Including source correlation and atmospheric turbulence in a ground reflection model for rocket noise

Acoustic data collected in static rocket tests are typically influenced by ground reflections, but have been difficult to account for. First, the rocket plume is an extended radiator whose directionality results from source correlation. Second, partial coherence of the ground interaction due to atmospheric turbulence can play a significant role, especially for larger propagation distances. In this paper, a finite impedanceground, single-source interference approach [G. A. Daigle, J. Acoust. Soc. Am. 65, 45-49 (1979)] that incorporates both amplitude and phase variations due to turbulence is extended to distributions of correlated monopoles. The theory for obtaining the mean-square pressure from multiple correlated sources in the presence of atmospheric turbulence is described. The effects of source correlation, ground effective flow resistivity, and turbulence parameters are examined. Finally, the model predictions compared favorably against data from two horizontal firings of GEM-60 solid rocket motors – one involving snow-covered terrain – allowing effective removal of the ground reflection from far-field power spectral densities out to the maximum measurement distance of 1220 m. Close to the motor, more physically realistic corrected spectra are obtained by increasing the modeled fluctuating index of refraction by two orders of magnitude.