Header Ads

Breaking News

How Noise-Cancelling Headphones Work (and How We Test Them)


Bose first introduced its QuietComfort headphones in 2000, and since then the world of consumer-level active noise cancellation (ANC) has grown exponentially. Every year, we test scores of headphones and earphones that promise to block out the sounds of the world around you, whether it’s the rumble of a plane taking off or a talkative coworker with a mechanical keyboard. But what exactly is noise cancellation, and how do we test it? We’re here to help you understand what ANC can and cannot do, and what we look for in testing to determine the best models you can buy.


Passive Noise Cancellation (or Noise Isolation)

Headphones and earphones are almost always designed to block outside noise in some form. Active noise cancellation uses complicated circuitry (which we’ll explain below), but that isn’t the only way to do it.

Passive noise cancellation, or noise isolation, uses materials and physical engineering to manually insulate your ears from outside noises. Earplugs provide passive noise isolation, as do most in-canal earphones thanks to how their silicone or foam tips fit in your ears. Over-ear (circumaural) headphones can also provide passive noise isolation, using the earcups to create a reliable seal around your ear to block out a decent swath of ambient noise, enhanced with materials like memory foam. ANC headphones often rely on these elements to help block out noise in addition to the circuitry they use.

Bose Noise Cancelling Headphones 700

The Bose Noise Cancelling Headphones 700 offer the best active noise cancellation you can get right now

So, whether we’re talking about canal-sealing earphones or circumaural headphones, it starts with noise isolation before more complicated ANC circuitry even comes into play.


The Key to Active Noise Cancellation: Inverse Audio

The basic concept of how active noise cancellation works is simple. Think of a sine wave on a graph, with the height of its arc in positive territory. Now, imagine the exact inverse of that, with its lowest point lining up with the peak of the positive sine wave. Just like adding 1 to -1 results in 0, these two waves cancel each other out. So if the waves represent audio signals, they essentially nullify each other, creating silence.

Sine waves on a graph

Think of noise cancellation as being where the blue and green sine waves meet on this graph, cancelling each other out

That’s what ANC is. Microphones built into ANC headphones capture surrounding noises, and audio processing circuitry takes those noises and generate their inverse to cancel them out in real time.

If all the sounds around us were as simple and predictable as sine waves, active noise cancellation would be easy to implement, cheaper to manufacture, and highly effective nearly all of the time. But sound is rarely as steady as that, and its complexity can make it difficult for noise-cancelling circuitry to generate an accurate inverse. ANC usually works quite well for constant sounds, like the whir of an AC unit or the rumble of an airplane. It’s harder to precisely create inverses for sounds that vary in pitch and volume quite a bit, like someone singing, or a percussive sound, like someone clapping their hands. 

On top of that, while lower-frequency sounds are relatively easy to create inverse waves for, higher frequencies are more difficult to adjust to. Since ANC works best on low frequencies and steady sounds, manufacturers use some additional tricks to dial back highs and transient sounds.

One trick we often see, especially on less expensive pairs of noise-cancelling headphones, is that manufacturers will insert a white noise-like hiss into the signal that effectively masks the frequencies the circuitry can’t eliminate. This isn’t noise cancellation, though, it’s noise creation, and we note it in our reviews whenever we hear it.


Active Noise Cancellation vs. Adaptive Noise Cancellation

Ostensibly, adaptive noise cancellation refers to active noise cancellation that adapts to the sounds it is hearing in real time to create the best possible noise cancellation for that environment. If that sounds a lot to you like the basic definition of ANC, you’re not wrong. After all, ANC must be adaptive, otherwise it wouldn’t work. But some companies are pushing the idea that their noise cancellation can tell where you are, and adjust how the circuitry behaves accordingly. Recent Sony models, for example, work with an app that has the ability to “learn” the environments you frequent and offer the best ANC for those areas.

