A Experimental Analysis of the Basic Phenomena Involved in Modern Diffraction Theories.

This study utilizes direct experimental techniques to analyze the basic scattering mechanisms that are important to modern diffraction theories. The efforts are intended to supplement and guide the development of theoretical models by exploring parameter ranges and configurations that are intuitive extensions of the cases handled by current prediction techniques. A broad program of experimental investigation of rigid three-dimensional scatterers is presented. The thin prolate spheroid is used as a basic model for scattering from smoothly curved bodies. Baffles, cylinders, cones, and cone variants serve for an examination of edges and tips of various orders. In addition to standard pulse measurements with gated receivers, some data are obtained by a holographic imaging technique. The results show that the backscattered pressure from smoothly curved bodies is determined almost exclusively by specular effects, even at wavelengths that are relatively large compared to the appropriate dimensions of the scatterers. Small changes in or additions to the smooth surface, however, modify both the amplitude and directivity of the scattering. Evidence of surface fields very close to smoothly curved scatterers is also observed. These phenomena can, in general, be identified with Franz-type creeping waves. However, they appear to depend only loosely on the shape of the scatterer's surface, and a prominent wave-type effect is noted at the antipole. Other data show that very small discontinuities in the shape of a model's surface can be prominent scattering sources, while discontinuities in only the surface's slope or curvature scatter much less. The form of the edge on a baffle is seen to be an important scattering parameter for finite edges with thicknesses as small as a quarter of a wavelength. Furthermore, moderate surface damping on a flat baffle appears to significantly attenuate the diffracted field propagating at a grazing angle along the baffle. Finally, because of a weakness in the current literature, the Freedman theory of echo formation is carefully derived, and its predictions are compared with applicable experimental results. The technique is found to be an exact (i.e., not asymptotic) variant of physical optics that produces a reasonably good model of scattering from baffles but that exhibits serious errors in cases involving three-dimensional bodies.