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654 JEROME R. COX, JR. average distance from the center of the room to the wall is ro -- v6 X 16/4rg- 2.75 feet. Figure 14 indicates that the difference between the sound preaaura the surface of the sphere (r/r0 = 0.28/2.75 = 0.1) and the wall is about 261, Thus, the level at the wall is 110 db -- 20 db = 90 db. -If The transmission loss through the plywood partition will be' about 19; (Figure 18) since 72-inch thick plywood has a surface density of about 1.5 lbff Plan of the Tvo Room Dleeussed In the Practical Zzanple Figure 21. Plan of the two rooms discussed in the practical example. square foot (Table 4). In order to evaluate the noise reduction from the ihsi the outside of this enclosure Figure 20, which requires a knowledge of S/$|a.. for the large room, must be used. The radiating area is Sw = 5 X 4 ft. X 4 ft. = 80 sq. ft., since it i/ that the sixth side of the enclosure lies on the floor and does not radiate/ijg into the room. The total surface area of the large room is: _ __ _ 8= (3o'ft7+TdfkT3oit:+46ft:)'i5'ft:+i _ __-;)* *4p (3orr> i- = 4500 sq. ft. ;`&a Thus, SK/S = 0.018. As indicated in equation 5, the average absorption coe may be calculated as follows: acelllnt See,line = 0.12 X 30 ft. X 40 ft. =144 a..u. Swan, = 0.03 X 140 ft. X 15 ft. = 63 cffioor sfl00r x x= 0.03 30 ft. 40 ft. = 36 aS = 243 NOISE AND THE CONSERVATION OP HEARING 655 ^Jdt-has been assumed that the floor is concrete with an absorption coefficient jg|jj|jj brick. Thus, a = 243/4500 = 0.054. The correction to be added to the 7^'||ssion loss is approximately 2 db yielding a noise reduction of 21 db. The Efjjttincjspressure level, therefore, drops from 90 db to 69 db. However, as pointed ^^en^ection II. J, the dashed portions, of the curves in Figure 20 indicate that ^fewund pressure may fall off as one moves away from the radiating partition. can be used to calculate the sound pressure level in the remainder of mSp as follows. in^ce the enclosure rests on the floor, the cubical source is best replaced by a ||tical one, hemispherical in shape and of equal surface area. The radius of j|misphere will be r = \/80/27r = 3.6 ft. The average distance from the Iff the room to the walls is r0 = V4500/4*- = 19 ft. Figure 14 shows that Ipm with a = 0.054 the sound pressure level at r/r0 = 3.6/19 = 0.19 is f db greater than the level near the walls. Because of this very reverberant $lon the sound pressure level at the partition separating the two rooms is Ijjust 1 db less than that at the outside of the enclosure. |nis partition is a 6-inch brick wall with a Vi-inch glass window occupying ||e feet of the total area of 300 square feet. The transmission loss of the brick i 47 db (Figure 18) based on a surface density of 66 lb. per square foot f/4). The transmission loss of the V4-inch glass window is about 25 db. the transmission loss of the window is 22 db less than that of the wall and ie fraction of the wall occupied by the window is 9/300 = 0.03, the trans. iloas of the composite partition (Figure. 1,9) is.about .7 db less than that Shriek wall, or 40 db. In order to compute the noise reduction through this Ion, we need to know the area ratio, S,,/S = 300/2000 = 0.15, and the |e absorption coefficient, 5: ^celling ^celling " 0.5 X 20 ft. X 20 ft. = 200 awaii. Swan. = 0.03 X 80 ft. X 15 ft. = 36 <*nuor sfl00r = 0.25 x 20 ft. x 20 ft. = 100 aS = 336 |s#i= 336/2000 = 0.17, and no correction need be added to the transmission figure 20). The noise reduction is, therefore. 40 db and- the sound-nressiire ||p?-exists in the room at the right in the 300 to 600 c.p.s. band is 68 db -- pit 28 db. A similar analysis should be performed in each of the seven other Ikbands. . p8 example and the charts used to compute the result demonstrate some ilamentaF-prdperties-of-tKe-tranemissien-of-soundrA-n-understairding-ofProperties should be of help in intelligently assessing most common noise The transmission of sound through solid materials,11 through acoustical !a. E. Crede, Vibration and Shock Isolation. Wiley, New York, 1951.