| Abstract | The coherence of a sound field in a refractive shadow near the ground has been examined for microphone separations which are transverse or longitudinal to the direction of propagation. Single tone test signals at 1 kHz, 500, and 100 Hz were measured at a range of 700 m by a microphone array, spanning 270 m longitudinally and 15 m transversely. Estimates of the spatial correlation, based on 10-minute averages, indicate transverse coherence lengths less than 2 m and longitudinal correlation lengths less than 7 m for moderately strong upward refraction conditions. Numerical simulations were done which emulate the experimental conditions. The Green's-function method for the parabolic equation was used to generate two-dimensional sound fields within a refractive shadow. An upward refracting atmospheric model with homogeneous isotropic Gaussian turbulence was used. The coherence in the simulated sound fields compares well, qualitatively, with the experimentally measured coherence. Both simulation and experiment indicate that the longitudinal coherence length within a refractive shadow is dramatically less than is typically observed outside of a shadow. The reduced coherence will restrict the useful aperture size of beamforming arrays deployed in acoustical non-line-of-sight conditions. |
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