How do noise-cancelling headphones work?

The power of active noise-cancelling lies in using sound waves against their own kind. Jake Port explains.

Travis Chambers / EyeEm

If you’ve ever tried listening to music on a train or on a commercial flight, you probably found the quality of sound produced by your headphones is not the same as when you use them in a silent room.

This is because a lot of the finer details of sound are lost among ambient noise – the noise produced by the surrounding environment.

This is easiest to notice when flying. The drone of the engines drowns out a lot of the sound coming through the headphones, whether the spoken word or music.

While there's no doubt it's irritating for passengers, for pilots, who need to clearly hear the voice of an air traffic controller or their co-pilot, it could be a disaster.

Enter active noise cancellation.

cosmos magazine

Sound waves can most easily be shown as a series of waves, with their height – or amplitude – indicating how powerful the wave is.

Ambient noise, such as a jet engine, generates a fairly consistent sound wave. Active noise cancellation uses a built-in microphone to listen to this sound wave, and then through a small computer chip, produce a sound wave with an opposite shape. For instance, when the engine sound wave peaks, the headphone-produced sound wave will have a trough.

When this artificial sound wave, known as an anti-phase, meets the sound of the engine it neutralises it. It’s just basic maths – when a wave with an amplitude of, say, 2 is met with a wave of amplitude -2 at the same point, they cancel out: 2 - 2 = 0.

In other words, no more engine noise.

So why doesn’t the music you’re listening to also get blocked since it's also produced using sound waves? Wouldn’t an anti-phase sound wave interrupt it too?

The ambient sound-sensing microphone is outside the headphone system, so only picks up and cancels external noise. The sound produced inside your headphones is safe.

There are limits, of course.

Active noise cancellation works best with consistent noises. You will still be able to hear loud conversations, traffic and other sudden noises over your music because the headphones simply can’t predict and produce anti-phase waves quickly enough to block out these sounds.

Nevertheless, active noise cancellation offers pilots, air travellers and everyday listeners the opportunity to almost magically cancel out a lot of the common noises experienced every day without having to uncomfortably turn up the volume.

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