A study of the phenomenon of “crackle” associated with the noise of high-performance aircraft at afterburner

Abstract

Ffowcs Williams et al were the first to discover crackle in the noise of the Concorde when the Olympus 593 engine which propelled Concorde was operating at afterburner power. Ffowcs Williams et al described crackle as a transient phenomenon. The primary objective of the present investigation is to show that the noise of F-18E aircraft has crackle. The fundamental elements in the noise of a crackling jet described by Ffowcs Williams et al are sawtooth-like pulses, bursts and trains of bursts. All these characteristic features of pressure pulses are identified in the noise of F-18E aircraft. By including a minimum pulse amplitude requirement based on the level found in the noise of the Concorde by Ffowcs Williams et al, a set of sufficiency conditions for the presence of crackle is established. It is found that crackle exists in the noise of an F-18E aircraft over a fairly large angular sector in the downstream direction. In the literature, the existence of sawtooth-like sound pulses in a noise field is often taken as an indicator of the presence of crackle. The importance of pressure pulse waveform on its impact on human hearing is briefly investigated by using a simple mechanical model of the ear.