Document bO9bxB5n0kqq4be1Q20XybgbO

644 JEROME R. COX, JR. Under certain conditions it is possible to calculate the approximt, pressure level produced in various parts of a room as a result of a simg source near the center of the room.8 The results of these calculations^! having various average absorption coefficients, a, are shown in Figurel horizontal axis is the distance, r, from the source to the pressure-rtf microphone divided by the radius, r0, of a sphere whose surface area is as that of the room: that is, r0 = Vsurface area/4 v. For example, the ' bar* r la tht Mituc* Trm thn Srarca ndr, 7 Total tarfua Ana of Korn/*' Figure 14. Curve3 showing the variations in sound pressure level in roorrl3..v^itii ..... .. .... sorting waifs. '* ,, ^ &, ~ a sphere whose surface area is the same as that of a cute "will: \/6!2/4ir = 0.691, where l is the length of a side of the cube. The veffi Figure 1.4 shows .the .increase -in sound pressure level that. :PYrgfe:i the room over the sound pressure level that would exist at ri> with; *H. F. Hopkins and N. R. Stryker, A proposed loudness-efficiency rating for|i___ and the determination of system power requirements for enclosures PtocI 36, 315 (1948). . *" NOISE AND THE CONSERVATION OP HEARING 645 iinfo free space. The various curves on this chart are drawn for average Jpiloefficients, between 0.01 and 1. ' in Figure 14 show that there are two distinct parts of the sound jift|jhfaopm. In the reverberant part of the field the sound pressure level is 8|$j|n$|qf location. However, closer to the source the field behaves like that 8Pace- For many practical sources this free field does not extend v^aolthe source. In these cases a third kind of field exists called the near detailed knowledge of the sound source itself,, it: is impossible sound pressure level in the near field of a sound source, rulves shown in Figure 14 will hold only when both the sound source and ^ufe-measuring microphone are located at least one half of a wave length HiiM|he`walls. Thus, if this room is very small or the frequency very low, Iljjbe large portions of the room in which Figure 14 will give incorrect addition, the curves will be more nearly correct for a source that pde-band noise than for one that radiates a pure tone. ||G. the transmission of sound through partitions l.the discussions have centered upon the transmission of sound from a 'sbme point in the space surrounding the source. If the sound source is, in partitioned off from the measuring microphone, the transmission of If;, as a rule, be drastically reduced. : [Ili^iare important exceptions to this rule. For example, "acoustical mate- u*'w||linot cause large reductions in the sound transmitted through them. irately, there is some confusion about this matter resulting from the fact Pod sound absorber is often a good heat insulator. Thfe heat-insulating |#of the material are derived from the air trapped in the many tiny air Mis difficult for a current of hot air to move through the material and, erefmse|;it is possible to take advantage of the poor heat, conductivity of a sta- Sr mass. Some materials have no connections between the. air spaces and jmb are very good heat insulators, but poOft-Sound^^ J'T:te,,word insulation should have no control. XT]!js'oftPi-.erroneously associated with quieting because man^'M'anUfa'cibrers claim & (heat) insulating material has good acoustical properties. They mean good sound absorber. The public, however, often concludes that the " i^OT'transmiFbut'a small amount of the sound incident upon it. In other iHne consumer assumes that the heat-insulating material.v.'will provide ex- jijjciund isolation. As we can see from Figures. 15, 16, and.^^|this is pot the J|Tinrap iinnftpennssiiKtyr noff +th.hep RsomumnHd iiss reduc'ed^^llv^ipMdPpS&ge tthhrrmouivghh tthhee .^S^matenaT/.tiutiebnBiderabl.jcgreater-r.eductiondfSarPb^Sfetalaed^ivith-less- materials. Figure 15 demonstrates the effect of a 4-inch thick glass fiber $hladl&||veighing 6 pounds per cubic foot. The frequency is 250 c.p.s. Most sound- materials are either thinner or less dense so that a larger fraction of the ' ss^.l1* sound is transmitted than is shown. in Figure 15. i