Finely resolved spatial variation in F-22 spectra

Abstract

Examination of the spatial variation in the spectrum from ground-based microphones near an F-22 Raptor has revealed spectral features at high engine power that are not seen at intermediate power or in laboratory-scale jet noise. At military and afterburner powers, a double peaked spectrum is detected around the direction of maximum radiation. In this region, there is not a continuous variation in peak frequency with downstream distance, as seen in lab-scale studies, but a transition between the relative levels for two discrete one-third octave bands. Previous attempts to match similarity spectra for turbulent mixing noise to a few of these measurements split the difference between the two peak frequencies [Neilsen et al., J. Acoust. Soc. Am. 133, 2116–2125 (2013)]. The denser spatial resolution afforded by examining the spectral variation on all 50 ground-based microphones, located 11.6 m to the sideline and spanning 30 m, provides the opportunity to further investigate this phenomenon and propose a more complete formulation of expected spectral shapes. Special care must be given to account for the relative amount of waveform steepening, which varies with level, distance, and angular position. [Work supported by ONR.]