect the amount of sound transmitted through the wall including the type and thickness of material frequency of the sound and quality of construction Materials that are dense such as masonry block or wallboard or stiff such as glass are generally better at reducing transmitted sound than materials that are lightweight or flexible Increasing the thickness of a material reduces the amount of sound transmitted through it Finally the ability of a material to reduce transmitted sound depends on frequency High frequency sound is more easily reduced than low frequency so
47. ong this type of path through the supply ductwork return ductwork or an open plenum E Breakout This type of path is typically associated with sound breaking out through the duct walls and into the space E Transmission This is a path where sound travels through walls floors and ceilings In its simplest form this path involves sound traveling directly through the air from the source to the receiver 34 TRG TRCO07 EN S TRANE period three Acoustical Analysis Examples of a Single Sound Path air cooled chiller Sound can travel between a single source and the receiver along one or multiple paths In the case of an air cooled chiller sitting on the roof of a building and a receiver located across a parking lot at the edge of the property sound travels along only one path Another example is a fan coil unit installed under a window in an office Sound travels primarily along one path from the fan coil to the receiver in the same room Example of Multiple Sound Paths source return sul y pply airborne y airborne In other cases there may be several paths for sound to travel from a source to the receiver This particular example shows the paths associated with an air handler that is installed in a mechanical equipment room adjacent to an occupied space Only one sound source is included in this analysis the fan located in the air handler The receiver is the person working in the office The sound t
48. oom example reducing the reflected sound energy lowers the sound level in the equipment room Given a fixed transmission loss for the walls this will result in a decrease in sound that travels to the adjacent space Said another way if it is quieter in the equipment room it will be quieter in the adjacent spaces 45 S TRANE period three Acoustical Analysis Receiver Sound Correction Receiver sound correction also called room effect is the relationship between the sound energy sound power entering the room and the sound pressure at a given point in the room where the receiver hears the sound This reduction in sound is due to a combination of effects including distance and the absorptive and reflective properties of the surrounding surfaces In an outdoor environment such as a field or parking lot the absorption of sound is nearly perfect Sound leaves the source in all directions and diminishes as it travels away from the source Only the portion of the sound that travels in a direct line from the source ever reaches the receiver In this environment the receiver sound correction is mainly a function of distance between the source and receiver In contrast sound entering a room bounces off walls and other surfaces Therefore the receiver will hear sound reflecting off the surfaces as well as the sound coming directly from the source The amount of sound that reaches the receiver is dependent on the size of the room an
49. ound pitch A subjective quantity used to describe a sound It is primarily based on frequency but is also dependent on sound pressure level and composition Pitch is not measured but is described with terms like bass tenor and soprano receiver sound correction The relationship between the sound energy sound power entering the room and the sound pressure at a given point in the room where the receiver hears the sound This reduction is due to a combination of effects including distance and the absorptive and reflective properties of the surrounding surfaces Also called room effect reflected sound The sound that bounces off or is reflected by a barrier back toward the source regenerated sound The noise caused by turbulent flow in air and water systems TRG TRC007 EN TRG TRC007 EN S TRANE Glossary reverberant field A uniform or diffuse sound field that is the opposite of a free field In a perfectly reverberant field the sound pressure level is equal at all points reverberant room A specially constructed room with reflective walls floors and ceilings When a sound source is placed in this room the sound waves bounce back and forth between the reflective walls many times In a perfectly reverberant room the sound pressure level is equal at all points in the room reverberant room method A common method for testing HVAC equipment It uses a specially constructed room to create a uniform or diffuse sound fie
