Acoustic reverberation time determination in solid medium

Abstract

Classical ultrasonic testing (UT) is a method used to evaluate the parameters of materials using mechanical wave. Most of these methods provide local information about the properties of the medium along the direct paths between emitter and receiver. Yet, some of these methods depend on the coda or late reflections, which provide global information about the medium. In this work, a method inspired by room acoustic techniques is proposed to determine medium properties. In fact, the reverberated signal is highly sensitive to mechanical parameters and boundary conditions. In acoustic room theory, which uses these reverberated signals, for determination of the reverberation time which is very useful to enhance the sound quality of rooms. However, this technique only uses one propagation mode, since the wave propagates through the air inside the acoustic room. Considering that a solid medium is analogous to the acoustic cavity, the walls are equivalent to boundary conditions. Firstly, a model is developed to estimate the acoustic intensity of each propagation mode taking into account the coupling due to mode conversion. It allows the reverberation time to be estimated in solid medium. Secondly, an experimental setup is proposed. A five piezoelectric (PZT) patches, randomly distributed on an aluminum block, one acting as an emitter and the others as receivers to perform experimental reverberation time measurements. Finally, a comparison between the simulated and experimental is done, and inverse problem solving is performed to retrieve the attenuations. A good agreement between the retrieved and literature attenuation values of both bulk waves has been observed in aluminum.