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. Theory shows that numerical values of the components of the complex parameters can be obtained experimentally for practical absorbing materials at a given frequency by measurement of the attenuation constant and of the velocity of propagation of sound in the medium, together with measurement of the normal acoustic impedance at the surface of a sample of the material of effectively unlimited depth. Experimental methods suitable for these measurements are described and results of such measurements are given for a typical sample of porous material. Validity of the general theory as applied to this material is suggested by the good experimental agreement obtained on applying the theory to measurements of the normal acoustic impedance of specimens of various finite depths of the material. From the measured values of the complex parameters referred to above, calculations of effective inertia, compressibility and porous resistance are made for the material used in the experimental work and the manner of variation of these quantities with frequency is deduced. The results throw interesting light on the mechanism of the propagation of sound in the material.