The high noise levels of early turbojet aircraft forced attention to be given to the mechanics of the noise generation process and ways were urgently sought for silencing their high speed propulsive flows. The subject of aeroacoustics was born with this problem, the subject being the variety of ways in which sound and flow interact, with the early emphasis being on sound generated aerodynamically. Sound was thought to be a by-product of the flow arid believed to have a negligible back-reaction on it.
The search for noise suppression has been extremely successful, so much so, that engine noise is no longer a problem likely to constrain future aircraft operations. Silencing technology has made enormous strides by avoiding as much as possible the very high speed jet flows that were a feature of early engines. But that avenue is not available to supersonic aircraft, which still wait for ways of making their operation acceptably quiet to be identified.
Aeroacoustics has brought into being a much deeper insight into the interactions of sound and flow, and some of the early presumptions about the mechanisms of noise creation have had to be rethought. Some of the advances have come from entirely new directions, for example, from anti-sound, a silencing idea based on the destructive cancellation of interfering waves, which has advanced to a state that provides a useful new technology. Some new understanding follows the discovery that flow-acoustic interactions are not always the one-way process previously assumed. Sound can affect flow, especially so when the flow is unstable. It is of course the very unstable flows that make most noise so, taken together, these developments indicate that control might one day be exercised by actively managing the unsteady flows, making them more useful and desirable than before. Indeed examples are now known where control has made possible the avoidance of a powerful performance-limiting instability that previously made that flow regime a prohibited operational zone, a development of aeroacoustics that is probably far more significant than its noise suppression role. That line of thought is expanded in this paper, which concludes with the speculation that active control might now be offering an avenue for approaching die high speed jet noise problem from a completely new direction. Of course it is far too early to know whether that approach can provide a way for removing the take-off noise constraint currently threatening the viability of supersonic transports, but it might; that makes it very important indeed, for it is hard to see any alternative.