Eli5: How does active noise cancelling work on headphones?
In: 6
Waves have an intuitive property where if two overlap (called inteference), the wave can be modelled as simply the addition of the two. You know [how a soundwave looks?](https://www-users.cse.umn.edu/~rogness/math1155/soundwaves/a.png).
When two waves interfere, you usually get some **Constructive Interference** (parts adding to get bigger) and **Destructive Interference** (two parts adding to get closer to the 0 line in the middle). The former makes things louder, the latter makes them quieter.
Now, given that, what if we had two waves that were exactly opposite one another? All the points above the middle on one are the same distance below on the other? Then they’d all add to zero!
That’s the idea behind Active Noise Cancelling. Make a wave that cancels the noise from outside the headphone so that they interference when you’re going to hear them. Usually this is done with some small microphones on the outside of the headphone, picking up the sound and then a microcontroller inverting it.
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