Quantifying nonlinearity in the propagation of noise from military jet aircraft

Because of the high noise levels radiated by military jet aircraft, it has been hypothesized that nonlinearity influences the propagation of the noise. A numerical model, which accounts for second-order cumulative nonlinearity, atmospheric absorption and dispersion, and geometrical spreading, has been developed to propagate jet noise waveforms. Numerical propagation of recorded waveforms from recent static engine run-up measurements demonstrates significant waveform steepening and an accompanying transfer of spectral energy to high frequencies that agrees well with measured spectra. Furthermore, the measured and nonlinearly-predicted waveforms are perceived to be significantly "louder" or "more annoying" than linearly-predicted waveforms, despite the fact that standard metrics such as overall sound pressure level (with flat, A, and C weighting) and Mark-VII perceived loudness show little difference between linearly-and nonlinearly-predicted spectra. The results of this study demonstrate the need for additional investigations with alternate metrics that more closely relate to perceived annoyance or loudness of high-amplitude jet noise.