How do noise-cancelling headphones work?

If you’re in the market for a set of headphones, you’ve probably at least heard the term Active Noise Cancelling used in marketing materials, unless you’re looking at particularly cheap and simple headphones. The promise of Active Noise Cancelling headphones is that they will remove outside noise and distractions from your listening experience, whether you’re listening to heavy rock, classical violin or just the latest true crime podcast at work, in transit, while exercising or at home.

All you’ll be left with – if the marketing hype is to be believed – is pure, sweet audio for your ears only. So how is it possible to do this? It’s magical tiny goblins running in and out of your ears, right?

Not quite. The effect might appear magical, but it’s actually based in some pretty simple science.

Sorry about this, but we’ve got to quickly explain just what sound is in order to explain how ANC works. You’re probably familiar with the idea that sound travels in waves, but what those waves really represent are the movement of particles in air, and how your inner ear reacts to those movements and resultant changes in air pressure.

Whether it’s a loud sound with lots of amplitude, or a high-pitched sound with more frequency changes in that sound wave, that’s the basics of how sound works. When we’re talking about headphones, there’s sound you want to hear – your music, your podcasts, the explosions in your video game and so on – and then there’s sound you don’t, whether it’s the constant thrum of an airplane’s engines or the shrill tones of Janet in the cubicle over the way complaining about how the café got her latte order wrong.

Presenting the audio you want is what the basic speakers in your headphones – of any type, ANC or not – do already, but the way you deal with that extraneous sound is quite clever.

Let’s take airplane noise, because wanting to block it is a very common use for ANC headphones.

The drone of an airplane engine is a solid, generally repeating low frequency sound wave, which is a space where ANC headphones typically work quite well. That plane engine noise sound wave has its peaks and troughs, like any sound wave. If you added the exact same noise to the headphones, all you’d end up with is a sound wave that’s twice as amplified – which is to say it is twice as loud – which nobody wants.

However, if you take that soundwave and analyse it for peaks and troughs and present that audio exactly out of phase – so the high points of the original wave are presented at the exact same time as the low points of the added wave – then the two sounds cancel each other out. The peaks in pressure match the troughs in pressure, which means that (ideally) none of that pressure reaches your eardrums, so you hear none of that particular sound.

So how do you capture and analyse that sound? With microphones, that’s how. Any set of ANC headphones, whether they’re in-ear buds or over-ear cans will have microphones in place, often several outside the headphones, and in some cases inside the headphones as well. They’re constantly capturing outside audio and presenting a directly opposite (if you want to get technical, it’s “phase shifting”) sound wave to block out the audio you didn’t want to hear at all.

If you’re wondering why it’s “active”, it’s because this is an ongoing process; even on our theoretical airplane the noise of the engines does change over time depending on speed, outside air conditions and atmospheric effects, so it would be of limited use if they only blocked out one type of sound in a fixed pattern.

This is also why all ANC headphones require their power, because they’ve got to constantly monitor the sounds around your head as well as pumping sound into it.

What about Janet and her shrill complaints, though? Higher pitch sounds are harder to block out this way, because they involve much faster shifts in the soundwaves, which is why so many ANC headphones struggle a little in this respect.

Of course, Janet aside – she should really just try another coffee place if it’s that bad, honestly – often those spoken words are the parts of outside audio that you might want to hear, especially if it’s somebody warning you that you’re about to blithely walk out in front of a car.

If Active Noise Cancelling exists, is there such a thing as passive noise cancelling? Sort of.

Without the microphones, the only way you get passive noise cancelling is by putting some level of barrier between your eardrums and the outside world. Often referred to as noise isolation, you get this effect in the most profound way with earbuds, because they’re blocking out your ears direct at the source. 

Over-ear headphones involve a little more of an air gap between your ears and the world, so they’ll give you slightly less isolation as a rule compared to a properly fitting set of buds.

Of course, if you just want passive noise cancelling and you don’t want to listen to any audio sources at the same time, you don’t actually need headphones. Simply stick your fingers in your ears, but be aware that this is generally seen as socially inconsiderate, if not downright rude.

Some ANC headphones will also advertise a “transparency”, or sometimes an “aware” (or similarly named) mode to allow you to more fully hear the outside world even when you’re wearing your headphones.

The way this functions is quite simple, because any set of ANC headphones must have a set of microphones on board to grab those sound waves for cancelling purposes. All most transparency modes do is use those microphones to capture that sound.

Instead of phase shifting it, they instead send that audio through your headphone’s speakers, typically while also lowering the presented volume of whatever you were listening to at the time.

For some mid to high-end sets of ANC headphones you may also see the option for dynamic or adaptive transparency.  This involves using the external microphones both coming and going, analysing external sound while also waiting for the higher pitched sounds that typically indicate speech patterns. 

Adaptive transparency modes allow you to block out the everyday humdrum noises, but mean – when they’re working properly – that when your boss asks you a question from behind you, you don’t miss out on it at all.

There can be a lot of variance in ANC quality between any two sets of ANC headphones depending on price, overall quality, even generationally. If you’ve got an older set of ANC headphones that were cutting edge 10 years ago, you’d be surprised at how much better a newer and now much cheaper set might be. 

However, one issue that does have a lot of users divided is whether or not in-ear  or over-ear ANC headphones are better when what you want to do is block out the outside world. Some studies suggest that in-ear ANC headphones may be more efficient in terms of the overall neutralisation of external sound, but you’ll still find plenty of users who will state unequivocally that over-ear ANC headphones are better.

Again, it’s going to depend on the quality of the ANC in play in the first place – a cheap set of ANC headphones will typically perform worse than a premium pair, as you might expect – but then there are other issues at play here, and some of them are highly subjective. 

For some users, in-ear buds can be significantly less comfortable to wear over time due to sitting within the ear canal. For others, the pressure of a headband and the heat buildup of over-ear cans can be less comfortable. While that’s not an ANC measure, it can lead to distinct personal preferences when choosing an ANC pair.

Typically, the drivers in a set of over-ear headphones will be larger, and that can lead to a more impressive and immersive soundstage than many (though not all) bud type headphones.  Where this matters for ANC is in the way that the mix of cancelled external noise and presented and desired audio comes through to you. If you’re lost in a musical soundscape par excellence because the audio is so good, it can also be part of what’s blocking out external noise as a result.

The practical takeaway here is that while in-ear buds might have a technical edge when it comes to ANC, it’s nearly always down to the quality of the headphones as a whole and your own personal preferences and needs more than anything else.