N-2140 Sound Propagation

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2 Pink noise, in contrst, is defined s hving the sme mplitude for ech octve bnd (or third-octve bnd), rther thn for ech frequency intervl. Its octve or third-octve bnd spectrum is truly stright, level line over the entire udible spectrum. Pink noise genertors re therefore conveniently used to clibrte octve or third-octve bnd nlyzers. Both white nd pink noise sound somewht like the sttic herd from rdio tht is not tuned to prticulr sttion. N-2140 Sound Propgtion From the source to the receiver noise chnges both in level nd frequency spectrum. The most obvious is the decrese in noise s the distnce from the source increses. The mnner in which noise reduces with distnce depends on the following importnt fctors: Geometric Spreding from Point nd Line Sources Ground Absorption Atmospheric Effects nd Refrction Shielding by Nturl nd Mnmde Fetures, Noise Brriers, Diffrction, nd Reflection N-2141 Geometric Spreding from Point nd Line Sources Sound from smll loclized source (pproximting "point" source) rdites uniformly outwrd s it trvels wy from the source in sphericl pttern. The sound level ttenutes or drops-off t rte of 6 dba for ech doubling of the distnce (6 dba/dd). This decrese, due to the geometric spreding of the energy over n ever incresing re, is referred to s the inverse squre lw. Doubling the distnce increses ech unit re, represented by squres with sides in Figure N , from 2 to 4 2. Since the sme mount of energy psses through both squres, the energy per unit re t 2D is reduced 4 times from tht t distnce D. Thus, for point source the energy per unit re is inversely proportionl to the squre of the distnce. Tking 10 log 10 (1/4) results in 6 dba reduction (for ech doubling of distnce). This is the point source ttenution rte for geometric spreding. 24

3 Figure N Point Source Propgtion (Sphericl Spreding) Are = 2 Are = POINT SOURCE D D As cn be seen in Figure N , bsed on the inverse squre lw the chnge in noise level between ny two distnces due to the sphericl spreding cn be found from: dba 2 = dba Log 10 [(D 1 /D 2 )] 2 = = dba Log 10 (D 1 /D 2 ) (eq. N ) Where: dba 1 is the noise level t distnce D 1, nd dba 2 is the noise level t distnce D 2 Figure N Chnge in Noise Level with Distnce Due to Sphericl Spreding Source Rec. 1 Rec. 2 D 1 D 2 However, highwy trffic noise is not single, sttionry point source of sound. The movement of the vehicles mkes the source of the sound pper to emnte from line (line source) rther thn point when viewed over some time intervl (see Figure N ). This results in cylindricl spreding rther thn the sphericl spreding of point source. 25

4 Since the chnge in surfce re of cylinder only increses by two times for ech doubling of the rdius insted of the four times ssocited with spheres, the chnge in sound level is 3 dba per doubling of distnce. The chnge in noise levels for line source t ny two different distnces due to the cylindricl spreding becomes: dba 2 = dba Log 10 (D 1 /D 2 ) (eq. N ) Where: dba 1 is the noise level t distnce D 1, nd conventionlly the known noise level dba 2 is the noise level t distnce D 2, nd conventionlly the unknown noise level Note: the expression 10 Log 10 (D 1 /D 2 ) is negtive when D 2 is greter thn D 1, positive when D 1 is greter thn D 2, nd the eqution therefore utomticlly ccounts for the receiver being frther out or closer in with respect to the source (Log 10 of number less thn 1 gives negtive result; Log 10 of number greter thn 1 is positive, nd Log 10 (1) = 0). Figure N Line Source Propgtion (Cylindricl Spreding) Line Source Are = b Are = 2b D b 2D 2b N-2142 Ground Absorption Most often, the noise pth between the highwy nd the observer is very close to the ground. Noise ttenution from ground bsorption nd reflective wve cnceling dds to the ttenution due to geometric spreding. Trditionlly, the ccess ttenution hs lso been expressed in terms of ttenution per doubling of distnce. This pproximtion is done for simplifiction only, nd for distnces of less thn 60 m (200 feet) prediction results bsed on this scheme re sufficiently ccurte. The sum of the geometric spreding ttenution nd the excess ground ttenution (if ny) is referred to s the ttenution rte, 26

