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Message: <blockquote data-quote="quellish" data-source="post: 60763" data-attributes="member: 750"><blockquote data-quote="Jemiba">"You can use a sound generator to emit sound waves on a reciprocal frequency "This method works relatively good in a small environment, like a passengers cabin, but to reduce the noise outside, it won't work. noise is emitted by every single rotor blade, which is constantly changing its position in space and relative to the fuselage of the helicopter. You would have to emit the sound caused by the blades with a 180° phaseshift in all directions with the original, very high amplitude, not to mention phase shiftscaused by interactions between rotor and fuselage. Calculating this "contra sound" probably couldn't be done in real time, not ot mentione emitting it .</blockquote>Oh no, this last bit is not correct at all. Many modern consumer electronics have active noise cancellation, which is exactly what you are talking about. The signal processing requirements were a problem in the 1970s and 1980s, but are not today. In the 1980s and 1990s there was a lot of work done behind closed doors with exotic signal processing techniques to solve some of these problems. Merging sound and optical signal processing, all kinds of very exotic stuff. Now you can buy off the shelf chips for this sort of thing.Still, yes, trying to use this approach for the sounds generated by, say, a rotorcraft is not practical. For the *sources* of the noise, such as internal vibration, harmonics, etc. yes, this is actually a viable approach. Many of these sound sources are small in strength or amplitude but produce large effects, especially at a distance. Countering the sources inside the airframe using active techniques does work.The same principals also work for other kinds of active signature reduction.</blockquote>

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