On the attenuation of slow waves in air-saturated porous materials.

Abstract

Certain important properties of porous materials, such as porosity, tortuosity, permeability, and internal damping, are not directly accessible from conventional acoustic measurements made on the frame itself. These quantities can be better assessed from measuring the slow-wave velocity and attenuation, at least whenever this elusive mode is detectable at all. Recently, a novel experimental technique was introduced based on the transmission of airborne ultrasound through air-filled porous samples, which lends itself quite readily to such measurements, even in highly attenuating natural rocks [P. B. Nagy and L. Adler, J. Acoust. Soc. Am. Suppl. 1 87, S145 (1990)]. Depending on the average grain size and the inspection frequency, the attenuation coefficient is dominated by viscous and thermal losses or scattering. Experimental results show that, rather surprisingly, the scattering induced attenuation is linearly proportional to frequency and it does not depend on the grain size. Different physical explanations for this unusual behavior are considered and checked against experimental observations. [Work supported by U.S. Department of Energy, Basic Energy Sciences Grant No. DE-FG-02-87ER1374.9A000.]