Document LgoKVbvgjDyRvYQe75K3kVJ6w

American Society of Heating and Ventilating Engineers Guidi * 1935 Chapter 18--Sound Control Table 4. Compliance and Resistance Data for Typical Specimens n Flexible Materials* w The compliances and resistances given in the table are for specimens 1 in thick and 1 so cm in cross-section '. nod will now be 0.25 X 10"* X ^ == 0.39 X 10-9 cm per dyne, and the eS11" 'll be 0 15 X 10s X 645 = 0.97 X 107 mechanical ohms (or absolute units), ace w1*1 Material Description op Material Approximate Upper Saps Loading m Pounds per Square Inch Compliance c in Centimeters per Dtnb RfsisTu.c, r i AMOlDDn^ ? 10" (0.97 X 107)* + 4ic, x 1Q0 x 0 39 T^Tx 107)* + ( 2* X 100 X 4.54 X 10* - 10* 2x X 100 X 0.39 = 0.037 Corkboard 1.10 lb per board foot 12 0.25x10-* 0-15x10. ... therefore, that with the bearing surface on the cork reduced Corkboard 0.70 lb per board foot 8 0.50 x 10-* 0.25 x 10* \n0 sa in. (that is, with the cork loaded to 10 lb per square inch), the t 10U sq- js reduced to 0.037, or the amplitude of vibration trans- FlaX-li-num 1.35 lb per board foot 4 to 6 0.60x10-* 0.50 x 10> to the floor will be only about_ l/27_ of what_it_would_beif the h Celotex Carpet lining 10 0.40 x 10-* nutY? e were mounted directly upon the floor. These two numerical Celotex Insulating 12 0.18x10-* ma- jes will serve to show not only the manner of making the calcu- Insulite Masonite board Insulating board Insulating 15 0.16 x 10-* 15 0.12 x 10-* fS|ns but also the importance of selecting the proper type and design of Iwible' supports for insulating the vibrations of a machine from the ri^id structure of a building. board Anti-Vibro-Block Sponge Rubber 25 lb per 5 1 to 3 0.60 x 10-* 3.0 x 10-* 1.5 x 10` CONTROL OF NOISE TRANSMISSION THROUGH DUCTS Soft India Rubber cubic foot 55 lb per cubic foot 3 to 6 1.2 xlO-* The most troublesome sources of noise from ventilating and air con ditioning equipment are fan and motor noises which are transmitted Hairfelt 10 lb per cubic foot 1 to 2 1.5 xlO-* through the ducts. The reduction, in decibels, of noise transmitted through a duct, neglecting reflection from ends and bends, is proportional Architectural Acoustics, by V. O. Knudsen. p. 278. (1) directly to the length of the duct, (2) directly to the perimeter of the duct, (3) inversely to the area of cross-section of the duct, and (4) directly machinery vibrations are less than 150 vibrations per second, and the assumed frequency / of 100 is quite representative of typical machines). Suppose that a 1-in. slab of cork-/ board weighing 1.10 lb per board foot be placed between the machine and the floor.) (or at least approximately so) to the coefficient of sound absorption of the material which comprises the interior surface of the duct. It is apparent therefore that long narrow ducts, lined with highly absorptive material, The loading on the cork will then be only 50 lb per square foot, or slightly more than? H lb per square inch. (It is assumed that the compliance c in centimeters per dyne fora i specimen 1 in. thick and 1 sq cm in cross-section is 0.25 X 10"* and the resistance rin; mechanical ohms is 0.15 X 10s.) ~ will provide a high degree of insulation against the transmission of noise through ducts. In fact, small ducts (4 in. x 6 in.), made of material having a coefficient of sound-absorption of 0.50, will provide a noise The transmissibility 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 104. reduction of slightly more than 1 db per linear foot. As can be seen from an inspection of Table 2, noises of low frequency are difficult to absorb; on the other hand, these frequencies are easily reflected by elbows, branches, and duct ends whereas higher frequencies are little affected. Furthermore, the reflection effects are more pro Therefore, the compliance of the entire support, 1 in. thick and '20 sq ft in cross nounced in small ducts than in large ducts. Hence, by introducing into section, is 0.25 X 10-* X ^ gg ^ = 0.134 X 10~' cm per dyne, and the resistance of a duct a sufficient length of small, absorptive channels together with a the entire support is 0.15 X 10s X 1.86 X 10* = 0.28 X 10'mechanical ohms (or absolute i units). Therefore, number of elbows or other reflecting elements it is possible to reduce the transmitted noise to qny required degree. This applies not only to ducts between the equipment room and other rooms in a building, but also to V 10" (0.28 X 10*)* + 4*s X 100 X 0.134 (0.28 + 10>)` + (2* X 100 X 4.54 X 10s - 10" 2x X 100 X 0.134 , = 0.93 ducts connecting adjacent or nearly adjacent rooms. By the proper use of such filters it is possible to eliminate all of the difficulties which arise in connection with the transmission of sound through ventilating ducts. The problem is an engineering one which .can be worked out prior to the in 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. stalling of the equipment, and it can be calculated in such a way as to meet the most rigorous demands for silent operation. There is a need for 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 quantitative data regarding the attenuation or noise-reduction provided rent types of ducts, but even with the meager data available it is 310 311