The Absorption of Sound in Air at Reduced Pressures

Abstract

The decay of sound energy in spherical shells has been used to measure the absorption coefficient of air as a function of pressure and humidity. Radial modes are stimulated by introducing the sound through small tubes which terminate at the centers of the spheres. The decay of the sound intensity for these modes depends both on the loss at the boundary and on the absorption in the medium. By determining the reverberation times in pairs of similar spheres of different radii, correction is made for the boundary effect. The measured values of the absorption coefficients for frequencies in the audio range are compared with those calculated for classical and molecular absorption.