How can headphones make noise disappear?

Hook

Why and how can headphones make noise disappear? Put on a pair of noise cancelling headphones, and the roar of a plane cabin fades to a hush. They aren't just blocking sound. They're fighting it with more sound.

Waves

Sound is a pressure wave. It pushes air molecules together, then pulls them apart, over and over. Your eardrum feels those highs and lows and your brain hears them as sound.

Mics

Active noise cancelling headphones have tiny microphones that listen to the world around you. A chip measures the incoming noise thousands of times a second.

Anti

Then the speaker plays a mirror image of that noise, called anti noise. Where the noise has a high, the anti noise has a low. When the two waves meet, they cancel each other out. Physicists call this destructive interference.

Low

It works best on steady, low rumbles, like engines, air conditioners and train noise. Those long waves are predictable, so the chip can keep up. Sudden, high pitched sounds like voices and clatter are harder to cancel.

Passive

That's why good headphones combine two tricks. Snug ear cushions or ear tips physically block the higher sounds, and the electronics erase the low ones. Together they can make a big difference.

Transparency

The same microphones also power transparency mode. Instead of cancelling the outside world, the headphones play it back to you, so you can hear an announcement without taking them off.

Close

So noise cancelling headphones listen to the noise, and play its opposite so the two waves cancel out. ThatsWhyHow.