ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

3aPAa7. Scattering of sound by turbulence into a shadow region.

Michael R. Stinson

Inst. for Microstruct. Sci., Natl. Res. Council, Ottawa, ON K1A 0R6, Canada

The sound field above ground due to a point source in an upwardly refracting atmosphere has been computed to determine the nature of turbulent scattering into an acoustic shadow region. A Green's function parabolic equation (GFPE) method that includes turbulence via a phase screen approach [X. Di and K. E. Gilbert, J. Acoust. Soc. Am. 92, 2405(A) (1992)] is used for the simulations. Flat ground with finite surface impedance and a logarithmic sound-speed profile are assumed, and propagation ranges of 1 km are considered. Different realizations of a turbulent atmosphere were generated employing a Gaussian spectrum to describe the turbulence. For 500-Hz source tones, it was found that the components of the turbulence spectrum that contributed most to the scattered field in the acoustic shadow region corresponded to dimensions of the order of 2--5 m; this scale of structure can be understood in terms of a Bragg reflection condition. The sound pressure at a receiver position in the acoustic shadow appears to be dominated by contributions from a small number of localized scattering regions.