50. ound pressure Logarithmic Addition of Decibels I 0 5 10 15 dB difference between values being added add to the higher dB value 50 dB 44 dB 51 dB Measuring sound using a logarithmic scale means that decibel values cannot be added arithmetically Instead logarithmic addition must be used to add two or more sound levels This involves converting the decibel values into ratios of sound intensity adding these ratios and then converting the sum back into decibels The mathematics become rather involved the graph in Figure 17 has been developed to simplify the procedure To demonstrate the use of this figure consider the example of adding a 50 dB sound to a 44 dB sound The difference between these two sounds is 6 dB Therefore 1 dB is added to the higher of the two sounds 50 plus 1 to arrive at the logarithmic sum of 51 dB Also notice that the logarithmic sum of two sounds of equal magnitude 0 dB difference results in a 3 dB increase Therefore adding two 50 dB sounds would result in a combined sound level of 53 dB TRG TRC007 EN TRG TRCOO07 EN S TRANE period two Sound Perception and Rating Methods Fundamentals of HVAC Acoustics period two Sound Perception and Rating Methods The study of acoustics is affected by the response of the human ear to sound pressure Unlike electronic sound measuring equipment which provides a repeatable unbiased analysis of sound pressure the sensitivity of the h
51. ound sources such as return or exhaust fans and compressors be tested to determine their acoustical impact on the air handler Any secondary source that alters the sound spectrum of the supply fan must be included in cataloged ratings In summary ARI 260 addresses a wide range of air handling equipment with a consistent test method It ensures accurate verifiable sound data and focuses on the entire air handler in all of its common installation arrangements ARI 260 enables designers to make equitable product comparisons and to more accurately predict sound pressure levels for occupied spaces TRG TRC007 EN 61 amp Trane 62 period four Equipment Sound Rating Sound Power by Octave Band equipment octave center sound power band frequency Hz dB ref 10 12 W 1 63 103 2 125 104 3 250 100 4 500 101 5 1 000 98 6 2 000 93 7 4 000 88 8 8 000 85 Depending on the type of equipment sound ratings for HVAC equipment are typically given as sound power levels by octave band or as a single dBA rating Outdoor equipment such as an air cooled chiller or condensing unit may be rated in terms of A weighted sound pressure level dBA at a specific distance from the equipment This generally assumes a free field environment and may be useful for comparing equipment from various manufacturers Nevertheless the sound pressure level by octave band should still be available from the manufacturer for use in an acoustical a
52. rane com For more information contact your local district office or e mail us at comfort trane com Response Card We offer a variety of HVAC related educational materials and technical references as well as software tools that simplify system design analysis and equipment selection To receive information about any of these items just complete this postage paid card and drop it in the mail Education materials Air Conditioning Clinic series About me Lj Engineered Systems Clinic series Name LJ Trane Air Conditioning Manual Title LJ Trane Systems Manual Business type Software tools Lj Equipment Selection Phone fax m System design amp analysis E mail address Periodicals Lj Engineers Newsletter Company Other LJ Address Thank you for your interest TRANE Trane An American Standard Company www trane com For more information contact your local district office or e mail us at comfort trane com Fundamentals of HVAC Acoustics One of the Fundamental Series A publication of Trane an American Standard Company S TRANE Preface Fundamentals of HVAC Acoustics A Trane Air Conditioning Clinic Trane believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research testing programs and field experience The Trane Air Conditioning Clinic series is one means of knowledge sharing It is in
53. ravels from the source to the receiver along four separate paths TRG TRC007 EN 35 S TRANE 36 period three Acoustical Analysis Supply airborne through the supply ductwork and diffusers and into the space Supply breakout as the sound travels through the walls of the supply ductwork through the ceiling tile and into the space Return airborne through the air handler intake return ductwork and grilles and into the space Wall transmission as the sound travels through the adjoining wall and into the space These paths are typical of most centralized air handling equipment including packaged rooftop and self contained air conditioners Most other equipment types have a subset of these paths Identifying Sound Sources and Paths A One piece of equipment may contain several A sound sources a un x Y A Sound may travel from ka ES source to receiver along Y g multiple paths s A Total sound heard by the packaged rooftop receiver is the sum of all air conditioner sounds from all sources and all paths There are a few important points to remember when identifying sources and paths for a source path receiver acoustical analysis E One piece of equipment may contain several sound sources For example a packaged rooftop air conditioner shown in Figure 41 contains supply and exhaust or return fans compressors and condenser fans Sound may travel from a single source to the receiver along multiple paths This
