Document M4zn2rqKd8zY5QXREN6NgY3oz

Heating Ventilating Air Conditioning Guide 1939 Chapter 30. Sound Control If r, the mechanical resistance,- is very small, formula 1 may be written f nv particular set-up. The value.of c can be obtained by making static 1 - tmurements of the amount of displacement of the compressed support teach additional unit of the compressing force. If this be done for a x - i '**amen of the flexible material of. a certain; thickness and area of .cros?, o tjon the compliance can be determined for any other thickness or area |eC relation that c will be directly proportional to the thickness and where rto is the natural frequency of the machine upon the elastic pad, Versely proportional to the area1 of the flexible support. . When the .Internal resistance r is not too large, it can be determined by observing the lBccessive amplitudes of the free vibrations of a mass m which rests upon 2% ^specimen of the flexible material, and solving for r by the usual log- In most cases of design of resilient machine mounting the effect of frictional resistance is small, and Equation 2 may be used. In such cases it is only necessary to know the natural frequency of the elastic pad or decrement method. Or, if the damping be;so great that the free motion of m |s non-oscillatory, r can be obtained from measurements on the experi mentally-determined resonance curve of the forced vibrations of m, or from measurements of the rate of return of m when it is given an initial platform used under the desired loading and the transmissibility for any vibrational frequency of the machine may be obtained. However, this formula gives the theoretical maximum insulation which may be obtained and should be used with a liberal factor of safety. (A factor of 2 is common practice.) If the pad is to be of any value in the prevention of solid-borne vibra tions, the value of t1 must be considerably smaller than unity. If the fundamental frequency of vibration generated by the machine happens to coincide with the natural frequency of the mass of the machine resting on the elastic pad, a condition of resonance will be established, and the machine will exert a greater force upon the foundation than it would if the pad were completely removed. It is necessary, therefore, that the displacement. If the resistance of a certain specimen of material, as cork, felt, or rubber, has been determined by any of these methods, the resistance (or any other thickness or area of the material can be determined approxi- mately because the resistance will be inversely proportional to the thick ness and directly proportional to the area of cross-section of the flexible support. Thus, if the values of c and r for a flexible material be known, it is possible to calculate, by means of Equation 1, the amount of insu llaattiioonn tuhiaatt support for wwaiillllpibe-'Vec*eoobtfaeinqeudip-fm-r-o--e-m-n--"t t--1hh--ae---v.ui'n--sge of this material as a a----m--a--s-s---m-- . For the flexible routine In nrartire. nmrWted with onlv o elirrVif carrifirf* elastic support be sufficiently, compliant, and the mass of the machine sufficiently heavy, that the natural frequency of the mass m upon its' elastic support will be low in comparison with the frequencies which are Table 2. Compliance and Resistance Data for TypicalKSpecimens of Flexible Materials generated by the machine. Thus, if the principal vibrations in the The compliances and resistances given.in the table are for specimens 1 in. thick ^ machine be of the order of 100 vibrations per second, the natural frequency and 1 sq cm in cross-section of the machine mounted on its elastic support should not exceed about 50 vibrations per second, and for best results preferably 20. When the forced frequency is low, it is frequently impossible to insulate for the fundamental forced frequency due to connecting pipe work and Material Description op Material Approximate Upper Safe Loading in Pounds per Square Inch Compliance c in Centimeters per Dtne Resistance r. in AbsolutbUnits other relevant factors. In cases of this kind an effective installation of sound insulation may be obtained with a mounting which functions far above the fundamental forced frequency. For example, a compressor Corkboard Corkboard 1.10 lb per 12 0.25 x 10- 0.15 x 10s board foot 0.70 lb per 8 0.50 x 10- 0.25 x 10s operating at 500 rpm has a forced frequency of 8.3 vibrations per second. By designing a mounting having a natural frequency of 20 to 25 vibrations per second, it is possible to isolate practically all of the noise. ' The elastic support under the machine acts as a low-pass filter which passes all frequencies below about two times the natural frequency of the machine mounted on its elastic support, but prevents all frequencies above about ^ from reaching the solid structure of the building. The principal influence of the internal mechanical resistance r is to limit the Fiber Board Fiber Board Fiber Board Fiber Board Fiber Board Anti-Vibro-Block Sponge Rubber .1.35 lb per board foot Carpet lining Insulating board Insulating board Insulating board . 25 lb per 4 to 6 10 12 15 15 5 1 to 3 0.60 x 10-* 0.40 x 10- 0.18 x 10- 0.16 x 10rS 0.12 x 10-* 0.60 x 10- 3.0 x 10 0.50 x 10" 1.5 x10s f. vibration at the resonant frequency. It is generally advisable, therefore, to use materials which have an appreciable internal resistance. Soft India Rubber cubic foot 55 lb per cubic foot 3 to 6 1.2 x 10- - The values of c and r can be determined for any specimen of flexible material and, when known, can be used to determine the insulation value From Architectural Acoustics, by V. O. Knudsen, p. 278. 606 607 B S SBR