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Air Conditioning Clinic F undamentals of HV A C A coustics One of the Fu ndamen tal Ser ie s TR G-TRC007 -EN.
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P erforation 5.5” from bottom/top P erforation 0.75” from edge Response Car d W e of f er a variety of HV A C-related educational materials and tec hnical ref erences, as well as sof tware tools that simplify system design/analy sis and equipment selection.
F undamentals of HV A C A coustics One of the Fu ndamen tal Ser ies A publication of T rane, an American Standard Compan y F undamentals of HV A C.
Pr ef ace © 20 01 American Sta ndard In c. All ri ghts reser ved TRG-TRC007 -EN ii T rane belie ves that it is incu mbent on man ufacturers to se rve the indus try by regularly dis seminating informatio n gathe red through laboratory rese arc h, testing prog r ams, and fie ld experience.
TRG-TR C0 07-EN iii Cont ents period one Fundamentals of Sound .......... ...... ...... ...... ...... ... 1 What is S ound? ......... ...... ...... ....... ...... ......... ...... ...... 2 Octave B ands ........... ...... ......... ....... ...... ....
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TRG-TR C0 07-EN 1 note s period on e F undamentals of Sound P eople have become increasing ly conscious of a coustics as a comp onent of a comfor table environ ment. Sound levels, both indoor and ou tdoor , can be aff ected to varying degre es by HV A C equipment and systems.
2 TRG-TRC007 -EN note s peri od one F undamentals of Sound What is S ound? S ound is the aud ible emissi ons resulting from the vibration of mol ecules within an elastic me dium. It is g enerated by either a vi brating surfac e or the move ment of a fluid.
TRG-TR C0 07-EN 3 peri od o ne F undamentals of Sound note s Airborne soun d is transm it ted aw ay from a vibrating body through th e transfer of energ y from one air molec ule to the nex t. The vibratin g body a lternately compre sses and rar efies (expands) the air molecule s.
4 TRG-TRC007 -EN note s peri od one F undamentals of Sound The w aveleng th of the sound is the linear measureme nt of one complete cycle. The w avelength a nd frequen cy of a sou nd are rela ted by usin g the following equation: The spee d of sound tra nsmission is a ph ysical proper ty of the medium .
TRG-TR C0 07-EN 5 peri od o ne F undamentals of Sound note s The w ave form s hown in Fig ure 5 represents soun d occur ring at a singl e frequency . This is calle d a pure tone . A pure sinusoidal wave form, however , is very rare in HV AC acous tics.
6 TRG-TRC007 -EN note s peri od one F undamentals of Sound Again , a pure tone has a s ingle frequen cy . If a so und in a nar row band of frequencie s is significantly greater than the sound at a djacent freque ncies, it would be similar to a to ne. T ones that st and out enoug h from the bac kground sound ca n be objec tionable.
TRG-TR C0 07-EN 7 peri od o ne F undamentals of Sound note s Octave bands com press the range of frequencies be tween the upper and lo wer ends of th e band into a single value. S ound measured in a n octave ba nd is the loga rithmic sum of the so und level at eac h of the freque ncies within the ba nd.
8 TRG-TRC007 -EN note s peri od one F undamentals of Sound Middle groun d between octave-b and ana lysis and full-spectrum analysis is provide d by one-third octave-band analys is.
TRG-TR C0 07-EN 9 peri od o ne F undamentals of Sound note s S ound P ow er and Soun d P ressur e S ound power a nd sound press ure are two dis tinct and comm only confused c haracteristics o f sound. Both are genera lly described us ing the term d ecibel (dB), an d the term “so und level” is commonly substitute d for eac h.
10 TRG-TRC007 -EN note s peri od one F undamentals of Sound The foll owing compariso n of sound a nd light may help illu strate the d istinction between these two proper ties. Think of sou nd power as the wat tage rating o f a light bu lb. Both mea sure a fixed amount of energ y .
TRG-TR C0 07-EN 11 peri od o ne F undamentals of Sound note s S ounds enc ompass a w ide range o f vol umes, or levels. Th e loudest s ound the human ear c an hear wi thout damage due to prol onged exposure is a bout 1,0 0 0,0 0 0,0 0 0 t imes greater than th e quietest perceptible sound.
