The measurements are described of the absorption of sound by light porous compressible rubber wall-covering layers. The greater part of these measurements was performed according to the interferencemethod with normally incident sound. The method and the arrangement used are described. We investigated two kinds of cellular rubber (porous rubber with mutual non-communicating cavities) of various thicknesses and two kinds of sponge-rubber (porous rubber of which the cavities do communicate) of numerous thicknesses. A few well-known porous non-compressible materials were investigated for comparison. Reverberation-measurements were carried out on 6ne sponge-rubber specimen. The conclusions drawn from these meashrements can be summarized as follows:
a. Cellular rubber behaves in accordance with the previously published theory of the absorption of sound by compressible materia]s.
b. As was to be expected, cellular rubber turns out to be unsuitable for absorbing material, owing to insufficient damping.
c. The various kinds of sponge-rubber (with normal surface) are excellent absorbers and behave chiefly as porous non-compressible materials. ยง I. Introduction. In a previous article 1) we have developed the theory of the absorption of sound by compressible, very light, wallcovering layers. Considering their great advantages, attention was there chiefly payed to wall-covering layers coated with a light nonporous surface-layer. The purpose of the present publication, which is divided into three parts, is to give an idea, by means of the experimental results, of what can be attained in practice with compressible materials. We shall deduce the mechanism of the absorption from the way in which the specific acoustic impedance depends on frequency. From the behaviour of this impedance one can, namely,