How do noise canceling headphones work?

1the odd trick

Noise-canceling headphones have a strange trick: they don't wall off the noise, they add sound. Somehow, adding sound to your ear makes it quieter.

2listen and answer

Tiny microphones listen to the rumble around you. A chip makes its mirror image, and the speaker in the cup plays that mirror image into your ear.

3the hum

Sound is a wave of air pressure: crests squeeze the air, troughs thin it. Say the noise at your ear is a steady engine hum, crest after trough after crest.

4anti-noise

The headphones play the mirror image: a trough exactly where the hum has a crest, and a crest where it has a trough. Engineers call it anti-noise.

5they cancel

Add the two together and every crest meets a trough: the pressure stays flat. That's destructive interference. The hum is cancelled in the little pocket of air at your ear.

6long and short

It works best on low, steady rumble. At a hundred hertz the waves are three point four meters long, slow and predictable. At a thousand hertz, they're ten times shorter.

7the other half

Quick, sharp sounds like voices or breaking glass are harder to mirror in time. For them, the padded cup and the seal around your ear do the blocking.

8the whole idea

So the whole machine: a microphone hears the noise, a chip flips it, the speaker plays the flip, crest meets trough, and the foam seal handles the rest.