No matter how advanced this all sounds, at its heart, it’s what all ANC is supposed to do. ANC shouldn’t just work in train stations, it has to work everywhere. That requires sending out the best result based on real-time audio analysis from mics that are constantly monitoring your surroundings.

So, to put it bluntly, technology is always improving, and manufacturers need to invent jargon to market and differentiate it from previous versions. So yes, what is called adaptive noise cancellation is by and large an improvement upon ANC from previous years, but with varying degrees of performance and consistency, and a bit of salesmanship on top.

Sony WH-1000XM4

Many noise-cancelling headphones (like the Sony WH-1000XM4 pictured here) offer an ambient mode that lets you hear your surroundings

Common Noise Cancellation Issues

As we mentioned earlier, hiss is something to watch out for, and a common issue among lower-cost headphones. Another red flag is noise cancellation’s impact on the audio output of a given pair of headphones. Switching the ANC on and off shouldn’t change the sound signature, and on top-quality models, it doesn’t. But we’ve tested plenty of ANC headphones that have trouble in this regard. Sometimes, the change in sound signature is very subtle, but plenty of cheaper pairs sound wildly different when the ANC is on, and those models are best avoided.

In-ear pressure is another issue. Perceived pressure changes caused by ANC varies not only from headphone to headphone, but from person to person. One pair might feel fine to me, but deeply uncomfortable on your head. We’re all built differently, and so whenever possible, it’s ideal to test the headphones out for yourself to make sure the in-ear pressure isn’t going to bother you. Relying on what a review says is helpful to a degree, but ultimately, I can’t know whether your ears will react the same way mine do to noise cancellation from a particular pair of headphones.


How We Test Noise Cancellation

Believe it or not, our testing process for noise-cancelling headphones is still pretty much the same during the COVID-19 pandemic as it has been for years prior. After all, we weren’t taking a cross-country flight every time we test a new pair of headphones. And while New York City, where we do our tests, is quieter than usual, on-the-street testing is only a small portion of how we determine the efficacy of ANC headphones. In order to definitively say which pair handles which frequencies best, we need consistency, and we can’t easily get that on the streets.

So for the bulk of our testing, we use the same sound files, at the same volume levels, through the same speakers, in the same acoustically treated room to generate noise. We test with sound files of low-frequency rumble, files of high-frequency sweeps, and recordings of noisy restaurants and office environments. We also put the headphones through stress tests, playing sound files at high volume levels that no pair will fully succeed in cancelling out, to determine how effective they are under extreme conditions.

Apple AirPods Pro

Apple’s AirPods Pro bring top-notch noise cancellation to a true wireless design

By comparing the way headphones handle these sound files as well as outside noise, we can get a read on which models are more effective at dialing noise back. The answer isn’t always straightforward; some pairs are better than others at eliminating specific frequency ranges, some are inconsistent, and some do better at one volume level but markedly worse at another.

Then, there’s the aforementioned hiss issue, which we we test for in an exceptionally quiet room. Our semi-soundproof, acoustically treated testing space is ideal for this, letting us listen for the hiss without any outside noise whatsoever. When audible hiss is added to the audio signal in a super-quiet room, activating the ANC will actually seem to make things louder, not quieter.

Obviously, all of this testing is done with no music playing through the headphones, but we also test the audio performance, both to see how the headphones sound, and to see whether the ANC alters the sound signature at all. After all, these are headphones, and how they sound matters just as much as how well they block outside sounds. For more on how we evaluate audio performance, see our story on how we test headphones.

Finally, once all of our testing is done, we consider noise cancellation and audio performance in addition to other factors such as price and user experience to determine a rating. We test most of the major (and not-so-major) noise-cancelling headphones on the market, and each of our reviews is a deep dive into all aspects of their performance.

For a look at our current favorite models, check out our guides to the best noise-cancelling headphones, the best noise-cancelling true wireless earbuds, and a head-to-head comparison of the top ANC models on the market right now.

Source Link

No comments