Backscattering enhancements due to retroreflection of ultrasonic leaky Rayleigh waves at corners of solid elastic cubes in water

Backscattering enhancements can be important for interpreting acoustical images of scatterers. Experiments were performed on a solid stainless steel cube in water in order to investigate a significant mechanism for producing a large backscattered signal. Enhanced high-frequency backscattering is observed when the block is rotated such that the incident acoustic wave strikes one of its flat surfaces in the vicinity of the coupling angle for launching a leaky Rayleigh wave at the steel–water interface. This enhancement can be explained as a retroreflection of the leaky wave at a corner of the block, which results in a reversal of the incident wave vector and thus produces a large backscattered signal. The magnitude of the peak backscattered pressure is approximated using a convolution formulation [P. L. Marston, J. Acoust. Soc. Am. 97, 34–41 (1995)]. Observed backscattering enhancements support the validity of the convolution formulation to approximate the general magnitude of the peak pressure, as well as being consistent with the dephasing predicted by that theory as the block is rotated away from the coupling angle. Tilting the block while maintaining the coupling angle reduces the backscattered amplitude because of changes in the reflection coefficients at the corner as well as increased attenuation.