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I have heard (and seen in a movie) about devices that pick up a sound, invert it, and play it back out to create destructive interference resulting in silence.  Apparently this is how noise canceling headphone that Bose makes work.  I just want to try to make a small device that you can point at something (has to be close by to keep in sync) and make it silent.  Nothing complicated.
How do I use a transistor to invert the signal then how do I use a small 8ohm  .5w speaker to cancel it.
With a small speaker that has next to 0 watts I should be able to make fairly quiet noises become silent (like whispering).....right?
It's just an idea out there...

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Noise-cancelling headphones "reduce" noises, they don't completely cancel them. It works only at lower audio frequencies where the wavelengths are long enough so that the cancelling wave has the opposite phase of the original wave.

If you use a speaker instead of headphones then it will be nearly impossible to stop acoustical feedback howling.

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