54. rd octave bands by 5 dB or more TRG TRC007 EN 31 S TRANE 32 period three Acoustical Analysis Acoustical Analysis VAV box Fa ductwork diffuser air handler occupied space source receiver Source Path Receiver Analysis Achieving the desired acoustical characteristics in a space however requires more than selecting an appropriate single number descriptor Including a single number descriptor in a HVAC system specification means that someone must perform an acoustical analysis to determine if the proposed HVAC system and equipment will satisfy the space acoustical requirements To make such a prediction the analysis must convert the sound power level of the source the fan in the air handler in this example to the sound pressure level in the occupied space assessing the effect of installation and environmental factors along the way Sound that reaches the occupied space will be altered by ductwork wall and ceiling construction room furnishings and many other factors The validity of an acoustical analysis therefore depends on the analyst s familiarity with construction details TRG TRCOO7 EN TRG TRCOO07 EN S TRANE period three Acoustical Analysis Source Path Receiver Model Predicting the sound level in a given space requires making a model ofthe system source path receiver model provides a systematic approach to predict the acoustical characteristics in a space As the name sug
55. rol Association International AMCA and the American Society of Heating Refrigerating and Air Conditioning Engineers ASHRAE are three such organizations However methods for predicting sound data may still vary from manufacturer to manufacturer hampering comparisons of similar equipment Done properly collecting accurate sound data for an entire line of products is an expensive and time consuming endeavor A single product line may consist of many models and each model may come in a range of sizes with various options that alter the sound generated by the equipment The ARI AMCA and ASHRAE logos are registered trademarks of their respective organizations 57 amp Trane period four Equipment Sound Rating Ducted Air Handling Equipment supply air outlet return air frasi inlet coil outdoor air intake supply fan One of the best examples to demonstrate the complexity of gathering complete and accurate sound data is air handling equipment This involves any type of HVAC equipment that contains a fan and is used to condition and move air through a duct system Consider that each fan in an air handling product line may run at multiple speeds and within a range of flow and static pressure conditions The fact that each type of fan forward curved backward inclined and so forth has a different operating characteristic further complicates testing A fan performs differently inside an air handler than
56. s to brightness The following comparison of sound and light may help illustrate the distinction between these two properties Think of sound power as the wattage rating of a light bulb Both measure a fixed amount of energy Whether you put a 100 watt light bulb outdoors or in a closet it is always 100 watt light bulb and always gives off the same amount of light Sound pressure corresponds to the brightness from the light emitted by the light bulb in a particular location in the room Both sound pressure and brightness can be measured with a meter and the immediate surroundings influence the magnitude of each In the case of light brightness depends on more than the wattage of the bulb It also depends on how far the observer is from the light bulb the color of the room how reflective the wall surfaces are and whether the light bulb is covered with a shade These other factors affect how much light reaches the receiver but do not affect the wattage of the light bulb Similarly sound pressure depends not only on the sound power emitted by the source but also on the characteristics of the surrounding environment These might include the distance between the sound source and the listener whether the room is carpeted or tiled and whether the room is furnished or bare Just as with light environmental factors like these affect how much sound reaches the listener 10 TRG TRC007 EN S TRANE period one Fundamentals of Sound
57. ted by the sound source Unaffected by the environment A Sound pressure Pressure disturbance in the atmosphere Affected by strength of source surroundings and distance between source and receiver Sound Power and Sound Pressure Sound power and sound pressure are two distinct and commonly confused characteristics of sound Both are generally described using the term decibel dB and the term sound level is commonly substituted for each To understand how to measure and specify sound however one must first understand the difference between these two properties Sound power is the acoustical energy emitted by the sound source and is expressed in terms of watts W It is not affected by the environment Sound pressure is a pressure disturbance in the atmosphere expressed in terms of pascals Pa that can be measured directly Sound pressure magnitude is influenced not only by the strength of the source but also by the surroundings and the distance from the source to the listener Sound pressure is what our ears hear and what sound meters measure While sound producing pressure variations within the atmosphere can be measured directly sound power cannot It must be calculated from sound pressure knowing both the character of the source and the modifying influences of the environment S TRANE period one Fundamentals of Sound An Analogy A Sound power Correlates to bulb wattage A Sound pressure Correlate
