Surface roughness changes the acoustic backscatter signal due to the microstructure of a sample. These changes have been previously reported for the backscatter power under the restrictive assumption that the microstructural length scales are much smaller than any other length scale in the problem. In this study this restriction is removed and an approximate analytic series solution is presented that describes the effects of surface roughness on the power and mean square of the acoustic backscatter from samples whose microstructure is characterized by an autocorrelation lengthLm, which may be smaller than, comparable to, or larger than either the wavelength or the autocorrelation lengthLsthat describes the surface roughness. Both focused and unfocused phase-sensitive transducers are considered. A surprising result emerged. For focused probes at normal incidence and for moderate roughness, the backscattered power at the focal depth is almost entirely determined by the frequency and the rms surface height; it is independent ofLm,Ls, and the radius of the transducer.