How do noise-cancelling headphones work?
The short answer
Noise-cancelling headphones work by using microphones to pick up ambient noise and then producing sound waves that are the exact inverse, effectively cancelling out the unwanted noise.
The long answer
Whether or not you own a pair yourself, you've probably heard of noise-cancelling headphones.
While it may seem magical how noise fades away, the technology behind noise-cancelling headphones has to do with the physics of sound waves.
All the sounds we hear — from a train screeching to birds singing to the cracks of your knuckles — travel through the air as sound waves. The shape of a sound wave depends on its pitch and volume. Higher pitched sounds have higher wave frequencies and louder sounds have taller waves, and vice versa.
Things get interesting when sound waves overlap. Now imagine you're at a choir concert and the choir starts singing the same note. The result is a louder sound. This phenomenon is called constructive interference.
Here's a diagram of two sounds of the same pitch, with aligned peaks and valleys. The result is a taller wave, or a louder sound of the same frequency.
When the peaks and valleys of sound waves overlap, the result is a louder sound.
But what if you flipped the other sound wave upside down? That would mean Sound #1's peaks would align with Sound #2's valleys. This is what is known as destructive interference.
When the peaks and valleys of sound waves are inversely positioned, the result is a muted sound.
When you combine Sound #1 with its exact inverse wave, the result is no sound at all. Active noise cancellation, the core technology behind noise-cancelling headphones, relies on this principle of destructive interference.
Here is the step-by-step process of how noise-cancelling headphones work:
Step 1: Ambient noise is picked up by the headphones' microphone.
When turned on, your noise-cancelling headphones are always listening. Its microphone is picking up on the frequency and amplitude of any noise in your surrounding area, otherwise known as ambient noise. Noises like your neighbor's lawnmower, your fellow airplane passenger's snoring, or the espresso machine at your favorite cafe are all being captured on the mic.
Step 2: A digital signal processor calculates the inverse sound wave of the ambient noise.
Next, the digital signal processor takes the data provided by the microphone and calculates the exact inverse frequency and amplitude of the ambient sound waves.
The "anti-noise" determined by the signal processor is 180° "out of phase" of the incoming sounds. In other words, the inverse sound waves have valleys where the ambient sound waves have peaks.
Step 3: The headphones produce sound waves that "cancel" out the ambient noise.
"Active Noise Reduction" by Marekich is licensed under CC BY-SA 3.0.
The "anti-noise" sound wave determined by the signal processor then feeds this into the headphone speakers, along with any music you have playing. The opposing sound waves create destructive interference, cancelling out the ambient noise.
When does noise cancellation not work?
Noise cancellation can reduce noise by up to 80 decibels, but it isn't perfect. Sudden, high-frequency, or loud sounds can make it challenging for your noise-cancelling headphones' to keep up with producing the inverse sound waves. That's why noise cancellation is best for low frequency, predictable sounds, like the cabin of an airplane.
🧠 Bonus brain points
What is the difference between "noise cancelling," "noise blocking," and "noise masking"?
Noise cancelling uses destructive interference to eliminate ambient noise by generating opposing sound waves. Noise blocking relies on the physical design and materials of the headphones to muffle sounds. Noise masking produces white noise to drown out ambient noise.
Check out some other curious questions:
Sources
Burns, V. (2020, October 18). How do noise-cancelling headphones cancel sounds?. BBC Science Focus Magazine. https://www.sciencefocus.com/science/how-do-noise-cancelling-headphones-cancel-sounds
Darby, C. (2023, March 7). How do noise-canceling headphones work?. ZDNET. https://www.zdnet.com/article/how-do-noise-canceling-headphones-work/
National Center for Families Learning. (n.d.). How Do Noise-Canceling Headphones Work?. Wonderopolis. https://wonderopolis.org/wonder/how-do-noise-canceling-headphones-work
Sony. (2023, May 10). What is noise cancellation and what can I expect?. Sony. https://www.sony.co.uk/electronics/support/articles/00203389
Walsh, T. (n.d.). Superposition of Transverse Waves. oPhysics: Interactive Physics Simulations. https://ophysics.com/w3.html