Resonance reflection of acoustic waves from a submerged, perforated and bilaminar rubber coating model

Abstract

Each of the perforated rubber layers constituting the multilayered composites used to absorb underwater sound waves can be (ideally) modeled as a fluid layer having density ρi and sound speed Ci. The volume concentration of the gas-filled perforations controls the (effective) values of ρi and Ci in each of the layers. We consider a plane two-layer coating having a (possibly different) fluid on each side, and we study its reflection coefficient R(f, θ) as a function of frequency f and incidence angle θ, as plane sound waves are reflected by it. Resonances are evident in R(f, θ) both as functions of f and θ, and we display many of these in three-dimensional graphs that are as informative as esthetically pleasing to view. We then show how these resonances in R can be analyzed in the light of the resonance scattering theory (RST) in order to generate simple (and approximate) but quite accurate reflection predictions, which we then use to extract information about the material composition of the bilaminar composite. The method determines which of the reflection features is caused by which of the two interacting layers.