Document b5mq4EajDmeJx7R2zwBQvQeQZ

American Society of Heating and Ventilating Engineers Guide, 1934 Therefore, the compliance of the entire support, 1 in. thick and 20 sq ft in cross section, is 0.25 X 10"* X ~j~gg ^ = 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 109 mechanical ohms (or absolute units). Therefore V (0.28 X 109)9 + 10" 4*9 X 100 X 0.134 (0.28 + 109)9 + ^2x X 100 X 4.54 X 10* - 10" \2 = 0.93 2x X 100 X 0.134, 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 = 0.39 X 10"9 cm per dyne, and the resistance will be 0.15 X 10s X 645 = 0.97 X 107 mechanical ohms (or absolute units). Therefore V + ((0.97 X 107)9 + 10" 4x X 100 X 0.39 (0.97 X 107)9 2x X 100 X 4.54 X 10* 109 - = 0.037 O.39) 2x X 100 X 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 Table 4. Compliance and Resistance Data for Typical Specimens of Flexible Materials3 The compliances and resistances given in the table are for specimens 1 in. thick and 1 sq cm in cross section Material Corkboard Corkboard Flax-li-num Celotex Celotex Insulite Masonite Anti-Vibro-Block Sponge Rubber Soft India Rubber Hairfelt Description or Material Approximate Upper Safe Loadin'] in Pounds prr Square Inch eCompliance in Centihbtehs pee Dtue Resistance t in. Absolute Units 1.10 lb per board foot 0.70 lb per board foot 1.35 lb per board foot Carpet lining Insulating board Insulating board Insulating board 25 lb per cubic foot 55 lb per cubic foot 10 lb per cubic foot 12 8 4 to 6 10 12 15 15 5 1 to 3 3 to 6 1 to 2 0.25 x 10-* 0.50 x 10" 0.15 x 10* 0.25 x 10s 0.60 x 10~*: - 0.40 x 10"* 0.18 x 10~* 0.50 x 10* 0.16 x 10~* 0.12x10"* 0.60 x 10"* 3.0 x 10" 1.5 x 10* 1.2 x 10~* 1.5 x 10"* Architectural Acoustics, by V. O. Knudsen, p. 278. 252 Chapter 18--Sound Control transmissibiUty is reduced to 0.037, 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 lations, but also the importance of selecting the proper type and design of flexible supports for insulating the vibrations of a machine from the rigid structure of a building. CONTROL OF NOISE TRANSMISSION THROUGH DUCTS The most troublesome sources of noise from ventilating and air con ditioning equipment are fan and motor noises which are transmitted through the ducts. The reduction, in decibels, of noise transmitted through a duct, neglecting reflection from ends and bends, is proportional (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 (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, 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 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 nounced in small ducts than in large ducts.. Hence, by introducing into a duct a sufficient length of small, absorptive channels together with a number of elbows or other reflecting elements it is possible to reduce the transmitted noise to any required degree. This applies not only to ducts between the equipment room and other rooms in a building, but also to 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 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 quantitative data regarding the attenuation or noise-reduction provided by different types of ducts, but even with the meager data available it is possible to design filters which will suppress the ordinary noises incident to the ventilating or air conditioning of buildings4. In general, the motion of air resulting from the ventilating of rooms is not sufficient to introduce any appreciable difficulty in auditoriums, except where noise may originate from the issuing of high-speed air from nozzles. However, by proper stream-lining of the nozzles, it is possible to work with speeds which are adequate for all practical purposes without pro ducing any disturbing noises. Since sound is propagated with a velocity of more than 1100 fps, the velocity of the air would have to attain speeds , 4How Sound is Controlled, by V. O- Knudsen (A.S.H.V.E. Transactions, Vol. 37, 1931). 253