eli5 Noise cancelling headphones are magical but how does the technology work and does it drain the battery?
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When two sound waves happen at the same time, their magnitude adds together. I’ll try to draw a picture in ascii art.
Let’s say you have a wave x like so:
xxxx xxxx
xxx xxx xxx xxx
xx xx xx xx
x…………..x…………….x…………..x…………….
x x x
xx xx xx
xxx xxx xxx
xxxx xxxx
and a wave y, which is the same but slightly quieter, like so:
yyyyyy yyyyyy
yyy yyy yyy yyy
yy…………yy…………….yy…………yy……………..
yy yy yy
yyy yyy yyy
yyyyyy yyyyyy
When both occur at the same time, they add together to form a louder wave, z like so:
zzzz zzzz
zz zz zz zz
z z z z
zz zz zz zz
z z z z
z…………….z…………….z…………….z…………….
z z z
zz zz zz
z z z
zz zz zz
zzzz zzzz
But what if the two waves didn’t overlap so nicely, and in fact one was the inverse of the other, like so:
xxxx xxxx
xxx xxx xxx xxx
xx xx xx xx
x…………..x…………….x…………..x…………….
x x x
xx xx xx
xxx xxx xxx
xxxx xxxx
yyyyyy yyyyyy
yyy yyy yyy
yy…………….yy…………yy…………….yy………….
yy yy yy yy
yyy yyy yyy yyy
yyyyyy yyyyyy
Then at each point along the way the two waves are in opposite directions. So instead of adding to a bigger wave they add to a smaller one. They “pull” the air in opposite directions, dampening each other.
The resulting wave from that would be this subdued smaller wave:
zzzzzzzz zzzzzzz
zzzzzz……..zzzzzzzz……..zzzzzzzz………zzzzzzz…………
zzzzzzzz zzzzzzzzz
Noise cancelling technology is a device that “hears” wave x, then tries to emit wave y to counter it, resulting in the subdued wave z.
So there’s a little microphone in the headphones that hears the wave that’s about to hit your ear, and immediately emits a wave in the exact opposite direction through the headphones’ speaker to dampen it.
For the question: Does it drain the battery? Well, yes, because it’s driving the speaker that produces wave y.
The irony is that to dampen louder noises, you need a more powerful speaker.
The better noise dampening headphones also have an “awareness mode” that allows “new” sounds to leak through, not dampening them until they’ve been repeating for a little while. Thus if you’re on an airplane the constant nonstop whine of the airplane’s engines gets muffled, but the “bing bong” when an announcement is coming doesn’t. It’s not that the headphones “know” that the “bing bong” is important. It’s just that they “know” the “bing bong” is different from what they were hearing before and therefore not background noise, so let it through. If the sound kept going “bing bong bing bong bing bong” over and over, then the headphones might start dampening it.
That “awareness mode” is there so that wearing noise cancelling headphones in traffic isn’t quite as dangerous as it otherwise might be. You want to let sudden changes in sound like a driver honking their horn at you to come through at full volume. (I still don’t trust this and don’t like wearing the headphones when walking in traffic, but will wear them when sitting still next to traffic – like on a bench at a bus stop.)
I’ve heard that effect with church bells ringing out 12 o’clock. The first few bongs get through the headphones at full volume, but by the 12th bong, they’re starting to get quiet.
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