12 TRG-TRC007 -EN note s peri od one F undamentals of Sound A logarithm is the exponent power of the base. In this case, the base is ten. For example, the log 10 of 1 0 (or 1 0 1 ) equals 1, the lo g 10 of 1 0 0 (or 1 0 2 ) equ als 2, and the log 10 o f 1, 000 , 000 , 000 ( o r 1 0 9 ) equa ls 9.
TRG-TR C0 07-EN 13 peri od o ne F undamentals of Sound note s When a refere nce value is established an d placed in the denominator of the ratio, the dB can be calculated for any value e ntered into the numerator . The referenc e value us ed for calc ulating sound -power leve l is 1 pico wat t (pW), or 1 0 -12 wat ts.
14 TRG-TRC007 -EN note s peri od one F undamentals of Sound Again , these reference values can be considered the threshold of hearing. The multiplier 20 is used in the sound-pressure level e quation instead of 1 0 becaus e sound pow er is propor tional to the squ are of sound press ure.
TRG-TR C0 07-EN 15 note s The study of acoustic s is af fected by the re sponse of the human ear to s ound pressure. Unlike electronic s ound-measu r ing equip ment, wh ic h provides a repeatable , unbiased a nalysis of so und pressure , the sensi tivity of the human ear va ries by fre quency an d magnitu de.
16 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods The s ensation of loudness is principally a function of sound pres sure, h owever , it also de pends upon frequency . As a selec ti ve sensory o rga n, the human ear is more s ensitive to high freq uencies th an to low frequ encies.
TRG-TR C0 07-EN 17 peri od t w o Sound P erc eption and Rating Methods note s Additionally , tones evo ke a partic ularly strong response. R ecall that a ton e is a sound th at occur s at a s ingle frequenc y . Chalk sque aking on a blac kboard, for example , produces a tone that is extremel y ir ritating to ma ny peo ple.
18 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods shal l be X ,” where X is a sing le-number des criptor co nvey ing the desi red quality of s ound. The most fr equently used singl e-number des criptors are the A-weig hting network, n oise criteria (NC), and room criteria (R C).
TRG-TR C0 07-EN 19 peri od t w o Sound P erc eption and Rating Methods note s The follow ing step s describe how to ca lculate an A-w eighted va lue. 1 Star ting with the actual soun d-pressure levels for the eight octa ve bands, add or subt ract the d ecibel val ues represen ted by the A-w eighting curve shown in Figure 23.
20 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods A-weighting is of ten used to define s ound in o utdoor en vironments. For example, local sound ordinances typically regulate dBA levels at property line s.
TRG-TR C0 07-EN 21 peri od t w o Sound P erc eption and Rating Methods note s Noise criteria (NC) curve s are prob ably the most c ommon singl e-number descriptor u sed to rate s ound-pressu re level s in indoor en vironments. Like the equal-loudn ess contours on whic h they are based, the loudness along eac h NC curve is about the sa me.
22 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods The follow ing step s describe how to calcu late an NC rating. 1 Plot the oc tave-band s ound-pressure levels on the NC c har t. 2 The highest curve crossed by the plotted data determ ines the NC rating.
TRG-TR C0 07-EN 23 peri od t w o Sound P erc eption and Rating Methods note s Room criteria (RC) curves are s imilar to NC c urves in tha t they are used to provide a rating for sound-press ure levels in i ndoor environ ments. The m ajor dif ference is that RC curv es give an additio nal indicati on of sound c haracter .
24 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods The follow ing step s describe how to determine an RC rating. 1 Plot the oc tave-band s ound-pressure levels on the RC c hart.
TRG-TR C0 07-EN 25 peri od t w o Sound P erc eption and Rating Methods note s n P erceptible v ibration (R V): The sound level in the octave ban ds between 16 Hz a nd 63 Hz fa lls in the s haded regions ( A and B ).
26 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods If we plot the acoustical dat a for our example of fice sp ace on the RC cha rt, we find that it re sults i n a rating of RC-3 1(R).
TRG-TR C0 07-EN 27 peri od t w o Sound P erc eption and Rating Methods note s The phon is another descriptor used to indicate loudness with a single number . The loudne ss of a sound, ex pressed in p hons, is e qual to the s ound- pressu re level of a standard sound, at 1 ,0 0 0 Hz, that is co nsidered eq ually loud.
