Theoretical and experimental investigations of acoustic signatures of materials using scanning microscopes with variable lens illumination

The most important part of a scanning acoustic microscope system is the wide-opening-angle lens which could have many shapes and sizes. Such lenses include several interfering surface modes which may obscure the measured acoustic properties. In this paper, we investigate the conventional spherical wide-angle lens whose central part has been covered by a circular aperture stop of varying diameter, D, to absorb the energy of acoustic modes. The influence of increasing D of such absorbers on the acoustic signatures, V(z), of plastic materials in which the speed of sound is slow (plexiglass) and materials in which the speed of sound is relatively fast (silica and stainless steel) is investigated theoretically and experimentally, leading to similar behaviours: a decrease in amplitude, a broadening of periodic spacings and the complete disappearance of the oscillatory form of V(z) for large D, namely for large excluded angles. A novel formula for periods, Delta z', is proposed for covered lenses, which explains the velocity shift associated with the period variation as the excluded angle increases.