THEORETICAL AND EXPERIMENTAL INVESTIGATIONS OF ACOUSTIC SIGNATURES OFMATERIALS USING SCANNING MICROSCOPES WITH VARIABLE LENS ILLUMINATION

The most important part of a scanning acoustic microscope system is th e wide-opening-angle lens which could have many shapes and sizes. Such lenses include several interfering surface modes which may obscure th e measured acoustic properties. In this paper, we investigate the conv entional spherical wide-angle lens whose central part has been covered by a circular aperture stop of varying diameter, D, to absorb the ene rgy of acoustic modes. The influence of increasing D of such absorbers on the acoustic signatures, V(z), of plastic materials in which the s peed 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 de crease in amplitude, a broadening of periodic spacings and the complet e disappearance of the oscillatory form of V(z) for large D, namely fo r large excluded angles. A novel formula for periods, Delta z', is pro posed for covered lenses, which explains the velocity shift associated with the period variation as the excluded angle increases.