Showing papers by "Naval Surface Warfare Center published in 1987"

Acoustic resonance scattering by viscoelastic objects

Abstract: In this paper acoustic scattering from a viscoelastic sphere accurately modeled by means of the Kelvin–Voigt model is studied. The approach based on a s i n g l e impedance‐type or Cauchy boundary condition used in the past to account for losses in the body, is reconciled with the exact approach of viscoelasticity, which accounts for material losses via complex field equations, complex propagation vectors, and a set of t h r e e realistic boundary conditions on the surface of the sphere. Using the exact approach of viscoelasticity theory, the effect of viscoelastic losses on the various quantities of interest is determined. The ResonanceScattering Theory (RST) is ideally suited to isolate features of the acoustic spectrum which are dependent upon material composition. In order to use the RST for the combination of a viscoelastic object in a liquid medium, an impedance‐matched background is required, and it is developed here for the first time. Subtraction of this background successfully isolates the resonances in the present case. Finally, an exact expression for the specific surface impedance of the sphere, which depends in a complicated way on frequency, on mode order, and on the four parameters controlling the viscoelastic properties of the sphere is derived. The effect of all these quantities on the sonar cross section of the sphere, or on the modal contributions contained within it is studied, and many pertinent results are displayed.

Inverse acoustic scattering from an elastic sphere

Abstract: A technique is presented to determine the material properties of elastic spheres using the resonance features present in their backscattering cross sections. The method is based on an asymptotic solution of the elastodynamic equations governing the sphere’s response in the high‐frequency (x≡k1a≫1) regime. Our study shows that the modal resonances that an elastic sphere communicates to its echo fall into two distinct families. The set of broader resonances is the dilatational family. The narrower set is the shear family. They are both present in all the modes (n>0) contained within the sonar cross section except for the n=0 mode that contains only broad resonances. Both sets of resonances are separated by spacings that asymptotically (x≫1) tend to uniform values that we have respectively called Δ and Δ’. The dilatational and shear wave speeds in the elastic sphere are shown to be respectively proportional to these spacings, so that the two wave speeds can be immediately determined from them. The sphere’s d...