Acoustic properties of the interface of a uniform medium-2-2 piezocomposite and the field distributions in the composite

The acoustic properties of a medium-composite interface and the field distributions in a 2-2 composite are analyzed based on an approach developed recently. It is found that the effective input acoustic impedance Z_in of the composite shows marked dependence on the shear stiffness constant of the medium in contact, which is related to the non-uniform vibration distribution at the composite surface. Since for a piezoceramic polymer composite, it is the ceramic phase which performs the energy conversion between the acoustic and electric forms, the amount of acoustic energy which can enter the ceramic region is one of the most important parameters in a composite transducer design. We show that even though the effective transmission coefficient increases as the frequency is increased, the amount of acoustic energy entering the ceramic region actually decreases. From the fact that there is more than 180° phase change in the reflection wave from the medium-composite interface, it is shown that the matching layer thickness is no longer equal to the quarter wavelength, but smaller, and the theoretical predictions are confirmed experimentally.