The absorption of sound in a homogeneous porous medium

Recent theories relating to the wave-propagation of acoustic disturbances in homogeneous, isotropic porous media are discussed, and expressions are given of the wave-equation and for the oscillatory pressure in the material in terms of a " velocity potential of average flow ". In their most general form these expressions are closely analogous to those which correspond to the propagation of sound in free air. The general theory is expressed in terms of two complex parameters which respectively take the place of the wave-length constant and the mean density of the air, which figure prominently in elementary acoustic theory. Expressions for the complex parameters are also given in terms of the " effective " inertia, compressibility and flow-resistance of the material.