Shear modulus effect on acoustically transparent materials

Materials have been developed that match density and dilatational sound speed to the values of water as closely as possible while maintaining sufficient rigidity to serve for structural purposes. Matching of density and speed results in transparency for fluids only; the shear modulus in a solid admits the presence of a shear wave which causes deviation from ideal behavior. The effect of a finite shear modulus on the reflection of plane waves by an infinite plane is analyzed. The shear modulus of the material was measured following a method developed by R. L. Adkins in 1966. Examples are given of the reflection coefficient as a function of incidence angle for values of and close to those of the medium, and various ratios of plate thickness to dilatational wavelength. The condition of total reflection is shown to be related to the occurrence of thickness-shear resonances. Insertion-loss measurements on a structurally rigid panel are compared with results of the analysis.