How Do Noise-Cancelling Headphones Work?

On a flight, switch on noise cancellation and the engine roar fades to a whisper. The headphones aren’t blocking the sound — they are adding more sound.

Sound is a wave

Sound is a pressure wave: tiny squeezes and stretches of air. If you add a second wave that is the exact mirror image — pushing when the first pulls — they cancel. This is called destructive interference.

What the headphones do

1. Small microphones on the ear cups pick up outside noise (and inside the cup, what reaches your ear).
2. A chip calculates the opposite (“anti-phase”) wave in real time.
3. The speakers play that anti-noise mixed with your music.
4. The noise and anti-noise cancel at your ear.

Why engine hum disappears but voices don’t

Steady, low-frequency sounds — engines, air-conditioners, train rumble — are predictable, so the electronics can match them well. Sudden, high-pitched sounds like voices and clattering dishes change too quickly. For those, the ear cushions’ passive blocking does most of the work.

Transparency mode

The same microphones can do the opposite: feed outside sound in so you can hear announcements or a colleague without removing the headphones.

A note on volume

Noise cancellation lets you listen at lower volumes in noisy places — good for your hearing in the long run.