58. tended to acquaint a nontechnical audience with various fundamental aspects of heating ventilating and air conditioning HVAC We have taken special care to make the clinic as uncommercial and straightforward as possible Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text This particular clinic introduces the reader to the fundamentals of HVAC acoustics 2001 American Standard Inc All rights reserved TRG TRC007 EN TRG TRCO07 EN Contents period one period two period three period four period five Fundamentals of Sound 1 What is Sound ccc cceeceeeeceeeeeeeeeeeeeeesaaeeeeeeeeeeeeeeeees 2 OCTAVE BANGS daniisiiscssionsnnatdbde noedensnsinnnan needed nette 6 Sound Power and Sound Pressure ccceeeeeeeeeeeeeee 9 Sound Perception and Rating Methods 15 Human Ear Response 0 c cceeeeeeeeeeeeeeeeeeneeeeeees 15 Single Number Rating Methods cccccccceeeeeeeeee 17 Octave Band Rating Method c ccccccccceeeeseeeeeeee 28 Acoustical Analysis 29 Setting a Design Goal ccc cececeeeeeeeeeeeeeeeeeeeeenaes 30 Source Path Receiver Analysis oo 32 Sound Path Modeling ccceseessssseeeeeeeeeeeeeeeeeenees 37 Terms Used in Sound Path Modeling 41 Equipment Sound Rating 47 Fields of Measurement
59. terms of an absorption coefficient The absorption coefficient is the ratio of sound energy absorbed by the material to the sound energy incident upon the surface of the material Preferably absorption coefficients are reported for each octave band but may also be expressed in terms of a single Noise Reduction Coefficient NRC The NRC is simply the arithmetic average of the absorption coefficients for the 250 500 1 000 and 2 000 Hz octave bands TRG TRCOO7 EN TRG TRCOO07 EN S TRANE period three Acoustical Analysis Reflected Sound Finally some of the incident sound energy W bounces off of or is reflected from the material Reflected sound becomes especially important when the sound source and the receiver are located in the same room Consider a mechanical equipment room that contains a water chiller pumps and other sound sources Often the walls of the equipment room are constructed of masonry either cement block or poured concrete Neither of these materials absorb or transmit very much of the incident sound energy so most of it is reflected back into the room The reflected sound adds to the sound coming from the source greatly increasing the sound level in the room The best way to reduce reflected sound is to add an absorptive material to as much of the walls floor and ceiling as possible On occasion reducing reflected sound may also lower the sound levels in adjacent spaces Using the equipment r
60. tions that make the behavior of the sound waves unpredictable Sound pressure measurements should not therefore be made in the near field The size of the near field is dependent on the type of source and dimensions of the equipment TRG TRC007 EN 51 amp Trane 52 period four Equipment Sound Rating Reverberant Field source reflective walls A reverberant field is nearly the opposite of a free field Reverberant fields exist in rooms with reflective walls floors and ceilings When a sound source is placed in an enclosed room the sound waves from the source bounce back and forth between the reflective walls many times This can create a uniform or diffuse sound field In a perfectly reverberant room the sound pressure level is equal at all points within the room Special reverberant rooms are designed built and qualified for the purpose of measuring the sound emitted by a piece of equipment This type of facility will be discussed later in this period TRG TRCO07 EN S TRANE period four Equipment Sound Rating Semireverberant Field semireverberant field Most rooms in buildings are somewhere between a free field and a reverberant field environment Called a semireverberant field these spaces have some characteristics of both free field and reverberant field environments The walls floor and ceiling prevent the sound from behaving as it would in a free field These surfaces are not however
61. tz is equal to one cycle per second insertion loss IL The difference in sound pressure measured in a single location with and without a noise control device installed between the source and receiver in place near field The area adjacent to the source where sound does not behave as it would in a free field due to the fact that the source does not radiate sound equally in all directions NEBB National Environmental Balancing Bureau www nebb org noise Unwanted or obtrusive sound Generally people object to sound when it interferes with speech concentration or sleep noise criteria NC A single number used to describe sound in a room It uses a series of curves for plotting sound pressure by octave band and determining the NC value noise reduction NR A term used to measure the effect of a barrier on reducing the amount of transmitted sound It is the difference between sound pressure measurements taken on each side of a barrier noise reduction coefficient NRC A single number used to describe the sound absorbing characteristics of a material It is the arithmetic average of the absorption coefficients for the 250 500 1 000 and 2 000 Hz octave bands octave band A range of frequencies that is defined such that the highest frequency in the band is two times the lowest frequency The octave band is identified by its center frequency phon A unit of measure using a logarithmic scale used to describe the loudness of a s
