Propagation of crackle-containing jet noise from high-performance engines

Crackle, the impulsive quality sometimes present in supersonic jet noise, has traditionally been defined in terms of the pressure waveform skewness. However, recent work has shown that the pressure waveform time derivative is a better quantifier of the acoustic shocks believed to be responsible for crackle perception. This paper discusses two definitions of crackle: waveform asymmetry versus shock content and crackle as a source or propagation-related phenomenon. Data from two static military jet aircraft tests are used to demonstrate that the skewed waveforms radiated from the jet undergo significant nonlinear steepening and shock formation, as evidenced by the skewness of the time derivative of the pressure waveforms. To the extent that crackle is caused by the presence of shock-like features in the waveform, crackle's perceived quality is likely to be heavily influenced by propagation through the geometric near field and into the far field.