Angular and frequency spectral analysis of the ultrasonic backward beam displacement on a periodically grooved solid.

The ultrasonic backward beam displacement, which has been shown to occur when a bounded beam is incident upon a periodically corrugated liquid-solid interface, is studied experimentally. This effect has been previously studied on a periodic water-brass interface at one particular frequency (6 MHz) and one corresponding angle of incidence (22.5 degrees), but the question has remained whether it would also exist at other frequency and angle combinations. The knowledge of whether this phenomenon is a coincidence or whether it will occur for other frequency and angle combinations contributes to a better understanding of the interaction of ultrasound with periodic structures and diffraction effects, in particular. Potential applications exist in the study of phononic crystals and in the non-destructive evaluation of materials. The present work reports results from recent experiments on the same periodically grooved brass sample that was employed in the first investigations of this phenomenon. Through the examination of frequency spectra in the form of angular and classical spectrograms, the experiments reported here show the backward beam displacement to occur for multiple angles of incidence and frequencies. Furthermore, evidence is shown as to the exact cause of the backward beam displacement, namely, a backward propagating Scholte-Stoneley wave.