AEROACOUSTICS

The high noise levels of early turbojet aircraft forced attention to b e given to the mechanics of the noise generation process and ways were urgently sought for silencing their high speed propulsive Rows. The s ubject of aeroacoustics was born with this problem, the subject being the variety of ways in which sound and Bow interact, with the early em phasis being on sound generated aerodynamically. Sound was thought to be a by-product of the flow and believed to have a negligible back-rea ction on it. The search for noise suppression has been extremely succe ssful, so much so, that engine noise is no longer a problem likely to constrain future aircraft operations. Silencing technology has made en ormous 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 av ailable to supersonic aircraft, which still wait for ways of making th eir operation acceptably quiet to be identified. Aeroacoustics has bro ught into being a much deeper insight into the interactions of sound a nd flow, and some of the early presumptions about the mechanisms of no ise creation have had to be rethought. Some of the advances have come from entirely new directions, for example. from anti-sound, a silencin g idea based on the destructive cancellation of interfering waves, whi ch has advanced to a state that provides a useful new technology. Some new understanding follows the discovery that flow-acoustic interactio ns are not always the one-way process previously assumed. Sound can af fect Bow, especially so when the Bow is unstable. It is of course the very unstable flows that make most noise so, taken together, these dev elopments indicate that control might one day be exercised by actively managing the unsteady flows, making them more useful and desirable th an before. indeed examples are now known where control has made possib le the avoidance of a powerful performance-limiting instability that p reviously made that flow regime a prohibited operational zone, a devel opment of aeroacoustics that is probably far more significant than its noise suppression role. That line of thought is expanded in this pape r, which concludes with the speculation that active control might now be offering an avenue for approaching the high speed jet noise problem from a completely new direction. Of course it is far too early to kno w whether that approach can provide a way for removing the take-off no ise constraint currently threatening the viability of supersonic trans ports, but it might; that makes it very important indeed, for it is ha rd to see any alternative.