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How can a pair of headphones make

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a jet engine quieter by adding more sound?

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Flip the switch on a noise-cancelling pair,

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and the low roar of a plane cabin seems

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to drain away.

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Nothing extra

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was blocked; something was added.

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By the end, you'll know

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what the tiny microphones

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on each ear cup are listening

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for, why a droning engine fades

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while a voice across the aisle still gets

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through, and why the whole idea

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is almost a century old.

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But first, what exactly

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are the headphones fighting?

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Sound is a wave of pressure.

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A vibrating object squeezes

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the air beside it, then lets it stretch,

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and that pattern of squeezes

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and stretches travels outward

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until it pushes on your eardrum.

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How fast those squeezes arrive sets

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the pitch.

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That rate is called frequency,

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and it's measured in hertz,

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the number of waves per second.

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A deep engine hum repeats slowly.

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The hiss of air

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from a vent repeats thousands of times a second.

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In air at 20 degrees Celsius, sound travels

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at 343 metres per second.

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So the lowest pitch most people

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can hear, around 20 hertz,

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has waves about 17 metres long.

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The highest, around 20,000 hertz,

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has waves just 1.7 centimetres long.

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That size gap is the whole problem.

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Foam and a snug seal soak up short,

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high waves well.

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But the waves of a low rumble

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are huge next to any cushion, and stopping

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them takes something heavy, like a thick wall.

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So how do you stop

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a wave you can't block?

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You fight it with more sound.

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If a second wave pushes exactly

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when the noise pulls, and pulls exactly

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when it pushes, the two cancel out.

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Physicists call

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that destructive interference,

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and the second wave is often called anti-noise.

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Here's one way a headset does it.

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A microphone on the outside

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of each ear cup hears the noise coming

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in, and a chip flips that wave upside down.

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The speaker plays the flipped wave along

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with your music.

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Many headsets also

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have a second microphone inside the cup,

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which listens to what's left

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and fine-tunes the mix.

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On a graph, the noise rises and falls,

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and the anti-noise is its mirror image:

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when the noise reaches a peak

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of 1, the anti-noise sits at minus 1.

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Add the two together,

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and the sum stays flat at 0.

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No change in pressure means no sound.

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It's a bit like a tug of war

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between two equal teams: everyone pulls hard,

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and the rope doesn't move.

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Here's where the comparison breaks.

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Anti-noise doesn't quiet

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the whole room, only a small zone around

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the microphone, right at your ear.

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It sounds almost too neat,

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so has anyone actually measured it?

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They have, and the idea is older

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than you might think.

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In 1933, a German inventor named Paul Lueg filed

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a patent on exactly this: pick up a sound

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with a microphone, flip it, and play

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it back through a loudspeaker.

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Twenty years later,

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in 1953, two engineers, Harry Olson

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and Everett May, built what they called

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an electronic sound absorber:

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a microphone,

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an amplifier and a loudspeaker, wired

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so the speaker cancels

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what the microphone picks up.

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They even imagined

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it in an airplane or a car.

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Near the microphone,

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the sound dropped by 10 to 25 decibels,

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but only for low sounds.

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Decibels count in powers of ten:

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every 10 decibels less means one tenth

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of the sound intensity.

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That's real, measurable quiet.

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Then the idea went flying.

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In December 1986, two pilots flew

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a light plane called Voyager around

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the world without stopping

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or refuelling: nine days in a cramped cabin

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with an engine at each end.

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To protect their hearing,

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they wore noise-cancelling headsets

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that aviation histories describe

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as early prototypes.

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So the path runs from a patent filed

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in 1933 and granted in the United States

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in 1936, to a working lab device

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in 1953, to headsets

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on a record flight in 1986.

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But if it works that well,

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why can you still hear

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the person sitting next to you?

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The answer is timing.

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Cancelling a wave means matching it push

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for pull, and the shorter the wave,

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the smaller the mistake it forgives.

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A slow hum is easy to match.

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A quick, high sound is not.

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Researchers have put numbers on it.

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Cancel a sound at one microphone,

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and the zone that gets

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at least 10 decibels quieter is only

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about a tenth of a wavelength across.

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For a 100 hertz hum, that's

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about 34 centimetres.

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For a 10,000 hertz hiss,

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it's just 3.4 millimetres.

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A headset adds a second limit.

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Every tiny delay in its electronics

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and speaker pushes the anti-noise out of step,

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and the higher the pitch,

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the further it falls behind,

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until it would add

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to the noise instead of cancelling it.

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So headsets hold back on high sounds:

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in a 1993 review, commercial ones cancelled

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from about 30 up to about 500 hertz.

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That's why a common belief is wrong.

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Noise-cancelling doesn't block

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every sound: it's best

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at steady, low sounds, while voices

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and sudden clatters carry much

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of their sound higher up.

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So what's really happening

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at your ear when it all works?

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So, how can adding sound make things quieter?

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Your headphones listen to the world

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through tiny microphones, build

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an upside-down copy of the noise,

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and play it into your ear

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at the same moment,

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over and over, as the noise changes.

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At your eardrum,

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every push from the noise meets a pull

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from the speaker,

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and the low roar flattens out.

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The snug, padded seal does the rest,

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soaking up much of the high hiss

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the electronics leave alone.

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Put together, that's a clear split.

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Cancelled well: engine drone,

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the rumble of a train,

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the hum of an air conditioner.

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Getting through: voices, clattering dishes,

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a sudden bang.

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Next time you flip

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that switch, you're hearing two sounds

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that add up to less than one.

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Quiet isn't only something you block.

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It's something you can add.

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So noise-cancelling headphones

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don't just shut the world out.

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They answer it, wave

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for wave, right at your ear.

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If something here surprised

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you, or you think I got

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a detail wrong, tell me in the comments.