THE COMPLETE GUIDE TO NOISE CANCELLING HEADPHONES

Active noise cancelling enables you to block out the world and melt into your tunes. It feels like magic, but how does it really work? And what should you look for when buying a pair?

They’re getting more affordable than ever, so here’s everything you need to know about ANC

Science is no stranger to noise cancelling. Dr Paul Lueg submitted his first patent for wavecancelling technology way back in 1933, designing a system that could be used to reduce vibrations in loudspeakers; the theory was sound, but the technology of the time did not allow for the equipment to be made. In the ’50s and ’60s, Dr Laurence Fogel continued similar research and issued patents for the first viable devices, designed to cancel the potentially harmful frequencies of helicopter cockpits. This time the tech was transferred from speakers to ear directly, using earphones to reduce the risk of more damaging audio feedback.

Fogel, though considered the inventor of active noise cancellation, was not alone. Around the same time the US Air Force was working on a project to protect air crews’ hearing, culminating with the design and testing of an active noise cancelling project of its own. PD Wheeler’s design was the first to escape the lab, seeing its first experimental use in the then-new (and extremely loud) Hercules transporter. The tech focused on attenuating low frequencies, since the enclosed nature of a flight helmet already deflected a majority of high-pitched sounds. And it worked, slashing 18dB of noise from its intended range.

FLIGHT OF FANCY

These are the seeds, but we need to move further along the timeline to really see ANC come of age. The story goes that Amar Bose, irritated by the inadequate headphones he was offered on a 1978 international flight, decided he could do better. Pulling out a pen and paper, he sketched out his own noise cancelling design right there. Bose, an inventor known for flashes of inspiration (“These innovations are not the result of rational thought; it’s an intuitive idea,” he told Popular Science in 2004) had already popularised active equalisation in the company’s directional speakers. Bose’s ANC design – more practical, affordable and effective than previous attempts – soon made itself comfortable in cockpits the world over. The tech accompanied Dick Rutan and Jeana Yeager on the first round-the-world flight in 1986. Still, technology took its time to catch up; it was not until the early 2000s, over 20 years after that hasty in-flight sketch, that Bose brought ANC to the consumer.

Now look at it: noise cancelling is very big news. Miniaturisation, innovation and refinement means we’re in a new, more peaceful world. And everyone’s at it. Bose’s foothold has been eroded by countless other companies. ANC is now not so much a case of technological advancement but of finding the right finely-tuned algorithm. The right combination of microphone, driver and software.

There might be tiny variations between different implementations, but the principle of ANC stays the same whatever kit you buy. Now, here comes the science: sound is a waveform, travelling through air with peaks and troughs of

compression and rarefaction. Play the same sound at the same time and you amplify that original waveform… but switch the phase – invert the second waveform, turning its peaks into troughs and vice versa – and the two waves begin to counteract each other. Get the amplification of the inverted waveform correct and you can effectively cancel any unwanted sound by opposing it.

That’s the simple overview, but the chaos of reality isn’t quite so kind. Inverting a live waveform is fundamentally very difficult, since to do it perfectly you’d need absolutely no latency between the sounds of the real world and what’s pumped out of that speaker. Most often, ANC won’t generate an exact opposite of the sound it’s blocking out, instead creating an approximate muddling tone – this is why noise cancelling headsets are great at blocking out droning sounds that don’t change frequency much, like road noise or aircraft hum, but can’t deal quite as well with sudden sounds.

It’s also hard to cancel noise without overwhelming any other signal you might want to put through that speaker. For this reason, most ANC focuses on lower end frequencies, those which are most likely to penetrate the closed backs of cans or ear buds, and relies on isolating materials to squash the rest of the sound.

There’s also, despite the broad term ‘Active Noise Cancelling’ (also referred to, in some cases, as ‘Active Noise Reduction’) not one single form of the technology. In fact, there are three. Feedforward, the most common variety, places a microphone on the outside of the earcup or earpiece, catching the noise before it reaches your ear. It then pipes that signal into the ANC processor, before spitting out the antinoise within the ear cup. This has its advantages – notably that brief microsecond extra it has in which to perform its duties – but also has a couple of disadvantages. Improper angling of the microphone could, in some cases, cause the ANC engine to actually amplify unwanted sounds rather than muting them, and captured wind noise can be problematic. Feedforward also has no ability to correct itself – it must make assumptions about how to work with that outside noise, without knowing what real effect its actions are having on the sound inside the ear cup.

Feedback ANC moves the mic inside the earpiece. This certainly solves the selfcorrection issue (the microphone cannot help but pick up what’s coming out of the loudspeaker) and means the ANC engine can adapt itself to precisely what the user is hearing, even if the headset is sitting at an odd angle. It does require very precise calibration of the ANC engine, though, as overzealous cancellation can cut frequencies from the music playing through the loudspeakers or – as its name might suggest – cause a painful feedback loop.

Hybrid ANC combines the two, with a pair of microphones working in tandem: one outside the earpiece, one inside. This allows the system to pick up exact frequencies from both the real world and the tiny ecosystem close to the ear and react on both, but it requires strong processing and a very exactly tuned algorithm to get the ANC right

Overzealous cancellation can cause a painful feedback loop

without ruining the sound. And it’s more expensive than single-mic alternatives.

ANC requires power. While this can be drawn from a USB or lightning cable, it’s not possible to supply enough juice through a 3.5mm jack. Given that they already carry a battery, you’ll more commonly find ANC on wireless audio devices; if you’re looking at a wired solution, check to see where that battery is located. Sometimes, particularly on older devices, you’ll find it sharing space with the ANC hardware in an awkward module mounted on the headphone wire, which can mess with the balance.

Even if you’re going wireless, bear in mind that activating the ANC engine will increase your device’s battery draw, and so cutting the between-charges lifespan of Bluetooth headphones.

Beware, when shopping, that ‘noise cancelling’ can also (somewhat deviously) refer to non-active noise cancellation. Some kind of passive blocking is, as we have discussed, part of just about every active system – from silicone or foam earbuds to the solid closed back of ANC cans – but some sell these natural baffles alone as a feature. You’ll generally be able to sell by the price if something includes an active element.

It is, at least in theory, feasible to run an ANC system on open-back headphones. The problem is that a vast amount more filtering and cancelling needs to be done, over a wide range of frequencies, since the passive noise cancelling element so critical to other systems has been removed. That heavy processing tends to ruin the quality of the source audio, so open-back ANC isn’t common.

A better solution, and one which is becoming increasingly more common (and effective), is some level of audio passthrough, often called ‘transparency mode’ or similar. The presence of passthrough is no guarantee that it’ll work – we’ve heard some absolute stinkers in this department, devices which have turned the sounds of the outside world into a robotic mess while destroying the main audio signal – but that’s not the norm.

Recent advancements seem to have ironed it out. But it’s worth testing the passthrough functionality before purchasing if you can. If that’s not possible, we’d highly recommend looking for something with a number of levels, so you’ll be able to find a balance that suits you. Passthrough is, if anything, a neat side effect of ANC, using technology that is already there to add a useful new function to your headphones.

The final critical element of an ANC setup is the form factor. Passive noise cancelling is an essential part of the equation, but how well is it executed? How much sound will the foam earpieces of your noise cancelling cans let leak in? How much baffling can those tiny true wireless earbuds really perform? This is something that varies wildly; only when every element of an ANC device is properly balanced will it be truly effective.

Beware that ‘noise cancelling’ can also refer to non-active noise cancellation