Acoustic backscattering enhancements for circular elastic plates and acrylic targets, the application of acoustic holography to the study of scattering from planar elastic objects, and other research on the radiation of sound.

摘要

Backscattering enhancements on both circular elastic plates and acrylic targets are investigated as well as several techniques for the study of the radiation of sound. For sound scattered from a circular plate, two backscattering enhancements associated with the extensional wave are observed. The first of these enhancements involves extensional wave excitation along the diameter of the plate. When the extensional wave strikes the plate edge, reflection occurs which produces radiation into the backscattering direction. For those portions of the leaky wave which strike the edge at oblique incidence, there is mode conversion into a trapped shear wave. For certain angles of incidence on the plate edge, this wave can undergo multiple reflections and convert back into a leaky wave directed in the backscattering direction. Each of these enhancements are modeled using quantitative ray methods. Acoustic holography is also used to image the surface motion of the plate to identify the causes of these enhancements and to assess the validity of the ray model. Backscattering enhancements associated with antisymmetric Lamb wave excitation are also investigated. Scattering at the first-order antisymmetric wave coupling angle is studied using acoustic holography. Significant mode-conversion between the zeroth and first-order antisymmetric waves is observed which plays a significant role in the scattering processes. Quantitative ray models were also used to examine the backscattering from acrylic targets. Polymer solids typically have shear and Rayleigh wave phase velocities which are less than the speed of sound in water. For solid acrylic spheres, low frequency resonances are observed both experimentally and in the exact backscattering form functions which are due to coupling between the incident field and the subsonic Rayleigh wave on the sphere. The effects of material absorption, which is generally high in polymers, is examined in both the exact solutions and the quantitative ray models. For spherical shells, the subsonic bulk wave velocities produce a bifurcation in the lowest symmetric mode of the shell. The implications of this bifurcation are examined for backscattering from the evacuated spherical acrylic shells. Although material absorption greatly decreases the magnitude of these shell enhancements, backwards wave enhancements are also detected which are not as strongly affected by the damping. In order to examine the radiation from vibrating shells, an electromagnetic transduction technique is developed to drive the shell resonances in water while measuring the radiation acoustically. It was possible to excite both the torsional modes and the quadrupole resonance with this method and detect acoustic radiation from each. A helicoidal transducer is also developed which produces a wavefront possessing a screw dislocation along the axis of the wavefield.