SOUND-PROPAGATION OVER CONVEX IMPEDANCE SURFACES

Theoretical calculations for the diffraction of sound by large spheres and cylinders with finite impedance surfaces are reported. The differ ences between existing two-dimensional and new three-dimensional resul ts are made explicit and are shown to involve a simple correction fact or in the case of a large sphere. The results for propagation over an infinitely long cylinder have a bearing on the widely used analogy bet ween sound propagation over a curved surface and sound propagation in a refracting atmosphere above an impedance plane. Specifically, it is found that there is a rigorous analogy between sound propagation above a large circular cylinder and propagation in a medium where the sound speed varies exponentially with height. This differs from the bilinea r profile that is often used when exploiting the analogy [see, for exa mple, J. Acoust. Sec. Am. 83, 2047-2058 (1988)]. Predictions for both profiles are found to agree well with each other and with the publishe d data in the shadow zone, but considerable discrepancies are found in the penumbra region. (C) 1998 Acoustical Society of America. [S0001-4 966(98)05311-9].