Anti-noise is a sound vibration that vibrates exactly the opposite way to the ambient sound. Two vibrations in the same place add up, so if they are opposite they largely cancel each other out. Noise-cancelling headphones measure the sound with a little microphone, turn the vibration round and let a little speaker send it out.
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You put on headphones and press a button. No music comes out, and yet it is quieter than it was just now. What that button does is make sound.
Two sound vibrations in the same place add up, because the air particle there can only be in one place at a time. If they vibrate in step, the amplitude gets bigger and it sounds louder. If they vibrate exactly the opposite way, they cancel each other out. That is anti-noise. In this video you see how noise-cancelling headphones do it: a little microphone on the outside measures the sound coming at you, the electronics turn that vibration round, and a little speaker on the inside sends it out. At your ear a small amplitude is left, and you see why the sound never goes away completely.
For upper secondary physics, in the chapter on sound. Halfway there is a question and a silence: pause it there and think for yourself first.
In this video
The full explanation, in writing
The button on the headphones
0:00You put on headphones and you press a button. No music comes out. And yet it is quieter than it was just now. What that button does is make sound.
Two vibrations added together
0:14Two sound vibrations in the same place add up. After all, the air particle there can only be in one place at a time. If they vibrate in step, they reinforce each other: the amplitude gets bigger, and that is louder sound. If they vibrate exactly the opposite way, one going up while the other goes down, then they cancel each other out.
0:36That is not a trick with the volume. Sound is added, and less is left over. Your turn. Here is a sound vibration. What must that little speaker in the headphones send out to remove it?
What happens inside the headphones
0:52Exactly the same vibration, but exactly the opposite way. We call that anti-noise. On the outside of the headphones is a little microphone that measures what is coming at you. The electronics turn that vibration round and send it to the little speaker on the inside.
1:08At your ear, ambient sound and anti-noise arrive together, and together they have a small amplitude. Small amplitude is quiet sound. It does not go away completely, for that, the anti-noise would have to match every vibration exactly.
Summary
1:26Two things. One: two sound vibrations in the same place add up, and if they vibrate exactly opposite, they largely cancel each other out. Two: anti-noise is that inverted vibration. A microphone measures the ambient sound, a speaker makes the inverted vibration, and what is left at your ear has a small amplitude.
The question
1:48One question. You have two identical sound sources vibrating exactly opposite, and they stand side by side. Someone adds another such pair. Does it get louder?
The answer
2:01No. Each pair cancels itself out, so you can add as many as you like and almost nothing is left over. More sound added is not automatically louder. It depends on when those vibrations arrive.
Closing
2:15You now know why two sounds together can be less than one of them. On the channel there is a separate video about how much sound there is in a difference of a few decibels.
Frequently asked questions
How do noise-cancelling headphones work?
On the outside there is a little microphone that measures the sound coming at you. The electronics turn that vibration round and send it to the little speaker on the inside. At your ear the ambient sound and that anti-noise arrive together, and together they have a small amplitude. A small amplitude is quiet sound.
What is anti-noise?
A sound vibration that vibrates exactly the opposite way to the sound you want to get rid of: one goes up while the other goes down. In the same place those two add up, and then they largely cancel each other out.
Why does the sound not go away completely with anti-noise?
For that, the anti-noise would have to match every vibration exactly, exactly as large and exactly opposite. That never fully works, so a small amplitude is left over. The sound gets quieter, but it is not gone.
Two sound sources vibrate exactly opposite. Does it get louder if you add another such pair?
No. Each pair cancels itself out, so you can add as many as you like and almost nothing is left over. More sound added is not automatically louder: it depends on when those vibrations arrive.
More sound added is not automatically louder. It depends on when those vibrations arrive.
Physics with Thomas makes physics explainers for secondary school. No words on screen, so what you see works in any language.
In this series
Sound and vibrationwhat you hear is not the air from his mouth6:53
Frequency, amplitude and pitchlouder is not higher5:57
Resonance and vibrating alongit is timing, not force6:17
Fundamental, overtones and timbresame tone, other sound6:06
The tone of an air columnf = v divided by 4L3:15
Decibels and loudness80 dB is not twice 40 dB3:31
Loudness and distancetwice as far, four times quieter3:27
Anti-noisemore sound and yet quieter2:31
The ear and hearing thresholdzero decibels is not silence5:38
Hearing damage and risk thresholdno pain does not mean safe6:23
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.