Eli5 How exactly does Noise cancellation work? That too in such small airbuds
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If you imagine sound as an airwave when it picks up sound that it isn’t making it immediately plays an opposite airwave (sound) that cancels out the two sounds in your ear
The speaker sends out a tone at a very high frequency. That frequency is particular that it actually interrupts noise outside.
Think of if you run your ear aggressively as if you’re itchy, you might get a high eering tone afterwards. You may also have noticed you don’t hear anything for a few seconds. That’s the whole concept of noise cancellation.
Hope this helps.
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Imagine a wave of water coming towards you in a wave pool.
Ignore anything to do with things like currents under the water, etc.
If you slap the water as the other wave is coming your way, and make a wave the exact same size, shape, etc, but going the opposite way to the wave that’s about to hit you, one stops the other.
Sound in air is much the same, it’s just waves that you can’t see. The headphones or earbuds with active noise cancellation, simply (or really, not so simply) detect other sounds, and play a sound that is the exact opposite, or close enough.
The waves hit each other, and the sound of both goes away!
The sound you hear is a wave that is a sum of all sounds around you. Waves have a few relevant properties: They travel at a known velocity, and they are additive.
To cancel such wave in your ear: we measure the wave just outside the ear and play its inverse with a small delay from the earphone. Notably, this only cancels the sound in a very small region around the inner side of the earphone. Everywhere else it adds its miniscule amount of more sound to the wave.
For best results: You need a good microphone in both earphones, and a good algorithm to slightly alter the wave, to mimic hiw it will be altered by the earlobe (as the in-ear earphone sound is not altered by the earlobe identically to the sound coming from the outside). Fortunately, we can tune this individually: place a second microphone inside each ear canal (near the very tip of the earphone), and measure which delay and which amplitude modifications reduce the sound the most.
A good analogue: Look at the waves in the see. Measure the height of the wave. If it is above the mean water level, push the water down with a paddle you have placed under the surface. If it is below, push the water up. If you move your paddle at just the right speed for a given measurement, you can destroy the wave around your paddle (whilst creating a new wave around your paddle, propagating outwards and adding a bit to the waves everywhere else in the sea).
You know how your headphones can reproduce any sound? Noise cancelling headphones have microphones to detect what outside sounds you’re about to hear and then make the exact opposite sound at exactly the right time. Any sound (no matter how complex or loud) + its exact opposite = no sound at all, much like 5 + (-5) = 0.
I could get into superposition and all that, but that’s probably beyond age 5.
If it’s cancelling the noise, it is it still considered a sound that is received by your ears?
I thought it was a frequency that our ears can’t pick up.
There are a lot of adults at the pool. Some of them are misbeahving by running clock wise around the pool. The lifeguard got annoyed and send out other adults that run counter clockwise. These adults smash into each others and stop being obnoxious.
Edit: so noise cancelling headphone detects those obnoxious adults, their locations and their behaviors. Then the headphone sends to these location exactly opposite adults and let them smash into each other and cancel each other out.
Edit 2: we can get a bit more technical (eli6 maybe?). The headphone detects the obnoxious adults by running the Fast Fourier Transform algorithm. The algo tells the headphone two things about the obnoxious adults: 1. the adult size (amplitude) and 2. its speed + direction around the pool (phase). The headphone produces the opposite adults with these characteristics: 1. same size (same amplitude) 2. same speed but move in a different direction(antiphase or phase-shifted by 180 degree ie clockwise vs counter clockwise). When the obnoxious adults and the opposite adults smash into each other (destructive interference), they both get knocked the F*** out.
Sounds can be drawn as a wave, so for one of the simplest examples let’s look at a sine wave – just a simple, regular wave shape.
As a simple maths experiment, if we mirror this wave around the horizontal axis so that where one wave goes up, the other wave goes down, and then add them together, they will cancel each other out.
This works with sound in headphones too. If we stick a little microphone on the outside of your headphones so that it detects the outside sound you will hear, then takes that sound and inverts it so it is upside down and plays that through your headphones, the two will cancel out and the outside sound will vanish. If you play the inverted sound alongside the music or other audio you actually want to hear, you get both at the same time – no outside sound, just the extra audio you are adding.
Incidentally, this works really well with headphones, because the microphone can easily detect exactly what your ear will. If you tried this with a whole room, the way sound reflects off of surfaces will mean that the ‘noise’ sound will be different depending on exactly where you stand, so it is impossible to cancel it out – you could set it up to cancel nicely in one place, but if you moved about in the room in other places it won’t work at all, or may make the noise worse.
Does it affect the quality of the sound your hearing?
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