Abstract
Like turbulent fluctuations in the atmosphere, variations in terrain elevations (surface roughness) and ground impedance may be regarded as diminishing the coherence of the sound field and introducing uncertainty into predictions. Here we examine, using a numerical approach, the characteristics and relative importance of terrain-elevation, ground-impedance, and turbulent variations on sound-field coherence. The calculations utilize parabolic equation solutions for a refractive atmosphere, with a Beilis–Tappert transform to incorporate terrain elevation variations and a phase-screen method for the turbulence. The relative importance of terrain variability and turbulence is studied for different refractive conditions. In particular, we examine whether surface roughness has a more significant impact on coherence in downward or upward refracting conditions. Interdependencies between the various mechanisms of coherence degradation are also investigated. The numerical calculations are compared to a recently developed theory for waveguide propagation conditions, which predicts existence of an effective spectrum with independent, additive contributions for turbulence and roughness. Limitations of the Markov approximation for modeling the elevation and impedance variations are also examined.
Published Version
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