5 Temperture nd humidity - Moleculr bsorption in ir lso reduces noise levels with distnce. Although this process only ccounts for bout 1 dba per 300 m (1000 ft) under verge conditions of trffic noise in Cliforni, the process cn cuse significnt longer rnge effects. Air temperture, nd humidity ffect moleculr bsorption differently depending on the frequency spectrum, nd cn vry significntly over long distnces, in complex mnner. Rin. - Wet pvement results in n increse in tire noise nd corresponding increse in frequencies of noise t the source. Since the propgtion of noise is frequency dependent, rin my lso ffect distnce ttenution rtes. On the other hnd, trffic generlly slows down during rin, decresing noise levels nd lowering frequencies. When wet, different pvement types interct differently with tires thn when they re dry. These fctors mke it very difficult to predict noise levels during rin. Hence, no noise mesurements or predictions re mde for riny conditions. Noise btement criteri nd stndrds do not ddress rin. N-2144 Shielding by Nturl nd Mn-mde Fetures, Noise Brriers, Diffrction, nd Reflection A lrge object in the pth between noise source nd receiver cn significntly ttenute noise levels t tht receiver. The mount of ttenution provided by this shielding depends on the size of the object, nd frequencies of the noise levels. Nturl terrin fetures, such s hills nd dense woods, s well s mnmde fetures, such s buildings nd wlls cn significntly lter noise levels. Wlls re often specificlly used to reduce noise. Trees nd Vegettion - For vegettive strip to hve noticeble effect on noise levels it must be dense nd wide. A stnd of trees with height tht extends t lest 5 m (16 ft) bve the line of sight between source nd receiver, must be t lest 30 m (100 ft) wide nd dense enough to completely obstruct visul pth to the source to ttenute trffic noise by 5 dba. The effects pper to be cumultive, i.e. 60 m (200 ft) wide stnd of trees would reduce noise by n dditionl 5 dba. However, the limit is generlly totl reduction of 10 dba. The reson for the 10 dba limit for ny type of vegettion is tht sound wves pssing over the tree tops ( sky wves ) re frequently refrcted bck to the surfce, due to downwrd tmospheric refrction cused by wind, temperture grdients, nd turbulence. Lndscping - Cltrns reserch hs shown tht ordinry lndscping long highwy ccounts for less thn 1 dba reduction. Clims of increses in noise due to removl of vegettion long highwys re mostly spurred by the sudden visibility of the trffic source. 33

6 There is evidence of the psychologicl "out of sight, out of mind" effect of vegettion on noise. Buildings - Depending on the site geometry, the first row of houses or buildings next to highwy my shield the second nd successive rows. This is often the cse where the fcility is t-grde or depressed. The mount of noise reduction vries with house or buildig sizes, spcing of houses or buildings, nd site geometry. Generlly, for n t-grde fcility in n verge residentil re where the first row houses cover t lest 40% of totl re (i.e. no more thn 60% spcing), the reduction provided by the first row is resonbly ssumed t 3 dba, nd 1.5 dba for ech dditionl row. For exmple, behind the first row we my expect 3 dba noise reduction, behind the second row 4.5 dba, third row 6 dba, etc. For houses or buildings pcked tightly, (covering bout 65-90% of the re, with 10-35% open spce), the first row provides bout 5 dba reduction. Successive rows still reduce 1.5 dba per row. Once gin, nd for the reson mentioned in the bove vegettion discussion, the limit is 10 dba. For these ssumptions to be true, the first row of houses or buidings must be equl to or higher thn the second row, which should be equl to or higher thn the third row, etc. Noise Brriers - Although techniclly ny nturl or mn-mde feture between source nd receiver tht reduces noise is noise brrier, the term is generlly reserved for either wll or berm tht is specificlly constructed for tht purpose. The cousticl design of noise brriers is covered in sections N-4000 (Trffic Noise Model) nd N-6000 (Acousticl Brrier Design Considertions). However, it is pproprite t this time to introduce the cousticl concepts ssocited with noise brriers. These principles loosely pply to ny obstcle between source nd receiver. Referring to Figure N , when noise brrier is inserted between noise source nd receiver, the direct noise pth long the line of sight between the two is interrupted. Some of the cousticl energy will be trnsmitted through the brrier mteril nd continue to the source, lbeit t reduced level. The mount of this reduction depends on the mteril s mss nd rigidity, nd is clled the Trnsmission Loss. The Trnsmission Loss (TL) is expressed in db nd its mthemticl expression is: TL = 10log 10 (E f /E b ) (eq. N ) where: E = the reltive noise energy immeditely in front (source side) of the brrier f E = The reltive noise energy immeditely behind the brrier (receiver side) b 34

