Design principles of transducers with matching layers based on admittance measurements

Basing on modified Mason's equivalent circuit of an acoustic probe the authors carried out the computation of admittances and pulses reflected from the ideal reflector immersed in water. The relation between the admittance of a PZT transducer with acoustic layers (matching the acoustic impedance of the transducer to the acoustic impedance of the human body) and the thickness of the layer is given. It is shown the optimal layer thickness should be determined as a quarter wave-length λe/4 (calculated for the electric resonance frequency). In such a case the admittance curve is symmetrical and reaches the minimum, and the reflected pulse is shortest. Small thickness changes of the layer causes more distinct changes of the admittance than of the reflected pulses. The influence of the acoustic impedance of layers and electromechanical coupling coefficient kt on the admittance curves is discussed. The admittances of the transducer measured for different thicknesses of the acoustic matching layer are shown. The results are in good agreement with computations.