Backscattering enhancements of ultrasound by tilted solid plastic cylinders in water due to the caustic merging transition: High angular resolution scans

Abstract

Bulk shear and longitudinal waves can make important contributions to the scattering of ultrasound by tilted finite plastic and rubber cylinders in water when the phase velocity of the wave is less than the speed of sound of the surrounding fluid. At a certain critical tilt angle, a backscattering enhancement is observed as a result of merging rainbow caustics associated with bulk transmitted rays internally reflected off the cylinder truncation [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 107 (2000)]. The backscattering away from this angle is otherwise weak. The critical angle is calculated using the Bravais effective refractive index for the projections of rays on the base plane of the cylinder. New ultrasonic observations are reported for a polystyrene cylinder over a wide range of tilt angles at high angular resolution. The backscattering amplitude angle scan takes on the shape of a Pearcey function with the peak amplitude offset from the critical angle. Backscattering amplitudes at the critical angle are also compared with a ray theory prediction as a function of frequency for both the polystyrene and a silicone rubber cylinder. [Work supported by ONR.]