PRECISION ULTRASONIC REFLECTION STUDIES IN FLUID-COUPLED PLATES

An approximate, but highly accurate, analysis is presented for the ref lection of bounded acoustic beams-and their detection by finite receiv ers-from fluid-loaded structures, demonstrating the relationship betwe en the plane-wave reflection coefficient and the transducer voltage. T he correspondence of the reflection coefficient zeros, in both angle a nd frequency, with the observed voltage minima is found to depend crit ically on the experimental geometry, especially the lateral transducer placement. In measurements with two identical transducers, the beam s hape, 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 re spect to the sample are the major geometrical factors determining the appearance, width, and depth of reflection minima. A series of precisi on, swept-frequency experiments on a uniform, homogeneous plate to val idate the calculation and expose any possible limitations is also repo rted here. In general, excellent detailed agreement between the effici ent approximate calculation and the results of the experiments has bee n obtained. An expression is derived to predict the optimal transducer positioning for accurate estimation of reflection coefficient zeros f rom voltage measurements. These results have significance for material property extraction from reflection measurements on fluid-loaded stru ctures. (C) 1996 Acoustical Society of America.