On the possibility of nonlinear reflections in shock-containing noise near a high-performance military aircraft

The skewness of the pressure time derivative, or derivative skewness, is a useful indicator of crackle perception in jet noise. Near a military aircraft, derivative skewness values are greater at ground-based measurements relative to nearby off-ground locations. As proposed in Vaughn et al. [J. Acoust. Soc. Am. 149(4), 2403–2414 (2021)], nonlinear reflections are a possible explanation for the increased derivative skewness values at the ground. For an ensemble of windowed shock events, the propagation angle, apparent source location, and angle of incidence relative to the ground are estimated using a two-point cross correlation. The shocks are then classified into shock reflection regimes based on their combination of angle of incidence and measured shock strength. A significant number of shocks measured along the ground array fall in the nonlinear reflection regime. A nonlinear shock reflection results in a pressure increase at the shock greater than twice the free-field pressure, which causes an increase in the derivative skewness value. Therefore, measurements at the ground may overpredict the crackle perception of a standing observer.