Beamforming of supersonic jet noise for crackle-related events
Crackle is an annoying perceptual component of supersonic jet noise. In the far field, crackle is related to the presence of acoustic shocks that develop due to nonlinear propagation, however, the intermittent source events that drive crackle generation are not well understood. This study investigates the apparent source locations of events related to crackle, which include high-amplitude or steepened, shock-like waveforms. The measured data were obtained through ground-array measurements near a high-performance military aircraft. The apparent source regions corresponding radiation angle, the skewness of the time-derivative of the pressure waveform (dSk), and overall sound pressure level are defined. Waveforms consisting of a dSk greater than 3 are considered to contain crackle. For 75% engine thrust request, the apparent source region for the top 1000 derivative events beamformed from locations with high derivative skewness, which corresponds to the potential for crackle, is 2-7 m downstream of the nozzl...