Eli5 How exactly does Noise cancellation work? That too in such small airbuds
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sounds waves go positive and negative. think of a graph of a sine wave (google it to see a visual). half the time, it’s above the line (positive), and half the time it’s below the line (negative). that represents the waves moving through the air. that’s all sound is. like when you push and pull really quickly on a slinky, you can see the waves ripple down the slinky. sound is like that.
sound waves have what’s called a phase. remember the positive and negative thing above? if you reversed the positives and negatives, did you know you still get the same sound? the sound will just be out of phase, but to the person listening, there’s no difference. a really really good way to actually hear noise cancelling for yourself is to find a living room setup that has big floor standing speakers. listen to a song with a lot of bass. then on ONE speaker, reverse the wires. meaning connect the + to -, and the – to +. listen to the same song. you’ll notice all the bass is nearly gone. that’s because waves that have the opposite phase cancel each other out.
think of the times the waves is above the line in the graph as +1. at the point it’s crossing the line, it’s 0. when it’s below the line, it’s -1. so one single period, meaning one full section of the wave before it starts repeating over and over, would look like this:
..–.._
or by the numbers we mentioned, like this:
0, +1, 0, -1, 0
now the wave with the opposite phase:
.._..–..
and the numbers for it:
0, -1, 0, +1, 0
now what happens if we add the two waves together?
(0+0), (+1-1), (0+0), (-1+1), (0+0)
0, 0, 0, 0, 0
if we graph that, we just get a flat line. the two waves completely cancel each other out.
in electrical terms, the positive and negative is just voltage, and it’s very easy to reverse that with electronics. noise cancelling headphones just have microphones at the edges of your headphones, reverse the wave, and add it to the sound you hear. so if you’re listening to music, it’ll add the reversed phase wave to your music. you don’t hear anything because its values exactly cancel out the original sound. also, this only works because electricity travels nearly at the speed of light, and that’s way way way way faster than the speed of sound, so even through the microphone and your ear are only an inch or two apart, it has plenty of time to record, reverse, and add the reversed signal to your audio output. it’s really neat stuff
But technically you are hearing 2 opposite sounds? So your hearing membrana in the ear hears both and vibrates and then you interpret it as scilence? Or the waves compensate each other and your ear membrana stays still?
There are two forms of noise cancellation. Active and passive. Passive essentially means just blocking sounds from getting by such as how over the ear hearing protection works. Active uses electronics and math. The active method is fairly simple to explain though may be more ELI 17 due to geometry and sinusoidal waves. A microphone picks up the ambient noise, and reproduces it in the speaker 180° out of phase which in turn nulls the sum of the noise’s amplitude. The out of phase thing, just means there is an ever so slight delay (which is dependent on the frequency).
Immagine a still lake. You throw a pebble in the water. Ripple spread out. Then quickly throw another pebble near by. It too makes ripple. Where the first p bibles ripples encountered the second ripples they kinda cancel each other out.
With the speed of sound traveling so fast, how can it reach the headphone, and the headphone have time to analyze and produce a “matching opposite” so quickly?
Sound is swinging air. Its like a wave. You have tops and bottoms in the wave.
Noise cancelation records the sound and gives back the opposite spike. Top spikes get cancelled with bottom spikes of same strength and other way round.
This is why you can feel some sort of pressure in your ears, when you have noise cancellation in your ears without actual music, when the outside is loud.
It’s wizardry dark magic that technology can “predict” outside noises and create the opposite sound in real time. There’s no other way to explain this!
The idea is somewhat simple, you simply measure (and in more expensive/sophisticated headphones predict) the noise coming into the ear and use that info to send noise into the ear which exactly counteracts the incoming noise you want to block.
There are several insanely difficult parts to this though:
1. How do we calculate what we need to counteract incoming noise fast enough that we actually counteract it in time? In airpods there may be a couple centimeters between the outer speakers measuring incoming noise and the inner speakers. This gives distance/speed of sound ≈ 3*10^(-2)/343 ≈ 1/10000 seconds, so 100 microseconds. That’s not a lot of time to calculate things.
2. The noise being measured at the outer speakers is distorted due to the shape of your ear. This needs to be taken into account or otherwise the inner speakers would not be cancelling out the correct sound.
3. Predicting what noise will be coming and using the prediction instead of actually measuring the noise. This works pretty well for a lot of signal processing purposes (separating noise from music, recreating ECG’s when only part of the data is available, or anything else where the input can be considered a signal) but real life noise is not always predictable.
A speaker works by shoving air forward…active noise cancellation works by sucking in the diaphragm at the same moment the ‘force’ of outward air to ‘suck in’ the energy. Microphones and knowing the speed that sound travels is what allows the noise cancellation circuitry to know exactly when to provide that ‘hollow’ space to cancel the noise.
No noise cancelling headphone cancels deeper frequencies as well as higher ones…the bigger the diaphragm the lower the frequency you can ‘catch’. Most noise cancelling starts with passive noise cancelling – which is a fancy word for ‘plugging your ears’ – ala foam/vinyl/leather to block the sound from getting in. In smaller air buds you’re automatically getting passive noise cancelling just by shoving them in your ears. Also your ears can only ‘hear’ the most powerful push…your eardrum is a diaphragm too. But it can only ‘hear’ the loudest thing at any given time. Many active headphones just produce a white noise ‘hiss’ which is amazingly effective at just drowning out the randomness of noise…and your brain just loves it as it only takes a few minutes for your brain to stop hearing that hiss/hizz noise.
Edit: Consider stereo systems…the bigger the speaker the deeper the bass. No one has ‘tweeters’ thumping the bass. And in ear phones have tricks to increase the bass – but the bigger the driver the better the bass sound you can get.
In two words: destructive interference.
If you and a friend hold a rope taut, and your friend raises and lowers their hand once very quickly, a wave will travel on ‘top’ of the rope to you. If you move your hand down and up quickly, a wave will travel along the ‘bottom’ of the rope toward your friend. If you and your friend did this at the same time, when the two waves meet in the middle, the rope will be completely flat for an instant.
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