Man wearing headphones in a busy office.

If you work in a busy office or have a noisy commute, noise-canceling headphones are a treat. But have you ever thought about how active noise cancellation works? While blocking out sound seems simple, it actually involves advanced computer processing.

What Is Active Noise Cancellation?

Active noise cancellation (ANC) is a feature in mid-tier to higher-end headphones that engages in something called “signal processing” to block outside sound. Many of our favorite headphones in fact offer ANC. To keep it simple, ANC takes a step beyond earplugs in making sure that you won’t hear that crying baby on a cross-country flight. At least, you won’t hear the crying nearly as much as you would without the headphones.

It’s not completely transparent, so if you turn on a set of headphones with ANC, you’ll hear it working in the form of a subtle hissing noise. This hissing is only the most audible portion of the signal. ANC plays lower frequencies as well, and these can actually hurt some users’ ears.

How Does Active Noise Cancellation Work?

ANC works using a series of microphones strategically placed around a given set of headphones or earbuds and a concept known as phase cancellation. The first step is using those microphones to determine what the listener is hearing.

Once the microphones have captured that sound, digital signal processing (DSP) hardware and software inside the headphones get to work. This is where it gets complex.

You’ve probably heard the term “sound wave” before. This is what sound is: a series of vibrations. If you take a sound, duplicate it, then delay it ever so slightly, this puts the waves out of alignment. The peak of one waveform plays at the same time as a peak in the opposite direction of the other waveform, so they effectively cancel each other out.

Waveforms in alignment

This rarely happens in nature, so it seems unnatural, but it works. The microphones in headphones use this exact technique. Some headphones use a rather rough implementation, while others use advanced hardware and software to intelligently adjust to your surroundings.

Two signals perfectly out of phase, cancelling out the sound

Some headphones, like earbuds, use microphones positioned on the outside. This is easier to do, especially on earbuds, but isn’t the most effective method because it doesn’t accurately mirror what you hear.

Larger headphones can use microphones mounted inside the ear cup, nearer to your ears. This means the sound the headphones cancels out more accurately reflects what you’re hearing, making the cancellation more effective.

Finally, other headphones use a “more is more” approach, and position microphones inside and outside the headphones, making for comprehensive noise cancellation.

Passive vs. Active Noise Cancellation

You’ll see some headphones claim that they offer “passive” noise cancellation. This isn’t really noise cancellation in the sense we’ve looked at so far, but is better referred to as isolation.

Passive noise isolation is a simple concept, especially compared to ANC. It works the same way that covering your ears with your hands does: putting something between your ears and a given noise makes that noise quieter.

Another part of this comes in as you start playing audio on your headphones. That sound is much closer to your ears, resulting in a higher relative volume, so it effectively drowns out sound in the background.

With headphones, as long as you have a good seal, this will block out some amount of background sound. The better the seal, the better the isolation. This is typically most effective with over-ear headphones or in-ear headphones. On-ear headphones and other styles don’t create a good enough seal for any real isolation.

None of this is fancy technology, but it is somewhat effective, and it’s definitely better than nothing. Just don’t confuse it with true active noise cancellation.

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The Downsides of Active Noise Cancellation

As nice as ANC is, there are a few issues with it. The first is that because of the microphones and processing, it requires power. This means that even wired headphones with ANC will need battery power.

ANC does also have a mild impact on sound quality. Because of the cancellation, you aren’t getting processing-free music. This is generally only a concern to audiophiles and music purists, but it’s still worth mentioning.

Finally, the biggest issue with active noise cancellation is that it, well, blocks out sound. This is great in a safe environment like an office, but if you’re wearing noise-canceling headphones while out and about, it can be dangerous.

If you cross the street without looking and can’t hear a car horn because of noise cancellation, that could be very bad. This is why more and more modern headphones are coming with built-in transparency modes.

What Is Transparency Mode?

airpods pro on grey background

Transparency mode originally had its major debut in Apple’s AirPods Pro, but it was quickly adopted by other headphone manufacturers. Some were even doing this before, but Apple’s name for it was the one that stuck.

The idea is very closely related to noise cancellation, as it uses the same microphones. The difference here is that instead of inverting the signal and using it to cancel out the noise, Transparency Mode plays that sound directly.

This means you can still listen to your music while hearing everything around you. If you’re a runner or cyclist, this is a major feature. Thanks to firmware updates, this could come to many headphones with ANC, so keep an eye on your manufacturer for updates.

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Profile Photo for Kris Wouk Kris Wouk
Kris Wouk is a freelance tech writer and musician with over 10 years of experience as a writer and a lifetime of experience as a gadget fan. He has also written for Digital Trends, MakeUseOf, Android Authority, and Sound Guys. At MakeUseOf, he was Section Editor in charge of the site's Mac coverage.
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