Precision ultrasonic reflection studies in fluid-coupled plates

Abstract

An approximate, but highly accurate, analysis is presented for the reflection of bounded acoustic beams—and their detection by finite receivers—from fluid-loaded structures, demonstrating the relationship between the plane-wave reflection coefficient and the transducer voltage. The correspondence of the reflection coefficient zeros, in both angle and frequency, with the observed voltage minima is found to depend critically on the experimental geometry, especially the lateral transducer placement. In measurements with two identical transducers, the beam shape, its sidelobes, and far-field or near-field conditions are found to play a relatively minor role in the receiver voltage. Instead, the size of the transducers and their relative lateral positioning with respect to the sample are the major geometrical factors determining the appearance, width, and depth of reflection minima. A series of precision, swept-frequency experiments on a uniform, homogeneous plate to validate the calculation and expose any possible limitations is also reported here. In general, excellent detailed agreement between the efficient approximate calculation and the results of the experiments has been obtained. An expression is derived to predict the optimal transducer positioning for accurate estimation of reflection coefficient zeros from voltage measurements. These results have significance for material property extraction from reflection measurements on fluid-loaded structures.