Measurement of Rayleigh phase velocity and estimates of shear speed by schlieren visualization

Abstract

A schlieren technique is used to give a direct accurate measurement of Rayleigh phase velocity on various materials. Plane solid interfaces underwater are insonified by finite acoustic beams. Sound incident at the Rayleigh angle produces a null strip in the radiated field which uniquely identifies the Rayleigh angle. This null strip is caused by the mutual cancellation of equal amplitude specular and Rayleigh radiations which are 180° out of phase. Rayleigh phase velocity is then calculated from the measured Rayleigh angle by a simple equation. The measured Rayleigh velocities are used to give estimates of shear velocity, and these estimates are compared with direct shear velocity measurements. The attenuation of the Rayleigh wave in the low-MHz region is due to radiation into the water and is found to be directly proportional to frequency and inversely proportional to material density as predicted by theory.