What type of ultrasonic wave travels along the surface of a material with elliptical particle motion?

Rayleigh waves are surface waves that travel along the surface of a material, and they are characterized by elliptical particle motion. This motion is a result of how these waves interact with the material's surface, causing particles to move in an elliptical path as the wave passes. This behavior is particularly significant in applications like non-destructive testing (NDT) because Rayleigh waves can detect surface and near-surface defects effectively due to their sensitivity to changes in the material.

In contrast, longitudinal waves feature particles vibrating parallel to the direction of wave motion, while transverse waves have particle motion perpendicular to the wave direction. Sound waves can refer to various types of waves, including longitudinal waves in fluids. However, none of these alternatives possess the specific characteristic of elliptical particle motion along a surface that is distinctive to Rayleigh waves. Thus, when considering the properties and behaviors of different wave types in relation to their particle motions, Rayleigh waves are the clear choice for this question.