A parametric analysis of attenuation mechanisms in composites designed for echo reduction

Abstract

The fundamental attenuation mechanisms operating in a particular class of composites are investigated for their viability as underwater anechoic materials. The type of composites of interest consists of dense (visco-) elastic inclusions in rigid, low-density, water impedance-matched, elastic hosts. Composites similar to this have been studied by Kinra2 and shown to attenuate transmitted elastic waves in a resonant regime of the imbedded inclusions. Our calculations indicate that the processes giving rise to the attenuation would also be appropriate for echo reduction. As a reference material, a composite of lead-loaded silicone rubber spheres in a rigid epoxy is studied. The processes operating at both the water–epoxy and epoxy–rubber interfaces are studied theoretically. Using the spherical elastic T matrix, effects due to resonant scattering are analyzed by reference to the scattered and absorption cross sections calculated for both a single rubber sphere and two rubber spheres imbedded in an infinite epoxy matrix. The dynamics at the water–epoxy interface is analyzed via classical elastic theory at a plane interface. The results of our analysis indicate that resonant compressional to shear mode conversion at the inclusions could be an effective way of producing the desired effects. Consequences of the mode conversion leading to attenuation include the trapping of backscattered shear waves and the enhancement of dissipation processes within the inclusions.