Determination of the size of spherical inclusions smaller than the width of the sound beam

Abstract

The current presentation introduces a new method for the determination of the size of balls with a diameter from nearly a wavelength up to the width of the sound beam (and higher). The basic approach is to analyse the directional pattern of the reflectors, which depends on size and shape. If the reflector is centered on the acoustic axis of the transducer, small objects cause approximately the same omni-directional echo, so that no differentiation of objects on the receiver is possible. For non-axial acoustic irradiation of the reflectors, differences of the directional pattern appear. While scanning spherical reflectors, these variations can be detected with a special structured array. Therefore a 3 MHz annular array is used, whose outer rings are divided in additional segments.In this work first measurements of spheres with different sizes where done. The sphere diameters varied about one up to six wavelengths and the lateral size of the sound beam in the scan depth was 2.8 wavelengths. All measurements are combined with sound field simulations, to provide algorithms for the determination of the size of reflectors.