A Device for Direct Measurement of Sound Absorption Coefficients

Abstract

This paper describes a small scale device for direct comparison of the intensity of sound reflected from a surface of unknown coefficients with that reflected under identical conditions from surfaces whose coefficients have been determined by reverberation methods. It consists of means for producing sound of fixed intensity and frequency at an angle of incidence of 30°, and measuring the energy reflected at this same angle. Calibration is effected by measuring the intensity at the “pickup” of sound reflected from a highly polished marble surface, and from surfaces of known coefficients, and finally when no reflecting surface is interposed in the path of the incident sound. These measurements show a strictly linear relation between the intensity of the reflected sound and the reflection coefficient of the reflecting surface. The intercept of this line on the coefficient axis shows the percentage of the stray energy directly from the source to pickup. This percentage is very constant as the intensity in the incident beam is varied. Coefficients obtained by this method show excellent agreement with reverberation coefficients obtained by measurement on identical samples.