Assessing the impact of ground reflection on measurement of the overall sound power level and acoustic radiation efficiency of rocket launches

Determining the sound power levels of a rocket is the first step in characterizing its radiation efficiency. Because the sound radiation from a launch vehicle is anisotropic, well-calibrated ground measurement stations are used, along with trajectory data, to obtain sound power. Historically, the effect of ground reflections appears to largely have been neglected in the literature, despite the potential to inflate OAPWLs and therefore radiation efficiency. This study investigates the likely effect of a finite-impedance ground on spectra, overall sound pressure levels, and power levels. The single-parameter ground reflection model by Embleton et al. [J. Acoust. Soc. Am. 74, 1239–1244 (1983)] is used to obtain an estimate for change in OASPL for a model spectrum based on space vehicle launches. When the correction is applied to the OAPWL it produces in a nearly 3dB reduction in level and reducing the efficiency by a factor of 2. This indicates the possibility that the 0.5% radiation power efficiency assumed for rockets in the past is too high.