Abstract
The development of the fast field and parabolic equation solutions to the wave equation has made it possible to solve for the combined effects of refraction in a layered atmosphere and the interaction of sound with a complex impedance ground surface. In many respects the numerical methods have advanced beyond an understanding of the basic phenomena. In an earlier study [J. Acoust. Soc. Am. 89, 107–114 (1991)], the residue series solution for upward refraction was investigated and provided insight into the nature of the interaction of refraction and ground reflection. In this paper, results of a similar normal mode solution are presented for downward refraction by a bilinear sound velocity profile above a complex impedance ground surface. This model is used to investigate when the surface wave is excited for downward refraction conditions and to develop criteria for the maximum range of cylindrical decay as a function of ground impedance phase and magnitude and the magnitude of the sound velocity gradient. [Work supported by NASA Langley Research Center.]
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