7 Figure N Altertion of Sound Pths After Inserting Noise Brrier Between Source nd Receiver. NOISE BARRIER A ABSORBED SOURCE DIRECT REFLECTED DIFFRACTED TRANSMITTED SHADOW ZONE RECEIVER Note tht E nd E re reltive energies, i.e. energies with reference to the energy of 0 db f b (see section N-2134). As reltive energies they my be expressed s ny rtio (frctionl or percentge) tht represents their reltionship. For instnce if 1 percent of the noise energy striking the brrier is trnsmitted, TL = 10log 10 (100/1)= 20 dba. Most noise brriers hve TL s of 30 dba or more. This mens tht only 0.1 percent of the noise energy is trnsmitted. The remining direct noise (usully close to 100 percent) is either prtilly or entirely bsorbed by the noise brrier mteril (if sound bsorptive), nd/or prtilly or entirely reflected (if the brrier mteril is sound reflective). Whether the brrier is reflective or bsorptive depends on its bility to bsorb sound energy. A smooth hrd brrier surfce such s msonry or concrete is considered to be lmost perfectly reflective, i.e.lmost ll the sound striking the brrier is reflected bck towrd the source nd beyond. A brrier surfce mteril tht is porous with mny voids is sid to be bsorptive, i.e. little or no sound is reflected bck. The mount of energy bsorbed by brrier surfce mteril is expressed s n bsorption coefficient α, which hs vlue rnging from 0 (100% reflective) to 1 (100% bsorptive). A perfect reflective brrier (α=0) will reflect bck virtully ll the noise energy (ssuming trnsmission loss of 30 dba or greter) towrds the opposite side of highwy. If we ignore the difference in pth length between the direct nd reflected noise pths to the opposite (unprotected) side of highwy, the mximum expected increse in noise will be 3 dba. If we wish to clculte the noise increse due to prtilly bsorptive wll we my use eq. N E in this cse is still the noise energy striking the brrier, but E now becomes f b 35

8 the energy reflected bck. For exmple, brrier mteril with n α of 0.6 bsorbs 60% of the direct noise energy nd reflects bck 40%. To clculte the increse in noise on the opposite side of the highwy in this sitution the energy loss from the trnsformtion of the totl noise striking the brrier to the reflected noise energy component is 10log 10 (100/40)= 4 dba. In other words, the energy loss of the reflection is 4 dba. If the direct noise level of the source t receiver on the opposite side of the highwy is 65 dba, the reflective component (ignoring the difference in distnces trveled) will be 61 dba. The totl noise level t the receiver is the sum of 65 nd 61 dba, or slightly less thn 66.5 dba. The reflected noise cused n increse of 1.5 dba t the receiver. Referring bck to Figure N , we hve discussed the direct, trnsmitted, bsorbed, nd reflected noise pths. These represent ll the vritions of the direct noise pth due to the insertion of the brrier. Of those, only the trnsmitted noise reches the receiver behind the brrier. There is, however, one more pth, which turns out to be the most imported one, tht reches the receiver. The noise pth tht before the brrier insertion ws directed towrds A is diffrcted downwrd towrds the receiver fter the brrier insertion. In generl, diffrction is chrcteristic of ll wve phenomen (including light, wter, nd sound wves). It cn best be described s the bending of wves round objects. The mount of diffrction depends on the wvelength nd the size of the object. Low frequency wves with long wvelengths pproching the size of the object, re esily diffrcted. Higher frequencies with short wvelengths in reltion to the size of the object, re not s esily diffrcted. This explins why light, with its very short wvelengths csts shdows with firly shrp, well defined edges between light nd drk. Sound wves lso cst shdow when they strike n object. However, becuse of their much longer wvelengths (by t lest hlf dozen or so orders of mgnitude) the noise shdows re not very well defined nd mount to noise reduction, rther thn n bsence of noise. Becuse noise consists of mny different frequencies tht diffrct by different mounts, it seems resonble to expect tht the greter the ngle of diffrction is, the more frequencies will be ttenuted. In Figure N , beginning with the top of the shdow zone nd going down to the ground surfce, the higher frequencies will be ttenuted first, then the middle frequencies nd finlly the lower ones. Notice tht the top of the shdow zone is defined by the extension of stright line from the noise source (in this cse represented t the noise centroid s point source) to the top fo the brrier. The diffrction ngle is defined by the top of the shdow zone nd the line from the top of the brrier to the receiver. Thus, the position of the source reltive to the top of the brrier determines the extent of the shdow zone nd the diffrction ngle to the receiver. Similrly, the receiver 36

