Acoustical Properties of Porous Materials

In this paper an experimental study is carried out to test the validity of the theories put foreward to explain the behaviour of porous materials when used for sound absorption. Measurements of absorption coefficient of sound and of acoustic impedance are made for samples of sponge rubber varying in thickness from 0.7 to 5.0 cm and for the frequency range from 150 to 5000 Hz. The comparison between the experimental results for the components of the acoustic impedance and those calculated using Beranek's theory and Scott's theory shows that the deviation of both theories are appreciable for very low and very high frequencies and that both theories are appropriate to use for the thicker samples while the divergence is obvious in case of the thinner samples. Discussion of the problem is given as well as reasons for the discrepancies between theories and experiment.