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Heating Ventilating Air Conditioning Guide 1939 of accuracy. Table 2 gives the values of c and r for a number of monly used flexible materials. Example 1. A machine weighing 1000 lb has a base area of 20 sq ft. Assume that th principal vibration of the machine has a frequency of 100 cycles per second (nZ? machinery vibrations are less than 150 vibrations per second, and the assumed frequen^ of 100 is cjuite representative of typical machines). Suppose that a 1-in. slab of board weighing 1.10 lb per board foot'be placed between the machine and the flor The loading on the cork will then be only 50 lb pier square foot, or slightly more tha yi lb per square inch. (It is assumed that the compliance c in centimeters per dyne for specimen 1 in. thick and 1 sq cm in cross-section is 0.25 X 10"* and the resistance . :* mechanical ohms is 0.15 X 10s.) ln The transmissibtlity is calculated in the following manner: Mass of machine in grams = 1000 X 454 = 4.54 X 10s. Area of base in square centimeters = 20 X 144 X 2.54 X 2.54 = 1.86 X 1(H. Therefore, the compliance of the entire support, 1 in. thick and 20 sq ft in cross section, is 0.25 X 10_t X i gg x = 0.134 X 10"" cm per dyne, and the resistance of the entire support is 0.15 X 10s X 1.86 X 104 = 0.28 X 10* mechanical ohms (or absolute units). Therefore, V1 (0.28 X 10s)* + 4*> X 1001 X (0.134 X 10-")* (0.28 X 10s)* + ((2. X 100 X 4.54 X 10s) - 2x x 100 x (0.134 X lO-^ V: y0.0784 X 10" + 4x* X 10". 10* X 0.018 0.0784 X 10" + f,2* X 4.54 X 10' - 2* 10s X 0.134/ = 0.935 Consequently, it is seen that the transmissibility is nearly equal to unity, and that the support therefore is not satisfactory for insulating 100 or fewer vibrations per second. If the amount of cork be reduced so that it is loaded to 10 lb per square inch, the total area of the supporting cork will be only 100 sq in. or 645 sq cm. The compliance of the entire support will now be 0.25 X 10~* X gjg = 0.39 X 10-* cm per dyne, and the resistance will be 0.15 X 10s X 645 = 0.97 X 10' mechanical ohms (or absolute units). Therefore (0.97 X 10')' + 4* X 100* X (0.39 X 10")* V ')' + (( *10 2 1 X 100 X 4.54 X 10s) - 2* X 100 X (0.39 X 10-"),/ 0.94 X 10" + 10" 4x1 X 0.1521 10' \2 v0.94 X 10" + ( 2* X 4.54 X 10' - 2* XX 0.39 ) 0.0375 It is seen, therefore, that with the bearing surface on the cork reduced to 100 sq in. (that is, with the cork loaded to 10 lb per square inch), the transmissibility is reduced to 0.0375, or the amplitude of vibration trans mitted to the floor will be only about 1/27 of what it would be if the machine were mounted directly upon the floor. These two numerical examples will serve to show not only the manner of making the calcu- 608 Chapter 30. Sound Control I tions, but also the importance of selecting the proper type and design of (Lihie supports for insulating the vibrations of a machine from the rigid structure of a building. Controlling Noise Through Room Wall Surfaces The ventilating equipment is usually housed in a separate room where the noise produced by the mechanical operation of the equipment can be isolated from the rest of the building. If the vibration of the machinery is absorbed by flexible mounting and is not transmitted to the building, the only noise to be eliminated by the walls of the room will be the air borne mechanical noise. Acoustical measurements on average brick, tile, lath, and plaster walls indicate that the usual wall of these types is sufficient to satisfactorily attenuate this air-borne mechanical noise. Three wall sections and two floor and ceiling sections which are satis factory for the wall insulation of the equipment room are shown in Fig. 1. \ ''4'Bncti I 5- IX *-g Plaster Insulation Value*47 db 4* Hollow Clay Tile 1** 2* Furring Strips Paper and Metal Lath ^ Plaster Insulation Value * 52 db. s| p" Absorptive Blanket f Fibre Board B Plaster I 'Staggered Wood hII Studs Insulation Value Greater than 50 db. ^Rough and Finish Flooring Absorptive Blanket ^Plaster on Lath Insulation Value * 50 db. Flooring !esilient Chairs Concrete Slab . agS- Resilient Hangers Plaster on Lath Insulation Value a GO db., or more Fig. 1. Three Wall Sections and Two Floor and Ceiling Sections which . are Suitable for the Insulation of Equipment Rooms3 Acoustical Problems in the Heating and Ventilating of Buildings, by V. O. Knudsen (A.S.H.V.E. Transactions, Vol. 37, 1932, p. 211). Attention should be given to the equipment room door, since this door may leak badly and allow sound to escape into parts of the building which should be quiet. Where the equipment noise is particularly severe, double doors should be used and in all cases, the doors of the equipment room should be fitted with tight thresholds and weather-stripping. The door itself may transmit considerable sound if it is thin but it will not transmit a tenth as much as will be transmitted by a J-in. crack between the door and the threshold. In cases where the equipment noise is extraordinarily high, it may be necessary to treat acoustically the walls and ceiling of the equipment room, if the equipment room is not entirely closed, partition walls may be necessary. NOISE TRANSMITTED THROUGH THE DUCTS . After noise reaches the air stream in the ducts it can be controlled by lining the ducts on the inside with a sufficient quantity of sound absorbing 609