How active noise-cancelling headphones work: the technology behind ANC

How things work: active noise-cancelling headphones
(Image credit: Apple)

Active noise-cancelling headphones: undoubtedly one of the breakthrough audio features in consumer headphones since the turn of the century. 

Pioneered by Bose and Sennheiser, active noise-cancelling (or 'ANC') headphones come mostly in wireless form (wired noise-cancelling headphones are becoming a rarer beast by the day), from the true wireless Sony WF-1000XM4 and AirPods Pro 2 to the over-ear Sony WH-1000XM5 and Bose QuietComfort 45. And it's a splendid bit of tech. Done well, it's the cherry on top of the wireless headphone sundae.

Some of the best noise-cancelling headphones offer levels of noise cancellation (not just 'on' or 'off'), while almost all have pass-through or 'ambient aware' toggles for letting sound in if you want to briefly hear your surroundings.

But how is it done? Believe it or not, the tech has been around since the 1970s and was first implemented in headsets made for pilots. So get ready for take-off, because we're about to explain it all, then tell you which models do it best...

Should you want active noise-cancelling headphones? 

How active noise-cancelling headphones work

Bose QuietComfort 45 (Image credit: Bose)

Picture the scene: Monday morning – at the table otherwise known as your 'home office'. Your other half's got a 10am Zoom meeting and an online yoga class at 11am. The chap next door has been fitting a new kitchen for, oh, around two weeks now. Currently a child upstairs is watching Frozen II on Disney Plus for the eighth time since Friday afternoon. Enter active noise-cancelling headphones – your silence, your saviour.

The best noise-cancelling headphones will ensure your headspace will become just that – yours, to fill with whatever music or podcasts you may fancy. And of course, when you need to be back in the room, you can simply turn the ANC off (and maybe ambient aware on).

Active noise-cancelling headphones are great for travel, too, letting you remove the thrum of the bus engine or the chatter in your train carriage with just the touch of a button. The latest models have ambient aware and pass-through features so, at the flick of a switch or the touch of an earcup, you can quickly reenter the outside world and hear that all-important train announcement or be aware of traffic when you're cycling through congestion.

How does active noise cancelling work? 

Sennheiser Momentum 4 Wireless

(Image credit: Future)

Short answer: with two or more tiny microphones on the outer housing of the headphones that "listen" to the noise around you and quickly create a mirror image of the compression and rarefaction of the air (i.e. external sound). 

Too much too soon? Let's expand. Disregard the sound coming from your headphones and going into your ears for a minute. Focus on the sounds outside them – and all around you. 

We can think of the soundwaves around us like peaks and troughs, or ripples in a pond. Aeroplane engine noise is an ideal example since the thrum you hear in the cabin is typically a soundwave of constant amplitude – the height of the peaks and the depths of the troughs is largely continuous. 

If you produce another sound wave with the same amplitude but opposite phase – with a peak where the engine sound wave has a trough, and vice versa – you get something called an antiphase. Added together, the two sounds cancel each other out. The result? Silence. And you're listening to your music in peace.

Wait: adding two things together to get nothing? Really? Yes. Think back to your school days (if you dare): adding a minus number to a number is the same as subtracting. So, 11 + -11 = 0. 

Understood at the back? Good work. Back to your active noise-cancelling headphones and your upcoming train journey. Toggle ANC on and the tiny microphones on your headphones pick up that irksome engine thrum. This is then quickly measured by the headphones' internal electronics to produce an opposite sound, which is fed into your ears. If the tech does a good job, all you'll hear is the chug of the train fading into nothingness. 

Try it out and you'll find that there's a second or so between switching ANC on and hearing it kick in. That delay is the time it takes for your headphones to listen to the noise that needs cancelling, process it and produce the appropriate 'anti-wave'. 

More advanced (read: more expensive) noise-cancelling headphones actually use a larger number of these small microphones to pick up external sounds, in a bid to more accurately determine which sounds need to be cancelled and which you might want to still hear. 

Noise cancelling at this level can sometimes be supplied with its own app. The Bowers & Wilkins PX7, for example, do a top of job of customising the external sounds you want to hear. This particular model boasts 'office', 'flight' and 'city' environment filters, as well as a 'voice pass-through' slider to customise how much you want to hear the person sat next to you, from 'off' (not at all, thanks) to 'amplified'. The Bose Noise Cancelling Headphones 700 have 11 levels of noise-cancelling intensity to suit your needs.

It's all done with the same antiphase calibration technology, and it's all pretty impressive. 

Active noise cancelling vs passive

How active noise-cancelling headphones work

Sony WH-1000XM4 (Image credit: Sony)

This is a fairly easy one, but it's worth explaining since some manufacturers may boast a "noise-isolating design" or "natural noise-cancelling abilities" in a product. And that 'passive noise isolation' isn't the same.

Active noise cancellation is a technology; it's an electrical feature that requires power to work. When you're using it, you'll soon notice ANC will drain your wireless headphones' battery at a faster rate.

Noise isolation, meanwhile, is a physical thing; a term used to describe headphones that block out external sounds – and reduce the amount of sound leaking into your ears – without the need for power. This is simply achieved through build quality and design. 

Closed-back designs, leather earpads, a good in-ear seal, sizing up or down in eartips, a heavier clamping force in the headband (meaning the earcups fit tighter over your ears), dampening in the earcups and even the shape or material of the driver housings all contribute to passive noise isolation. But remember: it's not the same as active (powered) noise-cancelling headphones.

