Noise cancelling headphones

How can headphones make a noisy plane go quiet?

Imagine sitting on a loud airplane, with a constant low engine roar all around you.

Sound is a wave. It travels through the air and pushes on your eardrum, like ripples on a pond.

Here is a simple sound wave, going up and down. Up is high pressure, down is low pressure. Now the clever part. The headphones listen to the noise with a tiny microphone.

• Noise = a wave

Then a tiny chip makes a mirror image of that wave, flipped upside down.

Here is the flipped wave. Where the noise goes up, this one goes down. When the two waves meet, they add together. Up plus down gives zero, so the sound cancels out.

• Opposite wave

Think of two kids pushing a swing. One pushes forward, the other pushes back, and the swing stays still.

This is called destructive interference. The two waves destroy each other, and you hear almost nothing. This is active noise control, because the headphones actively make sound. Padding alone is passive and just blocks sound.

• Destructive interference

It works best on steady, low sounds like engine hum, because they are easy to predict and mirror.

So one more try. What does the headphone play to cancel the noise? In short, headphones listen, flip the wave, and play it back, so noise and anti-noise cancel each other.

• Listen, flip, cancel