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,.3.2 Holes in Barriers
<br />such do holes cumhrumise the Transmission
<br />I of barriers.' More than would be expec-
<br />!'.�d, by far. For example, let us assume we
<br />:,ave 30 dbA at the source side of a barrier
<br />and that the TL of the barrier is 30 dBA.
<br />without holes, the noise on the opposite side
<br />:could be 50 dBA. Now let us open up one-
<br />tt:nth of the area of the barrier. The bar-
<br />rier surface is now 10% open. What is the
<br />rict TL of the barrier -plus -hole?
<br />I'irst, ninety pereent of the noise energy
<br />:1114 flllr bArrur its(Af and is reduced by
<br />i0 dBA. Ninety percent is converted to
<br />decibels using 'fable 5.6. From the table,
<br />ninety percent of 80 ABA is 80-0.5 = 79.5
<br />.litA. In deciboi.i, nc••arly all thr energy
<br />!,its the barrior. itself. This 79.5 is re-
<br />luced by 30, yielding 49.5 dBA.
<br />second, ten percent of the noise energy
<br />hits the hole, .and is increased by 6 dBA.
<br />From the table, 10 pt,rc:ent of 810 dIIA is 70
<br />dbA. This is 111creast•c1 by 6, yielding 76
<br />,1HA. Finally, titu totj: ent•rgy is the dB -
<br />.,,am of 49.5 dbA and 76 iOA, which is 76 dBA.
<br />L'he barrier- has provided only 4 dBA reduction.
<br />rno reason that the hole compromised the
<br />harrier attenuation sr) drastically is due to
<br />tint• logarithmic nature of noise.. The har-
<br />rlt,r it!Wti csscnLially eliminates 90 per -
<br />1. 14 the noi:+e en,rr,ly, I,ut. this is only
<br />duct ion 4)1 lU ,IIIA. I:v,•n more t•xtr,•mt•,
<br />ht• t..rrt It r yot rill of 9'1 ht•rcent of the.
<br />-l::tr ,•ncoly, Lir,- r,•,lrtct ton would be only
<br />.0 dIIA.
<br />The second rt .c:un !:,r t l •• boor p,rtorrnanco
<br />„1 the b.rrI I,•I-with-hr,1,. as lire b dHA in-
<br />ere.asc_ in noint: thr,,:agh the hole.
<br />HOLE AMPLIFICATION
<br />7 L moic • - 6 d8A
<br />""I'S inti nt ; a>; ,luc Lo :;o -cal led "prt>ssure-
<br />1011Mtr;:;" at tilt: !•Jrrt„r'., sur'ta"'.. More
<br />"111217% ptts::, d throu,lh 1. irr t,tr I t• than was
<br />tr:aignt-in,:rarutr. ::n it. The phenomenon i:s
<br />t•ump1,1x, b;,t rt al. '% • l”, ycxtd absorptive
<br />t reatment oto the _:curet :iidc of the uarrit•r
<br />,:an eliminate this 6 dBA amplification
<br />through the hole. In the examplt:, then, the
<br />net attenuation would be 10 dBA. The absorp-
<br />tion must be broad -band, rather than tontine(]
<br />to discreet frequencies, such as providt.•d by
<br />resonant absorbers.
<br />Table 5.7 combines these phenomena to indi-
<br />.ate the maximum 'rransmissirin Loss of a bar-
<br />rier with a holt. As can be seen, very
<br />rimall holes indeed can put low limits on the
<br />,f barriers.
<br />5-27
<br />Holes in barriers provide Lwo furth,•r cumpli-
<br />cations.
<br />• The 6 dBA amplification di:;,•ns,;t:d .above
<br />is due to an averaging over Lhe entire fre-
<br />quency range. Throughout most of the range,
<br />the noise is attenuated. But at: Lh,r reso-
<br />nance frequencies of the holes, it rs am-
<br />plified, sometimes by 15-20 dbA. The re-
<br />sulting noise through the holt, i,; not only
<br />amplified an average of 6 d8A1 b,;,: its cha-
<br />racter can be changed from a brcrri-!,and
<br />noise to one with discreet pure r.
<br />These pure Lones would be more objoct.ionJble
<br />than their A-level indicatt:s.
<br />• i•'iqure ;.24 demonstrates anotht•r com-
<br />plicaLlon clue to holes. When :r l,.arrier I.
<br />Slotted vorLically at regular int,•rvals, the
<br />sluts could behave as a diflrac:ti,,n grdtinq.
<br />The noist: emanatinq from a sing!,•
<br />ha�s,.s Lt',ruugh all the slots and •.rn pier -loci;
<br />sharp constructive interferent,., b.rnd_; on the
<br />rec,_•iver side: of the barrier. The more _iLoLs
<br />the inure lo,:alized would be these -,antis ->i
<br />cconstrucEive interference. As thu truck
<br />moves alonq Lhe highway, these han-k would
<br />n'.uV,_ with it, sweeping past the
<br />'rhe „ffe;:t might be similar to an ,,xplosiun
<br />or cannon shut as tyre truck passe: by.
<br />5.3.3 Absorptive Barriers
<br />In Frqure S. 1 above, the reflect,,,] encr,ty r••
<br />:shown to be impurtanL for rccc•ivtr:; on flat:
<br />ol,po-S i to side, of tht: roadway 1 roar a r'e t L,
<br />tiv„ harrier. In Example M6 J1,OV,•, Che ,•i
<br />1t•4:t of this rt•I ltuctt•d noise w,v c.ilt:ul.rt,•,1
<br />„xpl ic:i t ly. I I th,r barrier wal Is could ht•
<br />n,d,i, actru;tical ly at,sorptive, tilt r -!Ii--;c: ,. -
<br />t_
<br />1-cted omponcnt would be rt•du+ , , In inti
<br />ca:,•:,, ter:; would prcrvrcic: sign_t ic.int L,,•nt
<br />tit to tate opposite ret.eivcrs.
<br />Ilow much rs the ref lect.-d not :;o rctl,ac:ed;
<br />Thi:, •L_I„ nds upon the .ibsorption t-,;eftic:c•r,t_
<br />of th.: bdrrior wall. For a fu11 ,answer, Iii,!
<br />absorption coeffici,•nt must b(e knt:wn as a
<br />function of frequency. Then the traftic
<br />spectrum ImosL importantly the trlick spo -
<br />Lrum) is r,_duced by the absorption at ,•,t 1;
<br />fre,lu,•ncy, to obtain the retlect,r,l ripe, rrr,u,.
<br />After the A-level of tilts new spt:,c•ttu:n 1,;
<br />calculated, it is Compared to tilt-- or i•ttr,.,l
<br />A -levo) to obtain a reduction in ,,1::t. TI, t:,
<br />procedure is cumbersome, and ,:an ,..•n, rally
<br />60 simply tied as described b,_ lr,w.
<br />A >tr.yltr-r. un;iter absorption coot 11c i,•nt is
<br />araloqut d by the. Acoustical art -i In: ulating
<br />M•atorials Association. This simll,•-numbor
<br />coetticient is called the Noir:, P. luction
<br />Coet ti tient, NRC. It is an avcra•i• of the
<br />absorption coefficients in tht ir„•luency re-
<br />gion from approximately 200 to 3000 Hz.
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