9 loction reltive to the top of the brrier is lso importnt in determinig the diffrction ngle. From the previous discussion, three conclusions re cler. First, the diffrction phenomenon depends on three criticl loctions, tht of the source, the top of brrier, nd the reciver. Second, for given source, top of brrier nd receiver configurtion, brrier is more effective in ttenuting higher frequencies thn lower frequencies (see Figure N ). Third, the greter the ngle of diffrction, the greter the noise ttenution is. Figure N Diffrction of Sound Wves Source High Frequencies Low Frequencies Brrier The ngle of diffrction is lso relted to the pth length difference (δ) between the direct noise nd the diffrcted noise. Figure N illustrtes the concept of pth length difference. A closer exmintion of this illustrtion revels tht s the diffrction ngle becomes greter, so does δ. The pth length difference is defined s δ = +b-c. If the horizontl distnces from source to receiver nd source to brrier, nd lso the differences in elevtion between source, top brrier nd receiver re known,,b, nd c cn redily be clculted. Assuming tht the source in Figure N is point source,, b, nd c re clculted s follows: = [d + ( h h ) ] b = ( d + h ) 2 c = ( d + h )

10 Figure N Pth Length Difference Between Direct nd Diffrcted Noise Pths. TOP BARRIER SOURCE Diffrction Angle b h 1 h 2 d 1 c d 2 RECEIVER d PATH LENGTH DIFFERENCE (δ) = +b-c Highwy noise prediction models use δ in the brrier ttenution clcultions. Section N covers the subject in greter detil. However, it is pproprite to include the most bsic reltionship between δ nd brrier ttenution by wy of the so-clled Fresnel Number (N 0 ). If the source is line source (such s highwy trffic) nd the brrier is infinitely long, there re n infinite mount of pth length differences. The pth length difference (δ 0 )t the perpendiculr line to the brrier is then of interest. Mthemticlly, N 0 is defined s: N 0 = 2(δ 0 /λ) (eq. N ) where: N 0 = Fresnel Number determined long the perpendiculr line between source nd receiver (i.e. brrier must be perpendiculr to the direct noise pth) δ 0 λ = δ mesured long the perpendiculr line to the brrier = wvelength of the sound rdited by the source. According to eq. N , λ = c/f, nd we my rewrite eq. N : N 0 = 2(fδ 0 /c) (eq. N ) where: f = the frequency of the sound rdited by the source c = the speed of sound 38

11 Note tht the bove equtions relte δ 0 to N 0. If one increses, so does the other, nd brrier ttenution increses s well. Similrly, if the frequency increses, so will N 0, nd brrier ttenution. Figure N shows the brrier ttenution Δ B for n infinitely long brrier, s function of 550 Hz (typicl verge for trffic). Figure N Brrier Attenution (Δ B ) vs Fresnel Number (N 0 ), for Infinitely Long Brriers -25 Δ Β, db N 0 There re severl rules of thumb for noise brriers nd their cpbility of ttenuting trffic noise. Figure N illustrtes specil cse where the top of the brrier is just high enough to grze the direct noise pth, or line of sight between source nd receiver. In such n instnce the noise brrier provides 5 dba ttenution. Figure N Direct Noise Pth Grzing Top Brrier Results in 5 dba Attenution SOURCE NOISE BARRIER DIRECT, GRAZING RECEIVER ATTENUATION: 5 dba Another sitution, where the direct noise pth is not interrupted but still close to the brrier, will provide some noise ttenution. Such negtive diffrction (with n ssocited 39

12 negtive pth length difference nd negtive Fresnel Number ) generlly occurs when the direct noise pth is within 1.5 m (5 ft) bove the top of brrier for the verge trffic source nd receiver distnces encountered in ner highwy noise environments. The noise ttenution provided by this sitution is between 0-5 dba: 5 dba when the noise pth pproches the grzing point nd ner 0 dba when it clers the top of brrier by pproximtely 1.5 m (5 ft) or more. Figure N Negtive Diffrction Provides Some Noise Attenution SOURCE NOISE BARRIER NEGATIVE DIFFRACTION RECEIVER ATTENUATION: < 5 dba The forementioned principles of brriers loosely pply to terrin fetures (such s berms, low ridges, s well s other significnt mnmde fetures). The principles will be discussed in greter detil in sections N-5500 nd N N-2200 EFFECTS OF NOISE; NOISE DESCRIPTORS N-2210 Humn Rection to Sound People rect to sound in vriety of wys. For exmple, rock music my be plesnt to some people while for others it my be nnoying, constitute helth hzrd nd/or disrupt ctivities. Humn tolernce to noise depends on vriety of cousticl chrcteristics of the source, s well s environmentl chrcteristcs. These fctors re briefly discussed below: 1. Level, vribility in level (dynmic rnge), durtion, frequency spectrums nd time ptterns of noise. Exposures to very high noise levels cn dmge hering. A high level is more objectionble thn low level noise, nd intermittent truck pek noise levels re more objectionble thn the continuous level of fn noise. Humns hve better hering sensitivities in the high frequency region thn in the low. This is reflected in the A-scle (section N-2136) which de-emphsizes the low frequency 40