# Is it possible to use “negative sound waves” to “cancel out” a sound to create silence?

I saw youtube videos that claimed to do this, although I'm quite certain the videos just excluded sound and lied.

However, I am wondering if the physics of this is actually possible - to create a completely negative sound wave that cancels the sound out to a flat line i.e. complete silence.

p.s. I am not referring to white noise, which prevents the sound from being heard by overriding it with a different sound (white noise). Rather, I am referring to making a sound be completely silent by generating the inverse, opposite, or "negative" sound wave.

• You can cancel sound in one location in space but not everywhere without the second signal that causes the destructive interference also coming from the same point in space as the source. – Brandon Enright Oct 25 '14 at 6:05
• Look up "noise cancelling headphones". – DanielSank Oct 25 '14 at 6:06
• by using destructive interference of waves it should be possible (at some locations/conditions) – Nikos M. Oct 25 '14 at 10:58

Yes it can be done, and indeed it's a well established technology called active noise control.

The idea is based on destructive interference. If at some point two sound waves have the same amplitude and frequency and they're 180º out of phase then they will sum to zero and the sound intensity at that point will be zero. Your phrase negative sound just means sound that is 180º out of phase with the sound you're trying to cancel.

However it's rarely possible to cancel the sound over more than a very small region. The cancellation requires the amplitude of the cancelling sound to be precisely matched to amplitude of the noise. The trouble is that the amplitude of sound typically decreases as the inverse square of distance from its source. As a result it's hard to get the sound amplitudes to match over more than a restricted area. However noise cancellation is used in special cases like noise cancelling headphones.

• I think you should rewrite this part: "because the intensity of sound typically decreases as the inverse square of distance from its source" and partly what follows. – Walter Tross Oct 25 '14 at 9:47
• so, when it comes to light, intensity doesn't matter? Is it possible to have a light cancelling torch for any given torch? – Saravanabalagi Ramachandran Dec 27 '16 at 13:22
• @ZekeDran with sound it's easy because we can make active response fast enough (kilohertz is nothing to modern CPUs/DSPs). But with light we have petahertz oscillations, and they vary over less than a micrometer, so we are basically out of luck if we try to create such canceling devices — they'll need to be super-fast-speed and there'll need to be quite a lot of them to cancel a single torch. – Ruslan Dec 27 '16 at 14:28
• even in sound, we need to have the cancelling speaker right exactly at the source, so the intensity distributes so evenly, right? – Saravanabalagi Ramachandran Dec 27 '16 at 14:47
• Back in the 1970's the Grateful Dead had a sound system in which they used phase-inverted secondary microphones to minimize stage noise being picked up by the primary vocal microphones. They also used them to minimize feedback resonances (loudspeaker squeals). If you listen to live recordings from that era you can hear the phasing on the vocals because the secondary mics were only about 15 cm from the primaries. – Bill N Jan 2 '17 at 16:37

## protected by Emilio PisantyJul 3 '17 at 9:45

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