Flight test of a pure-tone acoustic source

Abstract

A unique experiment involving static and flight testing of a pure-tone acoustic source has been conducted. The objectives of the experiment were: to determine if a 4-kHz tone radiated by the source in flight and mixed with broadband aircraft flyover noise, could be measured on the ground with a high degree of statistical confidence; to determine how well a flight-to-static tone radiation pattern comparison could be made to the static radiation pattern; and to determine if there were any installation effects on the radiation pattern due to the flight vehicle. Narrow-band acoustic data were measured and averaged over eight microphones to obtain a high statistical confidence. The flight data were adjusted to an equivalent static condition by applying corrections for retarded time, spherical spreading, atmospheric absorption, ground impedance, instrumentation constraints, convective amplification, and the Doppler shift. The flight-to-static results are in excellent agreement with the measured static data. No installation effects were observed on the radiation pattern.