Correcting for ground reflections when measuring overall sound power level and acoustic radiation efficiency of rocket launches

Abstract

Historically the radiation efficiency of a rocket plume has been assumed to be 0.5%, but recent measurements have thrown this into some doubt. Determining the sound power level 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 largely to have been neglected in the literature, despite the potential to inflate overall power levels (OAPWL) and therefore radiation efficiency. This study investigates the likely effect of a finite-impedance ground on spectra, overall sound pressure levels (OASPL), and power levels. A single-parameter ground reflection model is used to obtain an estimate for change in OASPL for a model spectrum based on measured space vehicle launches. When the correction is applied to the OAPWL it produces in a nearly 3dB level reduction, therefore reducing the radiation efficiency by a factor of two. This indicates the probability that the radiation efficiency assumed for rockets in the past is too high.