28 TRG-TRC007 -EN note s peri od t w o Sound P erc eption and Rating Methods Octav e-Band Rating Method A more usefu l method of rati ng sound lev el is to use the octav e bands discuss ed earl ier .
TRG-TR C0 07-EN 29 note s period thr ee A coustical Analysis The primary acoustical desig n goal for an HV AC sy stem is to ac hieve a bac kground noise level that is q uiet enough s o that it does not inte rfere with the activity requi rements of the space an d is not obtrusive in sound quality .
30 TRG-TRC007 -EN note s period thre e A coustical Analysis Set ti ng a Desi gn Goal The first ste p of an acou stical des ign is to qua ntify the goa l. P eriod T wo introduced sev eral single- number descriptors that designers commonly use to define th e acoustical design g oal for a space.
TRG-TR C0 07-EN 31 period thre e A coustical Analysis note s When defini ng the acoustical des ign goal for an outdoor enviro nment, to mee t a local noise ordi nance for example, the A-weighted scale is typically used. This generally takes the form of a maxim um A-we ighted sound-pressure level a t the lot line of th e proper ty .
32 TRG-TRC007 -EN note s period thre e A coustical Analysis S ource–P ath–Receiv er Analysis Ac hieving th e desire d acoustic al c haracteri stics in a space, h owever , r equires more than s electing an appropriate s ingle-number de scriptor .
TRG-TR C0 07-EN 33 period thre e A coustical Analysis note s Pr edicting the sound l evel in a given s pace require s making a model o f the syste m. A source–path–receiver model pr ovides a systematic approac h to predict the acoustica l c haracteristics in a space.
34 TRG-TRC007 -EN note s period thre e A coustical Analysis The w ork, a nd ar t, of an acoustical ana lysis is in identifying and quantifying the various pa ths that sou nd travels from the source to the receiver . There are primarily three dif ferent types of sound p aths.
TRG-TR C0 07-EN 35 period thre e A coustical Analysis note s S ound can trav el between a single s ource and th e receiver along on e or multiple pa ths. In the case of an air -cooled c hiller sit ting on the roof of a building, and a re ceiver locate d acros s a park ing lot at the ed ge of the proper ty , sound trave ls along on ly one p ath.
36 TRG-TRC007 -EN note s period thre e A coustical Analysis 1 Supply airbo r ne through th e supply du ctwork and diffus ers and into the spac e 2 Supply break out as the sound travels through t he wa.
TRG-TR C0 07-EN 37 period thre e A coustical Analysis note s S ound- P ath Modeling When all the paths have been i dentified, the y can be individually mod eled to determine th e contribution of eac h to the tota l sound heard by the receiver . S ound-path mod eling st udies how sound from a source c hanges on its w ay to a receiver .
38 TRG-TRC007 -EN note s period thre e A coustical Analysis As mentione d previously , the total sound hea rd by the receiver is the su m of sounds fro m multiple so urces, following mu ltiple paths.
TRG-TR C0 07-EN 39 period thre e A coustical Analysis note s Theoret ical eq uations aid the analy sis of so me path elements, bu t predicti on equations based on test d ata an d experience prevail .
40 TRG-TRC007 -EN note s period thre e A coustical Analysis S olving thes e algorithms m a nually ca n be tedious and time consumin g, especia lly when o ne or more p aths need f ur ther at tenuation and the calculations have to be repeated.
TRG-TR C0 07-EN 41 period thre e A coustical Analysis note s T erms Used in S ound-P ath Model ing This sec tion introd uces sev eral terms tha t are fairly sp ecific to th e science of acoustic s. At tenuation refers to the r eduction in sound le vel as sou nd travels along the path from a s ource to a re ceiver .
42 TRG-TRC007 -EN note s period thre e A coustical Analysis The tot al sound en ergy that stri kes a surface (W i ) is either reflec ted (W r ), abs orbe d by t he m ater ial ( W a ), or t r ansmit ted through the material (W t ).
TRG-TR C0 07-EN 43 period thre e A coustical Analysis note s The ability of a material to reduce transmit ted sound is mos t commonly refer r ed to in te rms of its ins er tion loss, noise reduc tion, or tran smission loss.
