Bispectral analysis of high-amplitude jet noise

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.