What is adaptive ANC?

Active noise cancellation doesn't just remain static. Increasingly, modern headphones come with various ANC modes, one of the most popular of which is adaptive ANC (it can also be called hybrid ANC). As the name suggests, this mode adjusts the ANC depending on your fit and environment.

In a busy coffee shop, for example, the ANC will increase in order to block out unwanted background noise. Step out into a quiet street, and the ANC will reduce. All this happens without you doing a thing – you just pop on the headphones and go about your day.

Different brands call this function by different names, and they all work slightly differently. The Bose QuietComfort Earbuds II play a short, sweeping tone every time you put them in your ears – when this happens, a mic inside each earbud measures your ear canal's acoustic response. The audio and ANC performance is then altered accordingly, and is continuously tweaked in the background as the headphones monitor what’s happening around you. When in Aware mode (aka passthrough, the mode that allows in enough background sound to hold a conversation), they also activate ActiveSense, which cuts out loud background sounds so you can still hear what someone's saying.

The Apple AirPods Pro 2 do this too, but Apple calls it 'Adaptive Transparency' – it's effectively adaptive ANC during Transparency mode (which is what Bose calls Aware mode – confusing, right?). Their ANC is also adaptive, constantly listening out for noises to silence – the algorithm adapts the sound signal 200 times every second in order to react quickly (the Beats Fit Pro also have this feature). But they don't have different ANC modes, whereas the Bose let you designate different levels of noise cancelling for different scenarios.

Apple AirPods Pro 2 app

(Image credit: What Hi-Fi?)

This functionality also features in the Sony WF-1000XM4, but here it's called Adaptive Sound Control. Like Bose's, it gives you different presets for different activities  – sitting, running, walking and transport – each with their own volume levels and degrees of ANC. You can customise these using the excellent Sony Headphones Connect app.

Can ANC headphones cut out all sound, all of the time? 

Apple AirPods Pro 2

(Image credit: What Hi-Fi?)

Unfortunately, no – not yet, anyway. They do come close, but because your ANC headphones are unable to come up with the appropriate signal automatically, some sounds will creep in.

ANC headphones work best when the noise you're trying to cancel features low-frequencies and is constant – a train carriage, aircraft or office AC unit, say. Occasional noises like a yell, dog bark or horn beep will often still be heard, because even the best ANC headphones aren't quite quick enough to create an antiphase sound in real time.

But you might be surprised at how effective noise cancelling can be. We found the Adaptive Transparency mode on the AirPods Pro 2 works well even on the London Underground, "where the sharp, screeching sound of the tube rails is blissfully damped down" we wrote in our review. It also takes the "higher-frequency edges off environmental noise like traffic."

Bose's Aware mode on the QC Earbuds II is also a real breakthrough (this lets in background noise while still playing music, using ActiveSense to adjust the ANC on the fly so your conversation and music aren't overwhelmed by loud sounds). "It's fascinating to hear it in action," we noted in our review. "We try it out in the pub and it’s impressive how much control the earbuds can exert on outside noise while still allowing you to get the general essence of the track in your ears. It sounds much more natural and balanced than when you try to achieve similar results with lesser ANC earbuds."

Which are the best ANC headphones?

Sony WH-1000XM5

(Image credit: What Hi-Fi?)

As you'll know if you've been paying attention up 'til now, ANC headphones are a great option for those who commute or travel frequently. Because you're cancelling noise rather than just whacking up the volume to drown it out, you'll be listening at quieter, safer, less tiring volumes, too. 

Any downsides to ANC? There can be. Headphones featuring this in-built noise-cancelling technology are generally more expensive than their ANC-lacking counterparts, and toggling ANC on for long periods can diminish your headphones' battery claims. 

Lastly, when headphones feature active noise cancelling, there can be a compromise on overall sound quality. The best models available now largely avoid this pitfall but we've often found that when the active noise cancelling has been poor, the audio quality as a whole has been sub-par, too. Sometimes, a pair of headphones with a solid, noise-isolating design – where production has focused solely on sound at the price – might be a better choice, especially if you won't need noise-cancelling tech too often.

Still sold on ANC? Good on you – don't go anywhere just yet. We've tested in-ear, on-ear and over-ear noise-cancelling headphones from big-hitters and little-known brands alike. And you can find a list of them in our thorough best noise-cancelling headphones guide.

Our favourites include the What Hi-Fi? Award-winning Sony WH-1000XM5 (and the older but still superb XM4 are now even better value) or their in-ear brethren, the Sony WF-1000XM4 (and again, older XM3). The in-ear WF-1000XM5 are also rumoured to be coming soon.

If you have a bigger budget, there are none better than the Apple AirPods Max, while the B&W Px7 S2, Sennheiser Momentum 4 Wireless and Bose NC700 also top our over-ear picks.

On a really tight budget? Check out the Lindy BNX-60 or BNX-80.


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Joe Svetlik

Joe has been writing about tech for 17 years, first on staff at T3 magazine, then in a freelance capacity for Stuff, The Sunday Times Travel Magazine, Men's Health, GQ, The Mirror, Trusted Reviews, TechRadar and many more (including What Hi-Fi?). His specialities include all things mobile, headphones and speakers that he can't justifying spending money on.