How Noise-Cancelling Headphones Erase Sound

Put them on, flip a switch, and a plane's roar disappears. The headphones aren't just blocking sound — they're creating sound to destroy sound.

Sound is a wave

Sound is tiny pressure changes in the air: peaks (compressed air) and troughs (thinner air). If you add a second wave whose peaks line up exactly with the first wave's troughs, they sum to zero. Physicists call it destructive interference.

Cancel a wave yourself

Start at 100 Hz (engine hum). Then slide to 2,000 Hz without changing the delay. Then reduce the delay.

Inside the headphones

1. Outer microphones hear noise before it reaches your ear (feedforward).
2. Inner microphones hear what's actually left in your ear (feedback).
3. A chip computes the inverted wave thousands of times per second.
4. The speaker plays your music + the anti-noise.

“Silence, in your headphones, is a sound being played very precisely.

Why voices get through

A delay of 100 µs is nothing for a 100 Hz wave (one cycle = 10,000 µs), but at 2,000 Hz it's a big chunk of a cycle (500 µs) — the anti-noise arrives out of step. Voices and keyboard clicks are higher and change constantly, so they're hard to predict.

• A good ear seal (right tip size) blocks the high frequencies physically.
• Transparency mode does the opposite: mics pass outside sound in on purpose.