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Sound waves in opposite phases will cancel each other out.

Is it theoretically possible to create a device which will record the sound and output sound in opposite phase which will result in silencing someone.

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  • $\begingroup$ I've experimented with a subwoofer fed with a microphone in an attempt to control standing waves in a home theater I built. The problem is essentially that the device can (partially) cancel sound at the microphone location. To cancel sound within a region would require multiple microphones and transducers as well as significant signal processing. If one isn't careful, one will not create silence but deafening regenerative feedback instead. $\endgroup$ – Alfred Centauri Jan 24 '15 at 0:41
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Yes, it's called active noise control and it's exactly how those fancy noise cancelling headphones work.

So not only is it theoretically possible, but it's already a well-saturated market.

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    $\begingroup$ Yes I agree the technology of active noise cancellation has been around for decades now, and it can provide remarkable results. But as with any technology there are limitations. The cancellation is basically achieved by destructive interference and the nodes in space are localized and depend on the sound wavelength. So for example one may hear silence at some point and move 1 ft. to the left or right and silence is broken. The most successful noise cancellation applications are where the space can be confined - such as air conditioning ducts. $\endgroup$ – docscience Jan 23 '15 at 23:44

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