How do noise cancelling headphones actually work?
In: Engineering
They use little microphones to detect the sound outside your headphones, then they play sounds inside the headphones to cancel out the sound from outside. Kind of like the road noise in your car makes it hard to hear the radio!
I’m not sure how familiar you are with how waves work, so let’s start with a fact: you can cancel a wave with another wave that’s equal but “opposite”, that is where the first one has peaks, the other has valleys of the same shape and height. Basically if you flip it around. When you combine them, they average out to zero, nothing. No wave. That’s destructive interference.
That’s what noise cancelling does. Microphones on the outside pick up sound and very rapidly produce a matching but opposite wave inside the headphones to destructively interfere with the original, cancelling it. That leaves only the other sounds purposely made by the headphones.
It’s of course not perfect, so some noise always makes it through, but still very effective.
There’s two types of noise cancelling in headphones;
– Passive Noise Cancelling
– Active Noise Cancelling (ANC)
I’ll go over passive first. You ever put a pillow over your head? That’s passive noise cancelling. It tries to create as best seal it can with your skin, and then as much noise insulation that can realistically fit in the desired form factor. This is why they’re often associated with feeling sweaty, because they’re also stopping the air inside from escaping. That’s the simple one.
Active is a little harder to explain, but I’ll try to do my best.
So it works based on all the sounds we hear (or most) having an opposite noise, that will cancel them out, at least we won’t perceive them. So they’re always listening on the outside, and playing the anti-noise version of that on the inside.
It’s a little hard to visualise, since it’s not visual (duh), so I’ll try to make a visual equivalent. You remember when you learnt how to mix paints in school, and you mixed yellow and blue to make green? red and blue to make purple? yellow and red to make orange? What happens if you add the 3rd color to those paints? Technically you’re supposed to get black, but in the real world you always got brown, that’s kind of what we’re doing with ANC.
They will also often have “Transparency mode”, or something similar. This is just them swapping from playing the anti-sound, to replaying the normal sound that the microphone is picking up on the other side, kind of like how VR headsets like the new Apple one you may have seen, can flip between showing you a view of the world from it’s cameras, or just not showing you anything at all. Some of the more advanced ones, will also apply “noise cancelling” techniques to the microphones input when in Transparency mode, to help pickup voices, or road noises, depending on the situation they’re in, to give you a better idea of what it thinks you’re trying to pay attention to.
They have microphones on the outside which pick up external sounds. They also have microphones on the inside that pick up what is going to be played to your ear.
They amend the internal speaker sound wave with the opposite of the external noise and the opposite sound waves cancel each other out (more or less).
Plenty of videos on YouTube that explain it better than I can.
When two waves collide, their peaks with either add together or cancel out depending on their phase. Noise cancelling takes the audio of the surrounding from the microphones and inverts the waves and plays it back so it cancel out the noise.
All sounds are waves. So what happens if you have a wave that would normally be high meet a wave that is normally low? Well they would combine and cancel eachother out at that point.
What noise canceling headphones do is they use a mic to pick up sounds and they play the same sounds but inverted so that at each peak of the original sound there is a valley on the inverted wave, and at each valley of the original sound there is a peak.
Because headphones are a a pretty closed system you dont have to really worry about what happens outside of the point where each wave cancels out, and the point where things cancel out doesn’t move so you don’t have account for that.
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