Underwater characterisation of acoustic modes supported by structured and unstructured elastic plates

Abstract

Surface waves are supported by unstructured, or appropriately structured, elastic materials. Unstructured plates typically support a number of Surface Acoustic Waves (SAWs) that propagate at the boundary of the elastic materials—these can be bound or leaky. Whilst plates with appropriate structure can support bound Acoustic Surface Waves (ASWs) which propagate in the fluid at the fluid/solid boundary. In this talk, we present the underwater acoustic characterisation of both SAWs and ASWs supported by unstructured and structured aluminium plates. First, we measure the dispersion of the symmetric and antisymmetric leaky Lamb modes of an unstructured aluminium-alloy plate using a broadband ultrasound technique. Beaming of acoustic power over a narrow frequency band is observed. This is attributed to the first order symmetric leaky Lamb mode, which exhibits Negative Group Velocity (NGV) in a small region of its dispersion. Finally, we show that a perforated elastic plate supports a bound ASW. We characterize the dispersion of the supported modes using time-domain measurements of the surface wave pressure recorded as a function of position, and observe acoustic beaming of surface energy over a narrow frequency band. All results show close agreement with numerical models.