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Effects of Strong Diffraction on the Focusing and Scattering of Acoustic Waves.
PENNSYLVANIA STATE UNIV UNIVERSITY PARK APPLIED RESEARCH LAB
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In many wave propagation problems, diffraction is considered to be a second order effect that diffuses the sharp shadows and focal surfaces predicted by ray theory. However, if the frequency is low enough so that the wavelength is comparable to the size of the focusing region, structured fields of significant level can be found in classical shadows and smooth, featureless enhancement can replace the sharp, multiple convergences common at higher frequencies. This study examines the effects of strong diffraction on refractive focusing of acoustic waves. In addition, signal fluctuations caused by perturbations of the refractive index are considered where the spatial extent of a particular perturbation patch is comparable to the wavelength. Consequently, the perturbation mechanism is primarily diffractive also.
APPROVED FOR PUBLIC RELEASE