Lamb wave excitation in piezoelectric plates

The excitation of ultrasonic Lamb waves by an interdigital transducer (IDT) deposited on a piezoelectric plate is analyzed using the Green´s function method. The amplitudes of the generated waves are obtained in terms of the charge density on transducer electrodes. An electrostatic analysis that neglects piezoelectric coupling is used to relate this charge density to the voltage applied across the IDT. The analysis has been used to calculate the input conductance of an IDT as well as the coupling coefficient between the IDT and the various Lamb wave modes. It is found that when the plate is more than a few acoustic wavelengths thick the lowest order symmetric and antisymmetric Lamb modes are almost equally generated. The slight difference in the velocities of these modes gives rise to the beating effect whereby energy launched by the IDT on one surface transfers periodically back and forth between the two surfaces as a sinusoidal function of the distance traveled