62. uman ear varies by frequency and magnitude Our ears are also attached to a highly arbitrary evaluation device the brain The Human Ear middle ear nerves to brain Human Ear Response The ear acts like a microphone Sound waves enter the auditory canal and impinge upon the ear drum causing it to vibrate These vibrations are ultimately transformed into impulses that travel along the auditory nerve to the brain where they are perceived as sound The brain then analyzes and evaluates the signal 15 S TRANE 16 period two Sound Perception and Rating Methods Loudness Contours sound pressure dB ref 20 uPa j i 0 20 50 100 200 500 1 000 2 000 5 000 10 000 frequency Hz The sensation of loudness is principally a function of sound pressure however it also depends upon frequency As a selective sensory organ the human ear is more sensitive to high frequencies than to low frequencies Also the ear s sensitivity at a particular frequency changes with sound pressure level Figure 20 illustrates these traits using a set of contours Each contour approximates an equal loudness level across the frequency range shown For example a 60 dB sound at a frequency of 100 Hz is perceived by the human ear to have loudness equal to a 50 dB sound at a frequency of 1 000 Hz Also notice that the contours slant downward as the frequency increases from 20 to 200 Hz indicating that our ears are less sensitive to low fr
63. und TRG TRCOO7 EN S TRANE period three Acoustical Analysis Sound Transmission A Insertion loss IL A Noise reduction NR A Transmission loss TL The ability of a material to reduce transmitted sound is most commonly referred to in terms of its insertion loss noise reduction or transmission loss Insertion loss and noise reduction are both based on actual sound pressure measurements and are expressed in terms of dB reduction Insertion loss IL is the difference in sound pressure measured in a single location with and without a noise control device located between the source and receiver Using the air handler example Figure 42 assume there is a door installed in the wall separating the equipment room from the office space The difference in the sound pressure measured in the occupied space with the door open versus with the door closed is the IL of the door Noise reduction NR is the difference between sound pressure measurements taken on each side of a barrier For example the NR for this same door can be determined by measuring the sound pressure level inside the office space with the door closed and on the other side of the door inside the equipment room The difference in these measurements is the NR of the door Transmission loss TL is proportional to the ratio of the sound power level on the receiver side of a barrier to the sound power level on the source side Using the same door example the transmission
64. ured in an octave band is the logarithmic sum of the sound level at each of the frequencies within the band Unfortunately octave bands do not indicate that the human ear hears a difference between an octave that contains a tone and one that does not even when the overall magnitude of both octaves is identical Therefore the process of logarithmically summing sound measurements into octave bands though practical sacrifices valuable information about the character of the sound TRG TRC007 EN S TRANE period one Fundamentals of Sound One Third Octave Bands 9 vt amplitude 63 125 250 500 1 000 2 000 4 000 8 000 frequency Hz Middle ground between octave band analysis and full spectrum analysis is provided by one third octave band analysis One third octave bands divide the full octaves into thirds The upper cutoff frequency of each third octave is greater than the lower cutoff frequency by a factor of the cube root of two approximately 1 2599 If tones are contained in the broadband sound they will be more readily apparent in the third octaves The use of octave bands is usually sufficient for rating the acoustical environment in a given space One third octave bands are however more useful for product development and troubleshooting acoustical problems 8 TRG TRCO07 EN TRG TRC007 EN TRANE period one Fundamentals of Sound Sound Power and Sound Pressure A Sound power Acoustical energy emit
