The influence of matching layer material loss on radiation impedance conversion in ultrasonic transducers.

Abstract

PZT based thickness mode ultrasonic transducers for both air and water/tissue typically have a quarter wavelength front matching layer with impedance Zm. It is widely known that the lower acoustic impedance ZR of the propagation medium is converted to a higher impedance at the PZT surface by the relation Zmax = Zm2/ZR (quarter wavelength conditions). In this work, the converted impedance was accurately calculated using a transmission line model incorporating the mechanical quality factor Qm of the matching layer material. In a medical transducer, it was found that the peak value is 20% or 28% lower than Zmax = Zm2/ZR for Qm = 15 or 10, respectively. For air transducers, the peak value is one to two orders lower than Zmax for the same range of Qm. These Qm values are typically observed for the filled epoxy matching layer materials used in medical transducers and also for porous or air entrained materials used for air acoustic transducers. A simple impedance conversion equation for an air transducer has been proposed, making the design of air impedance matching layers easier and suggesting that neglecting material loss leads to serious errors.