An investigation of the accuracy of oblique incidence ultrasonic reflection coefficient measurements

Abstract

An investigation of the accuracy of ultrasonic reflection coefficient measurements from layered structures at oblique incidence is described. The measured reflection coefficients from a series of interfaces are compared with the predictions of a plane-wave (infinite transducer) model, and with predictions taking into account the finite size of the transducers. It has been shown that experiments with finite-sized transducers can be used to measure the plane-wave reflection coefficients from any interface within the system, provided that the angle of incidence is remote from any critical angle of the system, and that the receiver is placed at the center of the area insonified by the reflection of interest. If quantitative predictions of the full response measured by the real transducers are to be obtained, or if predictions at angles of incidence close to any critical angles are required, then finite transducer theory must be used. The reflection coefficients measured using a pair of 10-MHz center frequency, broadband transducers agreed with the theoretical predictions to within ±3% when the reflection coefficient from the top surface of a plate system was measured, and to within ±5% when an embedded interface was considered. The reproducibility of the measurements was found to be as good as ±1% provided that great care was taken in aligning the transducers when the rig was set up.