44 TRG-TRC007 -EN note s period thre e A coustical Analysis Absorptive material s work by c onve rting a c oustical energy into heat ene rgy . The abso r bed ener gy (W a ) is the p or tion of the inc ident sound energy (W i ) that is ne ither transmit ted through the material nor reflected of f the material.
TRG-TR C0 07-EN 45 period thre e A coustical Analysis note s Finally , som e of the i ncident sou nd energ y (W i ) bounces of f of (or is reflected from) the mate rial. R eflected sound beco mes espe cially impor tant when the sound so urce and the receiver are loc ated in the same room.
46 TRG-TRC007 -EN note s period thre e A coustical Analysis Receiv er sound correction , als o called r oom effec t , is the re lationship between the sound en erg y (sound power) e ntering the room and the sound pressure a t a given point in th e room where the receiver he ars the sound.
TRG-TR C0 07-EN 47 note s period f our Equipment Sound Rating As explain ed in P eriod One, sound pressure can be directly measured, however , sound power cannot. Bec ause sou nd pressure i s influence d by the sur roundings, the most accurat e sound data that can be provi ded for a piece of equipment is sound power .
48 TRG-TRC007 -EN note s peri od four Equipment Sound Rating Fields o f Measurement T o m easure sound pressure correc tly , it is im portant to understand the behavior of sound i n various en vironmen ts, or fields . In theory , a free field is a homoge neous, isotropic me dium that is free from boundaries .
TRG-TR C0 07-EN 49 peri od four Equipment Sound Rating note s This type of relationship betwe en distance and s urface area p rovid es the following simple m athematica l model for es timating how sound will c hange as the di stance from th e source increase s.
50 TRG-TRC007 -EN note s peri od four Equipment Sound Rating In practic e, this equa tion is com m only used to determine how loud a piece of equipment will be at a given distanc e. F or exampl e, the manufa cturer of a n air - cooled c hille r lists the s ound-pressu re level of the chi ller as 95 dB at a distance of 30 ft (9.
TRG-TR C0 07-EN 51 peri od four Equipment Sound Rating note s The near field is an area adjacent to the source w here soun d does n ot behave as it wo uld in a f ree field. Mos t sound sou rces, including all HV AC equip ment, do not ra diate sound in perfectly spherical w aves.
52 TRG-TRC007 -EN note s peri od four Equipment Sound Rating A reve rberant f ield is nearly the opposite of a free field. Reve rberant fields exist in rooms with reflective walls, floors, and ceilings.
TRG-TR C0 07-EN 53 peri od four Equipment Sound Rating note s Most rooms in buildings a re somewhere between a free field and a reverberant fiel d enviro nmen t. Cal led a semire ver berant field , these spa c es have some c haracteristic s of both free field and reverberant fiel d env ironments.
54 TRG-TRC007 -EN note s peri od four Equipment Sound Rating HV A C Equ ipment S ound Rating As mentione d earlier , becau se sound pressure is influence d by the sur roundings, of ten the best w ay for an equip ment manufac turer to pro vide sound dat a is to provide s ound-power le vels.
TRG-TR C0 07-EN 55 peri od four Equipment Sound Rating note s The mos t common te st method for HV AC equipment is the rev erberant-room method . The objective of a reverberan t room is to create a uniform, or dif fuse, sound fiel d by reflectin g and mixing the sound w aves.
56 TRG-TRC007 -EN note s peri od four Equipment Sound Rating The free-field method is comm only used fo r HV AC equip ment that is too large to be tested in a reverberant room. This includes water c hiller s, cool ing towers, and the outdoor sound from pa c k aged roof top air conditioners and air -cooled conden sers.
TRG-TR C0 07-EN 57 peri od four Equipment Sound Rating note s Forma l standards are writ ten by indu stry organizati ons to promote unifo rmity of data betw een dif ferent manufacturers.
58 TRG-TRC007 -EN note s peri od four Equipment Sound Rating One of the best e xamples to dem onstrate the com plexity of gathering complete and acc urate sound d ata is air -handling equipm ent. This in volves a ny typ e of HV AC equ ipment that c ontains a fa n and is us ed to condi tion and mo ve air through a duc t system.
