Physical interpretation of the far-field directivity pattern of lithium niobate bar transducers

In acoustic imaging and nondestructive testing applications, the quality of images obtained using linear arrays is closely related to the far-field directivity pattern of an elementary transducer bar. But, for some values of width-to-thickness ratio W/T (W being the width and T the thickness) these transducers exhibit parasitic modes with resonance frequencies close to their working frequency. In order to analyze and identify these modes and their parasitic radiations, the finite element method is used with the help of an original extrapolation method to compute the far-field radiation pattern. Measured far-field directivity patterns for (YZw)36 degrees transducer bar of LiNbO3 are presented for a large spectrum of W/T. To get a better understanding of the elementary transducer behavior, an approximate directivity pattern is obtained using a simple method and is compared to the numerical and measured directivities. This simple physical model is based on Sommerfeld's formula. It is shown that for some values of W/T, harmonic vibrations at the surface of transducer can be coupled to the fluid domain and produce a parasitic radiation (for example, for W/T=1, the first harmonic vibration introduces a strong parasitic radiation). (C) 1996 American Institute of Physics.