I thought iancmceachern asked a really good question. I was curious if it could work even theoretically.

I read the Wikipedia article on Active Noise Control. The third paragraph under Explanation: https://en.wikipedia.org/wiki/Active_noise_control#Explanati... mentions that if it's at the source, it needs to be just as loud as the source (as opposed to being by the ear, in which case even a whisper might be enough).

In addition to being as loud as the source, you need to blast it in all directions. So I guess it's hard/impossible to perfectly hit the same sound waves in all directions. If you ever miss, you'll just be doubling the noise.

Maybe that's why you'll be hard-pressed to find anything in a fixed installation that uses active noise cancellation at the source - even if it's heavy, periodic, and uses a lot of power anyway - such as a home air conditioner. Noise cancellation close to the ear seems much easier for this reason, and maybe at the source it's just impossible.

Not impossible, just impractical unless silence is incredibly important. If you want a quiet helicopter then you do what you can to reduce the noise signiture, then you might consider destructive interference. You wouldn’t use speakers, but that’s not the only way to produce a noise. In fact a clever country might even figure out how to use an arrangement of blades and fans designed to produce noise in such a way that it tends to cancel out the loudest contributions to the signature.

Buuuut... that’s not really viable for anything civilian or even commercial.

Like the Stealth Black Hawk helicopters that were used in the Osama Bin Laden raid!

(There existence is still not officially acknowledged of course, but many pictures exist.)

I was very curious about the technology they used to reduce the noise, as black hawks are very loud.

I believe I read that fan noise is chaotic, and hence impossible to cancel. Don't quote me on that, though.