Acoustic Refraction and Scattering with Compliant Tubes in Water

Abstract

Sound velocity in water may be reduced markedly with compliant structures which effectively increase the compressibility of water. Compliant tubes were derived from round aluminum tubes deformed into flat elliptical sections. Measurements in resonant chambers indicate uniform reduction in sound velocity at frequencies well below the lowest resonant frequency of a compliant tube in water. At higher frequencies, the velocity remains low but varies irregularly for frequencies up to the dipole mode of vibration. Reflection and scattering measurements with plane arrays of compliant tubes indicate high reflectivity over a frequency range that depends inversely on the center-to-center spacing between compliant tubes. The frequency for maximum reflection is also a function of spacing and approaches in the limit the resonant frequency of the compliant tube in vacuum. The characteristic behavior of a compliant tube to underwater sound is predicted with sufficient accuracy by treating it as a clamped-clamped beam and assuming that the distribution of the vibration on the equivalent beam is independent of the interaction with the fluid medium. The reflection coefficient for an array of compliant tubes is readily calculable by using an equivalent circuit, empirical expressions for the radiation load on an array and the mechanical impedance of the compliant tube.