How do noise-canceling headphones work?

1. Zoe is on a plane. The engines make a steady, deep hum that drowns out her audiobook. Then she presses a button on her headphones, and the hum fades away. How?
2. Sound is a wave in the air: a push, then a pull, again and again. On a screen it looks like bumps and dips. Taller bumps mean a louder sound.
3. A tiny microphone on the outside of each ear cup listens to the noise before it reaches your ear, and sends its shape to a small computer chip.
4. The chip makes a mirror image of the noise, called anti-noise. Wherever the noise has a bump, the anti-noise has a dip of exactly the same size.
5. The speaker in the ear cup plays the anti-noise. At your ear the two waves meet. Each push is met by a pull, they add up to almost nothing, and the hum disappears.
6. The trick works best on steady, low sounds like engines. Quick, high sounds, like a clap or a voice, change too fast for the chip. Thick ear cushions block those instead.

One ear cup has been cut open. Tap each part to see its job in erasing the noise.

• Outside microphone. This tiny microphone sits on the outside of the cup. It hears the noise a moment before the noise reaches your ear.
• Chip. The chip is a tiny computer. It takes the noise from the microphone and makes the opposite wave, thousands of times every second.
• Speaker. The same little speaker that plays your music also plays the anti-noise. The music stays, because the chip only flips what the microphones hear from outside.
• Inside microphone. A second microphone inside the cup listens to what your ear hears. If some noise is left, it tells the chip to fix the anti-noise.
• Ear cushion. The soft cushion seals around your ear. It blocks quick, high sounds that the chip is too slow to cancel.
• Battery. Making anti-noise takes energy, so these headphones need a battery. When it runs out, only the cushions block noise.

Drag the blue anti-noise wave, or use the sliders. Change its strength and its timing until the wave at the ear is flat. Then see what bad timing does.

Experiment: What happens if the anti-noise arrives half a wave late, so its bumps line up with the noise's bumps?

1. Set the strength to 100 percent and the timing to the middle. Look at the bottom wave.
2. Slide the timing all the way to one end.
3. Now try good timing with a strength that is too weak or too strong.

What we found: Timing is everything. When each bump meets a dip, the waves cancel. When bumps meet bumps, they add, and the noise gets twice as tall. The strength must match too.

Turn on JavaScript to try it yourself.

Three hums, each a different size. Set the strength and timing of the anti-noise to flatten each one, then press Lock in.

Ordinary headphones block noise the simple way: a padded cup covers your ear, like putting your hands over your ears. That works well for high, hissy sounds, but deep hums from engines slip through.

Noise-canceling headphones add a clever trick. Sound is a wave of pushes and pulls in the air. A tiny microphone on the outside of the ear cup listens to the noise. A chip makes a new wave that is the exact opposite: it pulls when the noise pushes and pushes when the noise pulls. The speaker plays this anti-noise into your ear.

When the two waves meet at your ear, they add together. A push plus an equal pull is nothing at all, so the hum fades away. It is like filling holes with the dirt from bumps: the ground ends up flat. Your music is not canceled, because the chip only flips the sound that the microphones pick up from outside.

The trick needs perfect timing. A deep hum has long, slow waves, so the chip has time to match them. Quick, high sounds have short waves that change too fast, and a sudden bang is over before the chip can react. That is why these headphones still have thick cushions, and why you can still hear someone call your name.

Key words

Anti-noise

A mirror-image copy of a noise that cancels it.

Amplitude

The height of a wave. A taller sound wave is louder.

Interference

What happens when two waves meet and add together.

Phase

Where a wave is in its up-and-down cycle.

Passive isolation

Blocking sound simply by covering and sealing the ear.

Check your thinking

1. A noise wave meets its exact mirror image. What do you hear?

   Show the answer

   Almost nothing. Each push is met by an equal pull, so they add up to almost nothing.

2. Why do noise-canceling headphones have a microphone on the outside?

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   To hear the noise so the chip can make its opposite. The chip needs to know the shape of the noise before it can make the mirror-image wave.

3. The anti-noise arrives late, so its bumps line up with the noise's bumps. What happens?

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   The noise gets louder. Bumps on bumps add up, so the wave gets taller and the noise louder.

4. Which sound do these headphones cancel best?

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   The steady hum of an engine. A steady, low hum has slow waves that the chip can match. Sudden or high sounds change too fast.

5. The anti-noise has the right timing but is only half as strong as the noise. How much of the noise's amplitude is left?

   Show the answer

   Half. With no phase error the leftover is 1 minus the relative strength: 1 minus 0.5 is 0.5.