Scattering Coefficients and the Absorption Edge of Longitudinal Coherent Sound Waves in Selected Inhomogeneous Materials.
NAVAL SURFACE WEAPONS CENTER SILVER SPRING MD
Pagination or Media Count:
The first three terms in the far field forward scattering amplitude for longitudinal waves impinging on spherical objects embedded in various matrix materials were studied as functions of frequency. The major resonances in these terms were used to obtain rough estimates of the position of the absorption edge in corresponding elastic and viscoelastic inhomogeneous materials. It has been generally known that in relatively compressible inhomogeneous materials the lowest resonance frequency is that of a monopole. It is achieved by means of highly compressible fluid inclusions in soft rubbery media. Not as widely known has been the fact that in incompressible materials the lowest achievable resonance frequency is that of the dipole. It occurs in relatively soft, rubbery media with heavy solid inclusions. By ordering our materials with respect to the lowest frequency at which significant scattering occurs, we put these observations into perspective. An estimate is given of the absorption edge for every material combination in this study. A special effort is made to trace the development of the first three resonances in elastic solids as functions of the material properties of the host and the inclusion. The monopole quickly moves to higher frequencies than the dipole as the compressibility of the inclusion is decreased, and that as a rule, these two terms interfere unless the mass density of the inclusion is much greater than that of the host.