Study of the Normal Modes of Vibration of the Piezoelectric Transducers Used in Acoustic Imaging Applications

Abstract

The performances of acoustical imaging devices are now much more limited by the insufficient understanding of the ultrasonic transducers behaviour rather than by imperfections in the associated electronics, as shown by the growing interest among transducer operation studies. For example, M. PAPPALARDO (1) measured the frequency spectrum of a freely vibrating transducer, in terms of the width (W) to thickness (t) ratio, but was unable to interpret all the observed vibration modes. For this purpose, the finite element mathematical model has been used by J. SATO (2) in a narrow range of W/t values, and the transducer behaviour at a single frequency was deduced therefrom. To our opinion, a simpler approach may be taken, by measuring and studying the eigen resonance modes, to give a more physical picture of the phenomenon. The eigen modes arise from the resonance of Lamb waves propagating along the three major orthogonal directions of the transducer. For acoustical imaging applications, like biomedical ones, the width and thickness are of the same order of magnitude, so that resonances along these two directions occur in the same frequency band.