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Noise concrof <br />Noise control as the name implies. is the <br />control of noise with the uldrrace goal <br />of reducing the cve ll sound level or <br />noise at some givrri point. Control may <br />be at the source, between the source <br />and the te¢iver, or at the receiver's <br />location. <br />For smpTirYry's sake, noise control <br />may be classified in five fundamental <br />groups: <br />A. Ratw.ol Of the offending noise <br />souRG <br />S. Ahsorbtion of the sound waves <br />be(Me, they reach tie receives. <br />C. Blocking the sound thezeby <br />preventing k from reaching the <br />rcrive" <br />D. Reducing the intercity of the . <br />[wise sourcc- <br />I:- When udtter methods fa4 sltoc- <br />ing the exposure time of person. <br />net. <br />Figure 26 <br />Removal of the Of fending <br />Noise Source <br />Obviously, the best medtod of noise <br />central would be to emanate th* source. <br />This Tn a increasing the dstartcc be- <br />ttveea the source and the receiver. In <br />Appe-,dix A it is shown that sound <br />ptesswe dim@tisltes by 6 dB each time <br />the distance bawccn the source and the <br />receiver is doubled. <br />Distance <br />(in feet) 20 40 80 160 <br />Sound <br />Pressure <br />Level loo db 94 88 82 <br />Figure 27 <br />It is important to recognize chat a 6 <br />dB reduction of soured pressure level <br />(SPL) is equal to halving the sound <br />pressure in a film field. it is generally im- <br />matenal whether the source of the <br />rsceivet or both arc movtel m increase <br />the matte, a long as the distance is <br />incmased. It is a simple matter to use <br />equation 8 to compute the additional <br />3istatue vfikh.must be moved is order <br />to achieve the given result. <br />SPLr - SPL, - 201"" R <br />E,,p-an 3 <br />Figure 28 shows values for 201ogha <br />l) for dtffetetu ratios of R <br />Z <br />Rr/Rh 20 1%. Ri/Rt <br />1 0 <br />2 6 <br />3 9.5 <br />4 12 <br />5. 14 <br />7.5 17.5 <br />10 20 <br />A lirde less obvious sac of this solu- <br />cion considers moving to new locaoons <br />which are less reflecrwe, more absorp- <br />tive, or under some circumstances may <br />crate a sicuauon whereby natural rer- <br />rain will have a beneficial result. <br />Under tcveaberant condition-, such <br />as in a gvarrr of pit, this rrmthod may <br />ba far less e(fecave- In facr, under some <br />circvtrrstames, increasing distance is <br />event de.=uncrual See i=W= 36 for a <br />chart showing tear field, free field, and <br />mverberaru field for an enclosed area. <br />Absorption of sound u,avcs <br />Absorpuon is a pros- whereby sound <br />energy its air is changed into hear <br />energy. This process occurs as a mSWE of <br />obscrucrioru, usually very small. that <br />e7vte friction in The air flow thereby. <br />creating hest. <br />The materials which are usually <br />used to absorb sound waves ate general- <br />ly fibrous, lightweight, and porous. The <br />fibers must be relatively rigid if they are <br />to be effective:. If a cellular material, <br />such as foam, is to be used, it must <br />have interconnecting passages from one <br />cell to the other. The more ,winding <br />these passages are. the better- <br />Some examples of materials which <br />ace ooceBcnr absorbers of sound arc <br />acoustic tiles, dtaperi fixes type of md- <br />fiers such as glass packs, urethane <br />foams, and foamed elasmmeTs. Probably <br />the moat important ehaxv tcriseie 6 B.- <br />t<sistanoc. The flow resistance must be <br />increased w rhickness is decreased in <br />order to nuintain a consircm level of <br />A-prion. <br />71te resistance. of flow can be rrrdi)y <br />served by Trying to blow through the <br />material thereby affording a tray to <br />compare the res;-t,ac to flow M the <br />ahaorpcivity of Elie material. Absorbent <br />materials are excellent for wall coverings <br />in order to ptevene reflection in a room <br />Fgwe Z8