Frequency-dependent directivity and sound power measurements of two launched United Launch Alliance Atlas V rockets

Abstract

The noise generated during rocket launches is both intense and directional. Historically, rocket noise directivity has been studied using static firing data. Although such tests result in greater averaging times at all angles, it is also possible to estimate directivity curves and sound power for a launched rocket so long as the trajectory data and microphone locations are known. Acoustical measurements were made during two United Launch Alliance Atlas V launches out of Vandenberg Space Force Base, CA. Multiple microphones were placed around the base ranging from 300–7000 m from the launch pad at different azimuthal angles around the pad. Frequency-dependent directivity curve estimates and sound power spectra are calculated and compared between measurement sites and launches. Directivity indices are compared with those found in the literature. The overall sound power levels agree well between the two launches, though a general trend persists where the sound power estimate decreases as a function of measurement distance. The sound power spectra are also nondimensionalized using Strouhal number scaling and compared with historical literature. [Work supported in part by the Oak Ridge Institute for Science and Education and the Air Force Research Laboratory.]