ACOUSTIC DOUBLE-REFLECTION AND TRANSMISSION AT A ROUGH WATER-SOLID INTERFACE

The double interaction of a wave with a rough interface is important i n many acoustic measurements. An acoustic beam that reflects twice fro m a rough water-solid interface is considered. The second reflection f rom the rough surface is accomplished after the transducer's buffer ro d is used as an acoustic mirror. Changes in the spatially averaged spe cular reflection are related to the phenomenon of ''enhanced backscatt er,'' previously observed for diffuse scattering. Simple, approximate closed-form analytic formulas for the strength of the specular double reflections are given as a function of frequency and the stand-off dis tance of the transducer. As one application of the analytic results, a method is proposed for determining the surface correlation function f rom the specular double reflections. The same physics (and appropriate ly modified analytic formulas) apply to doubly transmitted waves. Expe rimental measurements have been made of acoustic double reflection wit h a normally oriented broadband piezoelectric transducer near a rough water-solid interface. Measurements of (1) the wave reflected once fro m the rough interface, and (2) the wave reflected twice from the rough interface are given. The strength of the specular reflections was mea sured as a function of frequency and the transducer's distance from th e surface. It was observed that for large distances between the transd ucer and the rough surface and at low frequencies the scattering-induc ed loss of the doubly reflected wave was twice the loss of a singly re flected wave. However, for small distances between the transducer and the surface and at high frequencies the scattering-induced loss of the doubly reflected wave was four times the loss of a singly reflected w ave.