How do noise-cancelling headphones work?

TUREFF• •0

Put on a pair of noise-cancelling headphones on a train or plane and the low roar around you seems to drain away. It feels as if the headphones are simply blocking sound, but most of the effect comes from something stranger: they create extra sound designed to cancel out the noise. Here is how that works and why it is better at some noises than others.

Sound is a wave

Sound is a series of tiny pressure changes in the air: squeezes and stretches travelling outwards from the source.

A high point of the wave is a moment of slightly higher pressure, and a low point is slightly lower pressure.
Low-pitched sounds, like an engine hum, have long, slow waves. High-pitched sounds, like voices or clattering cutlery, have short, quick ones.

Cancelling a wave with its opposite

If you add two waves together, they combine.

When two peaks line up, the sound gets louder.
When a peak meets a trough of the same size, they cancel out and the air pressure stays roughly level. This is called destructive interference.
Noise cancellation exploits this. If the headphones can produce a wave that is a mirror image of the incoming noise, the two largely cancel at your ear.

What is inside the headphones

Microphones listen to the noise. Some sit on the outside of each ear cup or earbud, picking up sound before it reaches you. Others sit inside, near your ear, checking what is actually getting through.
A small processor analyses the noise and calculates the opposite wave almost instantly.
The speaker plays that anti-noise alongside your music or podcast.

Designs that use outside microphones are often called feedforward, those with inside microphones feedback, and many headphones combine both, which is known as hybrid noise cancellation. Each approach has strengths, and combining them can cancel noise across a wider range.

Why the hum vanishes but voices do not

Active noise cancellation works best on low, steady sounds.

Engine drone, air conditioning and the rumble of a train are predictable and change slowly, so the processor can keep up.
Higher-pitched and sudden sounds, such as voices, a baby crying or a door slamming, change too quickly and have such short wavelengths that cancelling them reliably is much harder.
That is where passive blocking takes over. The physical seal of the ear cushions or the tips of earbuds blocks a lot of high-frequency sound. Good fit matters: earbuds with the wrong size tips leak, and so do over-ear cushions pushed aside by glasses.

Transparency modes and pressure

Transparency or ambient modes do the reverse: they use the outside microphones to feed surrounding sound in, so you can hear announcements or traffic without taking the headphones off.
Some people notice a feeling of pressure or slight fullness in the ears with strong noise cancellation. It is commonly reported, harmless for most, and lowering the cancellation level usually eases it.
Wind noise can confuse outside microphones, and many headphones have a setting to reduce it.

Using them sensibly

They can protect your hearing indirectly. In noisy places, people tend to turn music up to drown out the background. With the background reduced, you can listen at a lower volume.
Stay aware of your surroundings. Cutting out noise while walking, cycling or near traffic means you may not hear hazards. Use transparency mode or take one side off when you need to hear what is around you.
Keep the volume moderate and take breaks. If you notice ringing in your ears or muffled hearing, give your ears a rest and see a doctor if it persists.

Have a question about how your headphones handle noise? Ask it in the comments.