How can noise cancel out noise?
Add a second sound. Can you make it quieter?
Or drag the dashed wave sideways.
Noise only. Pressure swing: 100% of the noise alone.
What adds up at this point
Noise only
Right now there’s only the noise. Use the controls to add your sound.
Pressure swing100%
Scale: 0–250% of the noise’s swing
One steady tone, slowed way down so you can see it. Each line shows the air pressure rising and falling at one listening point.
Sound is on. A quiet hum starts the first time you touch the mixer, and Mute turns it off. The real tone wobbles far faster than the drawing, and it may be faint on small phone speakers. Your device plays both sounds already added together. It doesn’t cancel noise in your room.
You made it quieter by adding sound.
Sound is air being squeezed and stretched. A speaker pushes the air beside it, that air pushes the next bit, and at your ear it arrives as pressure rising and falling. When two sounds reach you at once, they add up. So a fall can cancel an equal rise. Line the two tones up the other way, rise with rise, and the pressure swing doubles. That doesn’t make it twice as loud.
The air doesn’t follow the squiggle.
Air is made of particles too small to see. A speaker, or your voice, shoves the ones beside it, and they shove the next ones. The push travels outward, but each particle only shifts back and forth around its resting place. Where they crowd together, air pressure rises. Where they spread out, it falls.
The squiggle is a graph of how the pressure changes over time at one spot. It isn’t a path the air snakes along.
For sound to cancel, opposite changes have to reach the same spot at the same time. Move somewhere else and they may no longer match. A quiet spot doesn’t mean the sound has vanished everywhere.
Two snapshots of the same air, a moment apart, with one particle marked. Its movement is exaggerated so you can see it. The dashed line shows where it rests.
Noise-canceling headphones do the matching for you.
A microphone picks up the noise. A controller keeps adjusting what the speaker plays, so it pushes the opposite way near your ear.
Real noise is a jumble of many pitches at once. Each one needs its own opposing sound, sized and timed for the trip to your ear. Just replaying everything half a cycle late wouldn’t work.
Get the timing or strength a bit wrong and some sound gets through, just like in the mixer. Active cancellation usually works best on steady, low hums. Sudden noises and high sounds are harder.
The cushions also block sound just by being in the way. That’s called passive isolation. Adding the opposing sound is the active part.
A simplified view of one earcup. The match only works near your ear, and the microphone and controller keep updating it as the noise changes.
Talk like you’ve found the quiet spot
Phase
Where a wave is in its rise and fall at a given moment.
“Are they rising together, or one up while the other’s down?”Amplitude
How far the pressure swings above and below its usual level.
“How big is the pressure swing?”Destructive interference
When two waves overlap and partly or fully cancel each other out.
“Adding sound can make less sound.”