Unique acoustic material signatures (AMS) may be obtained in the reflection acoustic microscope. The proposed model shows that they result from interference between two components reradiated into the immerson fluid at the materials critical Rayleigh angle &Vthgr;R. The characteristic period &Dgr;zNof this interference signature varies as the square of the Rayleigh wave velocity and is empirically given by &Dgr;zN=&lgr;R/sin&Vthgr;R, where &lgr;Ris the Rayleigh wavelength. Materials covering a greater than 3 : 1 velocity range agree well with this physical model. Substitution of the longitudinal wave velocity in the expression extends the range of measurable AMS to acoustically slower materials. A variety of applications for AMS is suggested.