65. vel heard in the space TRG TRC007 EN TRG TRCOO07 EN S TRANE period three Acoustical Analysis Algorithms for Sound Path Modeling Theoretical equations aid the analysis of some path elements but prediction equations based on test data and experience prevail For example an acoustical lab may have measured the attenuation and regenerated sound from a number of different types of duct elbows at various airflow rates Data recorded from tests is used to generate an equation that can be used to model the test data ASHRAE collected and developed numerous prediction equations for path components in HVAC systems and subsequently published them in their Algorithms for HVAC Acoustics handbook Similar information can be found in the National Environmental Balancing Bureau NEBB publication titled Sound and Vibration Design and Analysis ASHRAE algorithms are widely used and generally provide good results When using the algorithms it should be remembered that they mainly come from test data As a result if they are used to extrapolate beyond the test conditions the accuracy of the algorithms will diminish 39 S TRANE 40 period three Acoustical Analysis Computerized Analysis Tools Solving these algorithms manually can be tedious and time consuming especially when one or more paths need further attenuation and the calculations have to be repeated Fortunately computer software tools are available to spare
66. vide different results 27 S TRANE 28 period two Sound Perception and Rating Methods Octave Band Rating Method equipment sound pressure octave center sound power in the space band frequency Hz dB ref 102 W dB ref 20 uPa 1 63 103 63 2 125 104 52 3 250 100 45 4 500 101 38 5 1 000 98 31 6 2 000 93 24 7 4 000 88 16 8 8 000 85 10 Octave Band Rating Method A more useful method of rating sound level is to use the octave bands discussed earlier While octave band data is not as simple to interpret as a single number rating it provides much more information about the character of the sound Both sound power levels and sound pressure levels can be presented in octave band format When equipment sound data is provided in terms of sound power level in each octave band an apples to apples comparison can be made between various pieces of equipment In addition this sound power data can be converted to sound pressure levels when the details of the environment are known This type of analysis will be discussed further in Period Three Sound pressure levels in each octave band whether predicted from sound power data or measured in an existing environment reveal much more about the character of sound than any of the single number rating methods It is important to note that any of the single number ratings described in this section can be calculated from octave band sound pressure data However octave ban
67. weighted sound pressure level providing a simple and objective means of verifying acoustical performance However as mentioned earlier one of the drawbacks of a single number descriptor is that data about the relative magnitude of each octave band is lost when the eight octave bands are combined into one value Therefore even if the target dBA level is achieved an objectionable tonal quality or spectrum imbalance may exist TRG TRCO07 EN 19 S TRANE 20 period two Sound Perception and Rating Methods A Weighting 4 Used for outdoor sound ordinances and indoor hearing related safety standards OSHA 4 Use with sound pressure data only not sound power A Express as a single number descriptor only not as octave band data A weighting is often used to define sound in outdoor environments For example local sound ordinances typically regulate dBA levels at property lines Hearing related safety standards written by organizations such as the Occupational Safety and Health Administration OSHA also commonly refer to A weighted sound pressure levels when determining whether hearing protection is required in a certain environment To avoid confusion we recommend that A weighting be applied only to octave band sound pressure data not to sound power data Also A weighting should be limited to expressing a single number descriptor Displaying sound data in all eight octave bands in terms of A weighted sound pressur
68. with the supply air ducted into the room In order to determine the sound that travels back down the return duct in a ducted inlet application the air handler is again installed outside of the reverberant room but the return air duct is connected to the reverberant room In order to determine the portion of the sound that is radiated by the casing the air handler is installed inside the reverberant room with both the return and supply air ducted outside of the room Finally in cases where the return air travels back to the air handler through an open plenum and into the open equipment room the combined free or open inlet plus casing radiated sound level must be known Therefore the air handler is installed inside the reverberant room with a free inlet and the supply air is ducted to outside of the room 59 amp ranw 60 period four Equipment Sound Rating Former Methods of Sound Testing 4 ASHRAE generic fan algorithm 4 Measured fan sound data plus prediction ay equations pret A AMCA Standard 300 Historically there have been several methods used to generate sound data for air handling equipment Though increasingly less common there are still cases where fan sound levels are based on prediction equations such as the generic fan algorithm published long ago by ASHRAE Using this algorithm is much less costly than using an acoustic test facility but results in much less accurate data In 1995 after tests

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