Temporal and spectral quantification of the ‘crackle’ component in supersonic jet noise

Measurements of the pressure waveforms along a grid in the far-field of an unheated Mach 3 jet were conducted in order to study crackle. A detection algorithm is introduced which isolates the shock-type structures in the temporal waveform that are responsible for crackle. Ensemble averages of the structures reveal symmetric shocks at shallow angles, while they appear to be asymmetric near the Mach wave angle. Spectral quantification is achieved through time–frequency analyses and shows that crackle causes the expected high-frequency energy gain. The increase in energy due to crackle is proposed as a more reliable metric for the perception of crackle, as opposed to the Skewness of the pressure or pressure derivative.