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Noise-cancellng headphones are a great thing, probably the epitome of electronic convenience devices. With a push of a button, all those pesky sounds, all those people muttering and harrumphing around you, the cars driving by on the street, even the birds chirping, all of them vanish at your command. This allows you to fully enjoy the music that is being transferred to your ears. And with a second push of that same button, reality seemingly creeps back into your skull.
Heck, some really expensive models even feature a multi-stage system, where you can select just how much of your environment you want to hear, and which parts to block out. But just how do noise-cancelling headphones work? Good question. Let’s find out, shall we?
How Do Noise Cancelling Headphones Work
A Word or Two about Amplitude
Before we get into the really techy stuff, we need to understand some of the fundamental physics behind sound.
As you probably know already, sound moves as a wave. In more real life-oriented terms, sound is simply vibrations moving in the air in a wave-like shape, or pattern, if you will. Now, when those vibrations do what they do, vibrate, they change the air pressure around them.
That change in air pressure is called amplitude. The higher the amplitude, the louder the sound is that our ears perceive.
On one of those fancy-looking waveform-style graphs, the amplitude is visualized as the ‘height’ of the wave.
Phases : In-phase and Out-of-phase
Now, let’s dig a little deeper and find out just what makes these noise-cancelling headphones tick.
Let’s start with a question: What would happen if you took two identical waveforms’”two vibrations in the air that had the same wavelength and amplitude, or, put even simpler, that vibrated ‘in the same pattern””and stacked them on top of each other?
The answer is, you get a louder sound. The two waves simply amplify one another. This is called being in-phase. However, the opposite can also happen.
If you line up two identical waveforms so that the peak point of one of the waves lies on the same spot horizontally as the trough point of the other, they’ll cancel each other out.
It does sound counter-intuitive to imagine something like ‘negative volume’, or ‘anti-sound’, as it is sometimes called, but this is how it works. This is called out-of-phase, and it’s the fundamental principle behind active noise-cancelling.
How Microphones Help Reducing Sound
You may not have noticed it, but, if your headphones have active noise-cancelling, they also have at least one, if not a whole lot of microphones. No, not the kind you speak into.
Noise-cancelling headphones possess mics, usually on the earcups, that detect sound coming from around you and record it constantly.
Some processing circuitry inside the headphones then takes the waveform from that sound and creates another one, akin to a mirror image, i.e the new waveform has a peak where the original would have a trough and vice versa, putting the sound out-of-phase.
This is then fed to the speakers, which of course play both waveforms simultaneously, cancelling them out. Of course, this needs some energy in the form of electricity to work, which is why you see this system being advertised as ‘active’.
Active Noise Cancellation
What’s so active about it? The fact that it needs an active, that is, constant, supply of energy to keep working, whereas truly passive noise cancelling diminishes outside noise via a completely different method.
That is why active noise-cancelling headphones have a rechargeable battery or two, and it’s also why they tend to weigh so much.
Sadly, all this doesn’t work perfectly all the time, especially when the sound in question is very sudden or random, and differences in the exact technology differ between manufacturers, but the rough idea remains the same across the board.
Passive Noise Cancellation
Active noise cancelling is not the only way to block those pesky external sounds, though. There’s also passive noise cancelling, even if you’re not actually supposed to call it that.
You see, the word ‘cancelling’ in ‘noise cancelling’ distinctly refers to the soundwaves being cancelled out, or put out-of-phase.
In passive noise cancelling, that doesn’t happen, so the proper term would be passive noise isolation or passive noise reduction.
This is why, generally speaking, when someone mentions noise cancelling alone, they probably mean the active method.
So, How Does Passive Noise Isolation Work?
Very simply, but effectively: You simply block your ears. Yes, that’s it. The idea relies on the headphones having a thick-enough padding and snug-enough fit on or around your ear so that any and all outside noises will simply not reach it, while the sounds coming from the headphones’ speakers will.
On in-ear headphones, this means being inserted at a sufficient depth and constructing the eartips out of a thick, noise-isolating material like silicone.
On larger headphones that sit on or around your ears, sound engineers have to play with cushioning, padding, earcup shapes and many more nifty little details to ensure that as few outside noises as possible ‘leak’ into your ear.
By definition, any and all passively noise isolating headphones are closed-back, meaning they are constructed to be sealed; no sound comes in, and none comes out in the best case scenario.
Just like active noise cancelling, though, this doesn’t work flawlessly. It’s simply impossible to make a device that suits more than very few pairs of ears perfectly without any issue whatsoever; we humans are just too unique! But hey, at least this method doesn’t drain any batteries, right?
So, now you now everything there is to know about the fundamentals of noise cancelling and noise reduction. Quite interesting, isn’t it? Perhaps you will now feel a little more confident when faced with these arguably very salesy terms. Oh well, on to explaining the next one.