Sound Propagation in Layered Media with a Rough Interface

Abstract

When sound propagates in a fluid layer of suitable thickness, overlying a much thicker layer of greater density and sound velocity, the normal-mode propagation expected ideally [C. L. Pekeris, “Theory of propagation of explosive sound in the sea,” in Memoir 27 of the Geological Society of America (1948)] is a result of total internal reflection. Anything preventing such reflection, such as a rough interface, should cause attenuation of the sound. A qualitative explanation is that some diffusely reflected sound strikes the interface at less than critical angles and escapes. Such attenuations have been studied with the apparatus of the accompanying paper by Ryan, Eby, and Williams. Small solid spheres were spread on the rubber-water interface; the extra attenuation caused by them was measured in the water layer as a function of frequency, water depth, sphere size, and area density of spheres. This attenuation is roughly proportional to area density of spheres, and to their acoustic scattering cross sections, and varies additionally with sphere size, water depth, and frequency, Frequencies were 100–250 kc; sphere radii 0.07–0.23 cm; water depths 0.5–2.0 cm. Glass, lead, and steel spheres behaved alike. (Supported in part by the Office of Naval Research.)