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How would you go about canceling out the effect of a radio wave by emitting another wave of same frequency, wavelength and amplitude

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  • $\begingroup$ It's really not possible to do this. $\endgroup$ – garyp Apr 16 '18 at 17:48
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    $\begingroup$ It happens in a limited form in real life. For example building locations with low or no radio reception due to interference of between primary wave and wall-echoed waves. $\endgroup$ – npojo Apr 16 '18 at 19:04
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It is possible to cancel sound waves by detecting incoming sound waves and then generating another sound wave of the same frequency and direction but opposite phase. Because electrical signals can travel much faster than sound, the sound wave that cancels the incoming sound wave can be generated a bit downstream from a microphone, just in time for the incoming sound wave to be canceled. In other words, an electrical signal can be sent ahead of the sound wave to "instruct" a sound generator to produce the cancelling sound wave when the incoming wave arrives, and that makes cancellation possible.

However, it is not possible to send a signal "ahead" of an incoming radio wave, because nothing can travel faster than a radio wave (or any other electromagnetic wave), so active cancellation the way you envision can't be done.

There is a kind of complicated situation in which it is "sort of" possible: if the incoming radio wave is moving very slowly because of the medium it is passing through. Paraffin, for example, slows the speed of radio frequencies to about 2/3 the speed of light. If the incoming radio waves had to pass through a block of paraffin hundreds of meters thick, but were detected as they entered and an optical signal were sent ahead via a "tunnel" through the paraffin, the optical signal could in principle get to the far side of the paraffin before the radio waves arrive, in time to "instruct" a radio source to generate the right radio wave to cancel the incoming wave just as it arrives. I know that's not what you had in mind, but it's about as close to what you want as is physically possible.

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  • $\begingroup$ Regarding your first paragraph: I thought once how nice it’d be to manufacture an apparatus doing that and thus create a sort of invisible window, letting breeze come in but not the slightest sound... The obstacle that I could think of was though: can you ensure that the sound travels only in the appropriate direction? $\endgroup$ – Sierra Apr 16 '18 at 19:44
  • $\begingroup$ Yes, you actually can ensure that sound travels in an appropriate direction, using a "phased array". It's not trivial, though, because in the invisible window, sound would be approaching from all directions. It would require an array of tiny microphones and tiny speakers, spaced closer than the wavelength of the sounds to be cancelled- and some fast signal processing between the microphones and the speakers. $\endgroup$ – S. McGrew Apr 16 '18 at 20:19
  • $\begingroup$ But why microphones and speakers? I googled a little and it seems that laser can be used as a sound sensor and it also generates at least underwater sound... Well, let somebody invent it some day... $\endgroup$ – Sierra Apr 16 '18 at 20:42
  • $\begingroup$ By "microphone" I meant anything that can sense phase & intensity of a sound wave; by "speaker" I meant anything that can launch a sound wave of controlled phase & intensity. Just didn't want to pack the answer full of unnecessary words. Right, lasers can be used to measure sound waves (via the change of refractive index due to compression & rarification); and lasers can be used to launch sound waves (via photo- thermoacoustics). But it's not very practical. $\endgroup$ – S. McGrew Apr 16 '18 at 20:58
  • $\begingroup$ I gather that it would be a very expensive window, but nice to have... thanks for the teaching! $\endgroup$ – Sierra Apr 16 '18 at 22:10

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