ACOUSTIC BACKSCATTER FROM MATERIALS WITH ROUGH SURFACES AND FINITE-SIZE MICROSTRUCTURE - THEORY

Surface roughness changes the acoustic backscatter signal due to the m icrostructure of a sample. These changes have been previously reported for the backscatter power under the restrictive assumption that the m icrostructural length scales are much smaller than any other length sc ale in the problem. In this study this restriction is removed and an a pproximate analytic series solution is presented that describes the ef fects of surface roughness on the power and mean square of the acousti c backscatter from samples whose microstructure is characterized by an autocorrelation length L(m), which may be smaller than, comparable to , or larger than either the wavelength or the autocorrelation length L (s) that describes the surface roughness. Both focused and unfocused p hase-sensitive transducers are considered. A surprising result emerged . For focused probes at normal incidence and for moderate roughness, t he backscattered power at the focal depth is almost entirely determine d by the frequency and the rms surface height; it is independent of L( m), L(s), and the radius of the transducer. (C) 1997 Acoustical Societ y of America.