TRG-TR C0 07-EN 59 peri od four Equipment Sound Rating note s In order to isolate these dif ferent paths, the air handler must be teste d using a number of differe nt configurations.
60 TRG-TRC007 -EN note s peri od four Equipment Sound Rating Historically , there have been s everal metho ds used to generate sound data fo r air ha ndli ng eq uipm ent .
TRG-TR C0 07-EN 61 peri od four Equipment Sound Rating note s The objective of ARI Standard 260–200 1, Soun d Rating of Ducted Air Moving and Conditioning Equipme nt , is to deli ver sound data that accurately represents the acous tical impa ct of the air -handling equipm ent af ter it is installed.
62 TRG-TRC007 -EN note s peri od four Equipment Sound Rating Depending o n the type of equipmen t, sound rating s for HV AC e quipment are typically given as sound-p ower levels by octave ba nd or as a single dBA ra ting.
TRG-TR C0 07-EN 63 note s W e will now r eview th e main con cepts that w ere co vered in this clinic on t he fundamenta ls of HV AC acous tics. P eriod One explained some of the basic concepts of s ound. S ound is the audible em issions resulting from the vi bration of molec ules with in an elasti c medium.
64 TRG-TRC007 -EN note s peri od five Review P eriod T wo di scussed how the hum an ear perceives sound. As a selective sensory organ, th e human ear i s more sens itive to high frequen cies than to low frequ encies. The se nsitivity of the human ear at a par ticular fre quency also c hanges with loudnes s.
TRG-TR C0 07-EN 65 peri od five Review note s P eriod Three wal ked through the step s of an acoustica l analysis , inclu ding set ting the des ign target for the indoor or outdoo r environment, an d performing a s ource–path –receiver analys is.
66 TRG-TRC007 -EN note s peri od five Review P e riod Four int roduced two common meth ods used by HV AC equipment manufactu rers to provid e accurate sound data. Because sound press ure is influence d by the sur roundings, t he most useful soun d data th at can be provide d for most pieces of equipment is sound pow er .
TRG-TR C0 07-EN 67 peri od five Review note s For m ore information, r efer to the following reference s: n Acoustics in Air Condition ing Applicatio ns Engineerin g Manual (T rane literature order nu.
68 TRG-TRC007 -EN Questions f or P eriod 1 1 What unit of measure is used to describ e frequency? 2 Define a ton e. 3 S ound _____ __ (power o r pressure) is what our ea rs hear and is influenced by the su rroundings.
TRG-TR C0 07-EN 69 Quiz note s Questions f or P eriod 3 7 T rue or Fals e: Sound can only travel from a source to the receiver along one path. 8 T rue or Fa lse: One piece of HV AC equipme nt may contain s everal soun d source s.
70 TRG-TRC007 -EN 1 Hertz (cycle s per seco nd) 2 A sound at a single freq uency . A sound at a nar row band of freque ncies that is s ignificantly greater than the sound a t adjacent f r equenci es wou ld be simila r to a tone.
TRG-TR C0 07-EN 71 abso rbed sound S ound energ y that strikes a material and is con ver ted from sound ene rgy to h eat energ y within the ma terial. absor ption coef ficient The ratio of the soun d energy absorbed by the material to the t otal sound e nergy in cident upon the surfa ce of that ma terial.
72 TRG-TRC007 -EN Glossary her tz (Hz) The unit of mea s ure for frequen c y . One her tz is equal to one cycle per seco nd. inser tion loss (IL) The diffe rence in sound pres sure measu red in a sing le locat ion, with and without a noise co ntrol devi ce (installe d between the source and receiver) in p lace.
TRG-TR C0 07-EN 73 Glossary reverberant fie ld A uniform, or d iffuse , sound fiel d that is the opposite of a free field. In a perfectly reverberant field, the so und-pressure level is equal at all points. reverberant room A speciall y constructe d room with reflec tive walls, flo ors, and ceili ngs.
74 TRG-TRC007 -EN note s Glossary the receiver , and ev erything it encounters as it travels along the w ay , constitute s the path. t one A sound in a single fre quency . A sound in a nar row band of frequ encies that is significan tly greater than the sound at adjacent freque ncies wou ld be similar to a tone.
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