Bispectral Analysis of High-Amplitude Jet Noise

Abstract

The overall sound pressure levels of noise radiated by military jet aircraft along certain angles are such that nonlinearity is likely to influence the propagation. Bispectral analysis of noise data from the F/A-18E Super Hornet has been carried out in order to provide further evidence that nonlinear effects are indeed present. The bicoherence, which is a normalized form of the bispectral density, has been previously used in a variety of applications to detect quadratic phase coupling (QPC) in a signal. In this case, the results of the bicoherence calculations indicate that QPC is indeed present at high-thrust conditions along the peak radiation angles, which means that nonlinearity does play a role. However, additional investigations are still needed to more fully understand the physical interpretation of the bispectral results for a random noise signal, which will also help better quantify the role of nonlinearity in jet noise propagation.

Measurement and Prediction of Noise Propagation from a High-Power Jet Aircraft

“…Recently, Gee et al [6,7] built upon Pestorius and Blackstock's [1] original work and compared the results of numerical propagation of F/A-18E waveforms via their algorithm against measurements that showed evidence of nonlinear propagation [8,9]. Although there were distinct similarities in the high-frequency rolloff trends for comparisons between predicted and measured spectra, there were notable differences as well.…”

Acoustic Shock Formation in Noise Propagation During Ground Run-Up Operations of Military Aircraft

“…These efforts often rely on metrics calculated from the waveforms. Such metrics may evaluate the time-domain [14,15] or frequency-domain [9,16] characteristics of the waveform, and have been applied to full-scale [17] and laboratory-scale [18,19] data. However, one of the issues that arises from the use of metrics is their interpretation.…”

Do recent findings on jet noise answer aspects of the Schultz curve?

“…9,10 Although there is currently no metric to quantify the perception of crackle, various techniques have been used to examine acoustical nonlinearities and the presence of acoustic shocks. These include bispectral analysis [11][12][13] and other indicators of shock strength. [14][15][16] Employed in this study are two techniques that have been used in previous jet and rocket noise studies and whose behavior for high and low-amplitude noise is at least qualitatively understood.…”