What makes you so sure?
Come on, tell us why it's impossible.
I can see a quiet and invisible drone in the future.
Making it inaudible is almost as difficult as making it invisible!!!
Its hard to explain why not if you don't have a thorough knowledge of the principles of sound.
Sound cancelation is less difficult when you have a simple sound source. that means only a few frequencies with and a more or less predictable time pattern.
Alas; the noise of the 4 props is a very, very complicated sound source and it is changing irregular al the time!
You need lots of microphones to pickup this sound and a very big number of little loudspeakers, amplifiers, computing power to produce the antisound (180degrees phase shift for every momentary frequency there is. And there are thousands and thousands of them.) You almost have to built the P3 into a tight case of microphones an loudspeakers.
And this is only a tiny little bit of a very long and complex story. The theoretical principle is very simple, the practical application in a real world is very complicated.
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