Formation of the Images of Spherical Objects in Acoustic Microscopy

Abstract

The problem of sound wave diffraction on sphere belongs to the fundamental problems of acoustic. This problem arise in acoustic microscopy when non-plane objects (such as spherical defects in solids, bubble, drops, biological cells, inclusion in composite materials etc) are investigated. At present there are a lot of experimental papers devoted to the investigation of spherical specimens in acoustic microscopy (Weglein, 1981; Edwards and Jarzynski, 1983; Poirier and Cheeke, 1982; Poirier et al., 1984; Sherar et al., 1987; Stockman and Nicholson, 1986; Stockman et al., 1989; Pavlin et al., 1990). Unfortunately the theory of acoustic microscopy of such objects is not practically developed. Until now only the theory of the convergent beam scattering on sphere and disk placed directly in the center of curvature of spherical transducer (Kanevskii 1977; Zinin et al., 1986) or the sphere located on the acoustical axes and in the focal plane of transducer (Lobkis and Zinin; 1990, Ueda and Morimatsu, 1990) has been considered. In order to investigate the processes of the acoustic images formation, it is necessary to know the solution of the problem of the focused beam diffraction on such objects placed in any points near a focal region. In the present work we have solved this diffraction problem for spherical objects. We have obtained this solution in Fraunhofer approximation in the form of a superposition of spherical wave functions. Geometry of the system of our interest is shown in Fig.l.