The analysis and measurement of the noise emitted by machinery

Abstract

The paper deals with part of a comprehensive investigation which is being carried out by a manufacturer of electrical plant on the noise emitted by machinery and other apparatus, a subject of great present and future importance. The need for treating the subject in a thorough and quantitative manner for engineering purposes is pointed out. The physical magnitudes involved in the measurement of the sound set up by a vibrating body are discussed generally, together with the characteristics of the ear. The characteristics that should be possessed by a sound-measuring apparatus intended for use in engineering problems are pointed out and a brief survey is made of existing types of apparatus. The principle, development and final form of the apparatus, which enables a complete analysis of a complex sound to be made, are described, together with the methods employed to verify the accuracy of the apparatus. The influence of the conditions under which sound measurements on engineering apparatus are carried out is discussed, and a special laboratory which has been set up for acoustical work is described. Some sound-test results on engineering apparatus are given as examples. A = strength of sound source in terms of maximum rate of emission of fluid (cm 3 per sec). a = area of diaphragm (cm 2 ). B = flux density (lines per cm 2 ). /? = elastic restoring force (dynes for 1 cm displacement) . K x = Rayleigh disc constant. d = linear deflection on scale. E = maximum instantaneous e.m.f. (volts), e = R.M.S. volts. F = damping resistance (dynes for velocity of 1 cm per sec). / = frequency, in cycles per sec. / = current, in amperes. 0 = angle between normal to Rayleigh disc and axis of tube. K 2 = NB x 2nr x 10" 8 ; also mA/mV/mH. k = a>/v -2TT/A. L = inductance. d ± = distance from scale to mirror. I = length of suspension (cm). A = wavelength (cm). M = mutual inductance (henrys). m t = total mass of diaphragm, m = correction factor for microphone dimensions. N -number of turns on coil. P = maximum value of alternating air pressure. p = R.M.S. excess pressure (dynes per cm 2 ). R = electrical resistance (ohms). r -radius (cm) of sphere surrounding source of sound or distance from centre of source to sound detector. r = radius of disc. r 0 = radius of source of sound (cm). p = density of air. *S = microphone sensitivity. TR = restoring torque, in dyne-cm per radian per cm. T = torque (dyne-cm). t = time (sees.). v d = velocity of translation of diaphragm (maximum). v = velocity of sound in air (cm per sec). v Q = velocity at 0° C. V = maximum air velocity. W = average power per cm 2 at surface of sphere surrounding source (ergs per sec. per cm 2 ). x = displacement (cm). x = ratio of thickness to diam. of Rayleigh disc x = fractional frequency change. Z = impedance (ohms). U) = 2TT/.

Noise measurement and evaluation

“…Equal loudness contours or phonon curves below 100Hz Stevens" Whittle et al' 3 * Y eowart'" 123 118 9. 4 Occurrence of man-made infrasound and low-frequency noise Generally speaking, high levels of infrasound and lowfrequency noise are found to occur in any situation where there is a large mass of air flowing at high speeds. Leventhall 137 Tempest and Bryan 140 and Williams and Tempest 141 have measured noise levels down to the 1 -2 Hz region in a large variety of different situations.…”