How do active noise canceling headphones work?

Almost every pair of good wireless headphones and the best wireless earbuds come with active noise cancellation (ANC). That’s a good thing. The world around us is a loud place, and sometimes you need to drown it out. Wherever you use those earbuds or headphones, whether at home, on the job, in the city, or on public transit, your listening experience will be better if less external sound makes it into your head.ANC helps accomplish that. With the press of a button, ANC headphones muffle incoming noise, letting you better enjoy the sounds you want to listen to. When it’s effective, it’s incredible. Dialing down the ambient noise around you like you’d adjust your media volume is an uncanny experience. But the way it works is even wilder.

What is sound, exactly?

Before we get into how tiny in-ear devices like the Bose QuietComfort Earbuds II can muffle the roar of jet engines, here’s some context. What many of us experience as sound is the product of changes in air pressure.

Our eardrums are thin membranes inside our ears that detect waves of changing air pressure, which cause them to vibrate. Those vibrations then travel through some delicate bones in our heads to eventually reach a part of the brain called the auditory cortex, which interprets them as what we perceive to be sound. (Bone conduction headsets cut out all the moving air, sending vibrations directly through your skull.)

These pressure changes are also why we can feel especially loud or bass sounds, like fireworks or the music at a concert. Loud sounds displace a lot of air over a short period, sometimes enough to feel the reverberations in parts of your body other than your ears.

Illustration of a waveform.

You may have seen sound waves illustrated as waveforms. The Y-axis on these undulating graphs represents a sound wave’s amplitude. In this context, it can be seen as a measure of how much air is displaced. More displaced air means louder sounds and taller waves on the graph. The distance between peaks on the X-axis is a sound’s wavelength. High-pitched sounds have short wavelengths; low sounds have long ones.

How noise-canceling headphones work: Anti-noise

Noise-canceling headphones use built-in microphones to listen to the sound in your environment. Processors inside the headphones analyze that incoming sound and create anti-noise that’s played to neutralize the noise, so you don’t hear it.

That may sound like science fiction but consider the sound waveform. The Y-axis, amplitude, technically plots the compression and decompression of air molecules. Points above the X-axis represent compression, and points below it represent decompression.

ANC, in a nutshell.

Anti-noise has the same wavelength as its target sound wave, but its amplitude’s phase is inverted. Their waveforms are like mirror images of each other. That means that when noise’s sound wave causes negative air pressure, anti-noise’s sound wave causes positive air pressure (and vice versa). This results, ideally, in blissful silence for the ANC headphone wearer.

The limits of active noise cancellation

There are limits to what ANC can do. If you’ve used active noise-canceling headphones or earbuds, you’ve probably noticed that while they’re great for drowning out the types of constant, low-frequency noise you might hear on an airplane, they typically struggle against other people’s music or the clatter of a coffee shop.

While a consistent, deep sound is relatively easy to predict and counteract with appropriate anti-noise, it’s harder to react to irregular, organic background noise in real time. If the ambient sound around you is unpredictable, your ANC headphones won’t be as able to muffle it as well.

Even uniform higher-frequency sound is trickier to counter than low-frequency sound. Because the physical distance between each discrete sound wave is shorter in high-frequency sounds than in bassy low-frequency sounds, factors like the distance between your headphones’ microphones and drivers, as well as the distance between those drivers and your eardrums, become more important in formulating effective anti-noise as the target noise gets higher in frequency. The upshot is that ANC gets progressively less effective the higher-pitched ambient noise is, and past a certain point, the headphones can’t keep up.

Active noise cancellation is also generally more effective in over-ear headphones than what you’ll get from in-ear buds. This is because headphones provide better passive noise cancellation than earbuds do. Headsets with bulky ear cups stop more sound from entering your ears by physically blocking more sound waves than tiny earbuds can. Whichever form factor you prefer, making sure you have a good seal is crucial to ANC performance.

Behind the ANC curtain

We hope this helped demystify noise-canceling headphones for you! You can now be free to block out the world around you. For more explanations of techy things, we covered the complexity of pixel binning in mobile photography and some exciting details on the up-and-coming Matter smart home standard.

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