Ultrasonic wave transducer

A method for generating a convergent ultrasonic wave beam comprising the steps of:

• positioning a piezoelectric substrate having a planar surface and at least one interdigital electrode group composed of a plurality of interdigital electrodes, each electrode of which has a plurality of longitudinally-extending fingers, said group being arranged on said surface of the substrate, in a liquid so that at least said interdigital electrodes contact with said liquid, said plurality of interdigital electrodes being arranged in a row extending in the direction of said interdigital electrodes, and each of said electrodes being applied on alternating voltage of a predetermined frequency with the phase difference defined below ##EQU3## where m is an integer, .omega..sub.O is an angular frequency of the electrical signal, v.sub.w is the sound velocity in the liquid, Y.sub.n is the length between the n'th interdigital electrode and the origin point, .DELTA..phi.(y.sub.n) is the phase of the electrical signal applied to the n'th interdigital electrode, and (y,x) are coordinates, and wherein the period between said fingers of said electrodes satisfies
• r.sub.n.sup.2 =k.sub.1 n.lambda..sub.f L+k.sub.2 n.sup.2 .lambda..sub.f.sup.2
• where, as defined with respect to cartesian coordinates, r.sub.n is the abscissa length between the n'th finger of the electrode and the focal point, .lambda..sub.f is the wavelength of the sound wave in a liquid, L is the ordinate length between the n'th electrode and the focal point, and k.sub.1 and k.sub.2 are constants.