ELECTROMECHANICAL COUPLING-COEFFICIENTS OF ULTRASONIC LAMB WAVES

The electromechanical coupling coefficients of fundamental and higher- order ultrasonic Lamb wave modes propagating in a piezoelectric plate have been calculated using two different methods. The first approach c alculates the coupling coefficient K from the well-known equation K2=2 (DELTAv/v) = 2(v0-v(m))/v0, where v0 and v(m) are the wave velocities on a free and a metallized surface, respectively. The second approach, which involves considerably more calculations, is based on the Green' s function method of analysis. In the case of surface acoustic waves, it has been shown that the two approaches give nearly identical result s. It is found that, in most cases, this is also true for Lamb waves. The only exception occurs for the lowest-order symmetric (S0) and anti symmetric (A0) modes, if the ratio h/lambda (h = plate thickness, lamb da = acoustic wavelength) is greater than 1. In this case the DELTAv/v method gives erroneous results. This occurs because, in this case, th e surface metallization causes strong perturbation of the mechanical d isplacements associated with the waves. In all other cases, the surfac e metallization produces a weak perturbation of the acoustic wave, and the simple DELTAv/v method can be used to calculate the coupling coef ficient.