Sound Propagation in Dissipative Layered Media

Abstract

When sound is propagated from a point source in a layer of fluid, comparable in depth to the acoustic wavelength and overlying a much thicker layer of fluid with greater density and sound velocity, “wave-guide” or normal-mode effects and cylindrical spreading are to be expected, according to well-known theory. If the thicker layer absorbs sound, total internal reflection is destroyed, and the sound in the thin layer should be attenuated according to relations derived by Kornhauser and Raney [J. Acoust. Soc. Am. 27, 689 (1955)]. If the thicker layer is also shear-elastic, this attenuation should increase, through conversion of compressional into shear waves [K. Ergin, Bull. Seismological Soc. Am. 42, 349 (1952)]. These predictions have been confirmed by measurements in a water layer 0.5–2 cm thick, over a slab of rubber for which the density, absorption, and sound velocities were known. The frequency range was 55 to 600 kc. Vertical modal patterns of sound intensity and interactions of two modes were also found to agree with theory. (Supported in part by the Office of Naval Research.)