eli5 Noise cancelling headphones are magical but how does the technology work and does it drain the battery?
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Sound is caused by very fast changes in air pressure. When you clap your hands, you compress the air and briefly create an area of high pressure. Right after you clap, that high pressure air wants to get back to normal pressure, so it expands out, causing a ripple effect in the surrounding air, and those air pressure ripples eventually expand out and travel to your ear. Your ear is designed to detect air pressure differences and interpret them as sounds.
Air pressure variations can be positive or negative, meaning they can either be higher-than-normal pressure or lower-than-normal pressure. Most sounds are actually a very quick oscillation back and forth between high and low pressure. For instance, if you sing a note, your vocal cords are vibrating hundreds of times per second, and that creates a small air pressure variation that varies between high pressure and low pressure several hundred times per second.
If you have two different sounds happening at the same time, their air pressure differences add together, since a region of air can’t be at two different pressures at the same time. In some cases, if the air pressure caused by one sound is positive at a certain moment, and the air pressure caused by a different sound is negative at the same moment (and in the same physical location), those two air pressures cancel out and you’re left with no change in air pressure. This is referred to as destructive interference. You can see the same thing in pond ripples. If you throw two rocks into a pond in different locations and watch the ripples spread out, when the ripples intersect there will be some small areas in the intersection where the ripples become twice as high or twice as low (due to constructive interference), and other areas where the water surface doesn’t move at all (due to destructive interference) despite all these waves passing through each other.
Noise cancelling headphones work by using this principle. They have a microphone that can record the ambient noise around you. They quickly process this noise and play the inverse of that noise through your headphone speakers, in such a way that the noise is cancelled out and you no longer hear it. Additionally, the fact that (many of) these headphones cover your ear entirely and seal it off helps to reduce the amount of noise that makes it to your ear. Furthermore, if you’re actively listening to something in the headphones like music or movies, that drowns out outside noises even more, because the music/movie sound is probably being played more loudly than the ambient noise level around you. All these things combine to reduce noise to a level where it is no longer noticeable.
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