Document gbGax5R4ae67rMj0KX34qgozN

652 JEROME R. COX, JR. exactly equal to that radiated into the room. The sound power absorbed square foot of wall surface is equal to the incident intensity times the absorpl coefficient. The total power absorbed by the walls, therefore, will be equal tpf average intensity times Sa, where S is the total surface area of the walls, ceil) and floor and a is the average absorption coefficient of the room. Equating* power absorbed by. the walls to the power radiated into the room we can solve1 average intensity of the reverberant sound in the room: Reverberant intensity XS = transmitted intensity X g Reverberant intensity = o--a (transmitted intensity) Combining the reverberant and the transmitted sound will give the a)T sound pressure in the secondary room. The difference in sound pressure levgl opposite sides of the partition is called the noise reduction and may be found)' the aid of Figure 20. The horizontal axis of Figure 20 gives the ratio (Sn,/^ +6 .jdl 1.08 = 0.8 +2 r* 0.5 35 -2 0.2 O.tl or 0.02' -It M '1 0.01 0.02 0.04 ---- 0.2 0.4 Ratio of Transmitting Wall Area (Sy) to Total Surface Area of Room (S) Figure 20. Curves for the calculation of the noise reduction between two rooms.*! NOISE AND THE CONSERVATION OP HEARING 653 jlsrpitting area to the total surface area of the room. The point on the f|l[axis corresponding to the intersection of the correct area ratio and the Mfethe average absorption coefficient, a, gives the number of decibels that IliS'dded to the transmission loss (Figure 18) to get the noise reduction. absorption (a= 1). in the secondary room yields a noise reduction ^an the transmission loss. The reverberant condition in the source room ^-jme incident intensity level to be 6 db less than the sound pressure level ^orULC), which gives rise to this difference. Since sound-measuring instruJ^^jreact to the sound pressure rather than the sound intensity, one measures reduction (difference in sound pressure level) rather than the transmisIndifference between the incident and transmitted intensity levels), tlipurves are dashed in the upper left-hand corner of Figure 20 to indicate pply to the noise reduction measured near the partition. The levels in the fair of the room are often less than near the partition.10 When the radiating IS:is relatively small (S^/S small), it is often possible to use Figure 14 to Sl^vel at locations away from the partition. luiination of Figure 20 shows that one must have a certain amount of ^ material in the secondary room in order to obtain the maximum benefit ^isolating partition. It should be noted, however, that there is a point of ' ping returns. It is economically wise, therefore, to add no more absorbing ||than necessary to bring the level in the secondary room within a few ,hat-obtained-for perfect absorption (= 1). K. A PRACTICAL EXAMPLE pure 21 shows a situation that lends itself to a demonstration of the use of i||i, 14, 18, 19, and 20. A small multislide machine that makes pins at the jabout 300 a minute is fully automatic except for infrequent stoppages. The Ifind power radiated by this machine is 80 mwatt (milliwatts), and the fre- fdigtribution is similar to that shown for the weaving room (Figure 9). The power radiated in the 300 to 600 c.p.s. band is 10 mwatt. The sound pressure ithis band that exists in the right-hand room is to be'c88i)Hitld. ' is convenient to replace th8-multi8l-ide<maohiii^?4yfliohii^d^iaaS:l ^^emg-the same"sound -power, but spherical in shape and BUspeiided in j|pbf the enclosure. If the area of the surface of this sphere is 1 square ^intensity there (at a radius r = s/l/iw -- 0.28 ft.) will be lOMttatt per |J86t. The corresponding intensity level is (see equation 3): {si109' = Intensity level = 10 log 110 db K||||pfQi,ee is operating inside an enclosure in which the average absorption co- VHtr of the wall is about 0.85 in the 300 to 600 c.p.s. band (Figure 12).The |||||L, Beranek, Acoustics. McGraw-Hill, New York, 1954. i % ith i