‘Crackle’: an annoying component of jet noise

Abstract

The paper describes an investigation of a subjectively distinguishable element of high speed jet noise known as ‘crackle’. ‘Crackle’ cannot be characterized by the normal spectral description of noise. It is shown to be due to intense spasmodic short-duration compressive elements of the wave form. These elements have low energy spread over a wide frequency range. The crackling of a large jet engine is caused by groups of sharp compressions in association with gradual expansions. The groups occur at random and persist for some 10−1s, each group containing about 10 compressions, typically of strength 5 × 10−3 atmos at a distance of 50 m. The skewness of the amplitude probability distribution of the recorded sound quantifies crackle, though the recording process probably changes the skewness level. Skewness values in excess of unity have been measured; noises with skewness less than 0·3 seem to be crackle free. Crackle is uninfluenced by the jet scale, but varies strongly with jet velocity and angular position. The jet temperature does not affect crackle, neither does combustion. Supersonic jets crackle strongly whether or not they are ideally expanded through convergent-divergent nozzles. Crackle is formed (we think) because of local shock formation due to nonlinear wave steepening at the source and not from long-term nonlinear propagation. Such long-term effects are important in flight, where they are additive. Some jet noise suppressors inhibit crackle.

Near-field shock formation in noise propagation from a high-power jet aircraft

“…Ffowcs Williams et al 16 indicated that a skewness greater than 0.4 distinctly crackled and subsequent experimental 17 and numerical 18,19 studies have focused on quantifying pressure skewness. The conclusion has been that positive skewness (i.e., an asymmetric waveform with relatively infrequent large positive values, and many smaller negative values) is generated at the source, and therefore crackle is also a source phenomenon.…”

“…That the pressure skewness (a) appears to originate near the nozzle, (b) has relatively low values downstream along the shear layer, and (c) has significant values toward the sideline all indicate that the waveform asymmetry is a high-frequency source phenomenon. Note that the It is important to note that the skewness values for the F-35A at military power, with the maximum region ranging between 0.3 and 0.4, would be considered "borderline" by the Ffowcs Williams et al 16 crackle criterion. They defined skewness values less than 0.3 to be non-crackling and greater than 0.4 to be distinctly crackling.…”

Screech tones from rectangular jets with spanwise oblique shock-cell structures

“…Interest in nonuniform jet exit geometries dates back to the early developmental stages of the Concorde (Westley & Lilley (1952), Ffowcs Williams, Simson & Virchis (1975), Smith (1989), Lilley (1991)). During noise reduction attempts with the Concorde, the Olympus engines were fitted with innovative variable geometry intake and exhaust nozzle assemblies.…”

Acoustic waveforms produced by a laboratory scale supersonic jet

“…When the convective speed of large scale turbulent structures is greater than the sound speed of the surrounding gas, Mach waves are emitted which propagate at angles relative to the jet axis determined by [4,9,10,22,27,36]. Because these Mach waves are generated by turbulence, they are classified as a component of turbulent mixing noise, which is the most significant source of noise for jets operating at supersonic acoustic Mach numbers.…”