ELI5: How do noise cancelling headphones/earphones work?

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There’s electronics inside that listen to the ambient noises around you, then they do what’s called phase cancellation where it makes a sound similar to it and puts it out of phase. The two noises then cancel each other out creating silence. Phase cancellation is wild but really cool


There’s two types of noise cancelling. Active and Passive. Passive noise cancellation just means the headphones are designed to try and *physically* block sound from getting in. Just like covering your ears with your hands when there’s a loud noise. Active noise cancelling /u/TheTalmidim explained better than I can. But, often active noise cancelling and passive noise cancelling tech are used together for maximum isolation.


Sounds moves in waves. A wave has a peak and a trough. Basically just a big up and down squiggle in the air and the “zero” is a line right down the middle. So part of the wave is above the line and part is below it. The part of the wave above the line is trying to push air up and the part below the line is trying to push air down. If the wave runs into another wave one of two things can happen: if the high part of one wave runs into the high part of the other wave their “up energy” combines and the peak gets even higher. So two three inch peaks become one 6 inch peak. Same if two parts of the down wave hit each other, they get even lower. If the waves hit a little different though they’ll fight each other instead. A 6 inch peak runs into a 2 inch trough you get a 4 inch peak instead. The “noise canceling” comes in when you get identical waves. A 4 inch peak hitting a 4 inch trough just cancels each other. They’re trying to move in opposite directions at exactly the same speed so nothing happens. What the headphones do is take in external noise with a microphone, wait a fraction of a second, and then replay it into your ear. The exact same sound is coming from the environment and the headphones but slightly different timing. If it’s all working correctly, the delay will be just right so all the highs of the environmental wave hit all the lows of the headphones wave. Since it’s the same wave then in theory they would cancel each other completely and you’d have dead silence so you would only hear the song or whatever from your headphones and no ambient noise. In practice it isn’t flawless so you still can hear a bit usually but a good quality set will help a lot.


I’ll take a crack at it! Imagine a pond. There are peaks and valleys of the water as it moves up and down. If you made an exact copy of the pond and inverted it (peaks are now valleys and valleys are now peaks)..then put the copied pond exactly on top of the original. It would be a flat calm pond with no valleys or peaks. This is what the headphones are doing… but with sound waves (which are really just 3d versions of a pond)


The concept is really old and the same as any sound mixing. It’s called de-emphasis. The magic is doing it real time in yo head…don’t forget also there is a small delay and buffer so the waves can be flipped