Source Localization of Crackle-Related Events in Military Aircraft Jet Noise

Crackle is a perceptual feature of supersonic jet noise that is related to the presence of acoustic shocks. This study investigates the apparent source locations of the steepest shocklike events in the noise field of a high-performance military jet aircraft using an event-based time-domain beamforming method. This method uses the cross correlation between adjacent microphones to determine the angle of propagation of an ensemble of shock-related events within the time waveform. This angle of propagation is then traced back toward the source to find the apparent source location. Based on the propagation angle, derivative skewness, and overall sound pressure level, the microphone positions along the array are sorted into six groups. Shock events from groups related to crackle perception in the near field originate anywhere from 2 to 14.5 m downstream along the nozzle lip line, with distributions that shift downstream and broaden with increasing engine power. The crackle-related events appear to be generated by Mach wave radiation and large-scale turbulence structure noise.