# How noise cancelling works: it fights noise with more noise > Noise-cancelling headphones don't block the noise: they add its exact opposite, so the two cancel to near silence. A mic outside hears it, a chip flips it and the speaker plays it, in tens of microseconds; a mic inside fixes what's left. A 49-second explainer, every figure sourced. - URL: https://oliverz.dev/videos/noise-cancelling - Author: Oliver Zhang (https://oliverz.dev) - Language: en - Published: 2026-11-01 - Tags: audio, how-it-works, explainer - Video: https://www.youtube.com/shorts/aywLhNwXALU > **In short:** noise-cancelling headphones don't block the noise, they add more sound. Sound is a wave of high and low pressure; add its exact opposite, low wherever it's high, and the two cancel out to near silence. A microphone outside the cup hears the noise coming, a chip flips it and the speaker plays the mirror image, all in the tens of microseconds before the noise itself reaches your ear. A second microphone inside checks what's left and corrects it. That race is why it works best on a steady, low hum and much less on voices and sudden sounds, which the cushions have to block instead. ## Sound against sound BBC Science Focus: "Noise-cancelling headphones cancel out unwanted sound by creating an opposing sound wave that mimics the noise you want to get rid of, but just 180° out of phase. This means that when one sound wave is at its highest peak, the other is at its lowest, effectively cancelling each other out. This is known as 'destructive interference'." ## High pressure meets low pressure The Physics Classroom: "Sound is a pressure wave that consists of compressions and rarefactions." A compression is "a high-pressure region", a rarefaction "a low-pressure region". When the two line up against each other: "The tendency of the compression to push particles together is canceled by the tendency of the rarefactions to pull particles apart; the particles would remain at their rest position as though there wasn't even a disturbance passing through them." So the "two sound waves would combine to produce no sound." ## Hear it, flip it, play it The Physics Classroom again: "Such earphones capture sound from the environment and use computer technology to produce a second sound wave that one-half cycle out of phase." BBC Science Focus on the parts: "First, there is a microphone on the earcup to listen out for annoying external sound." Then "the 'out-of-phase' sound is created and then fed into the headphone speakers, along with the music you're playing." ## A race of tens of microseconds IEEE Spectrum (2018): "having microphones located so close to the ears means commercial headphones get just tens of microseconds to process incoming sound, calculate an antinoise signal, and play it through their speakers before the sound reaches the ears." Sound in air travels at "v = 343 m/s" at 20 °C (The Physics Classroom). Over an illustrative 2 cm, the noise takes about 58 µs to cross the cup (our arithmetic: 2 cm ÷ 343 m/s; no source gives a cup's depth). ## A second mic checks what's left SoundGuys: "With feedforward technology, the noise-capturing microphones are placed on the outside of the headphones." And: "With feedback ANC, the microphone is located inside the ear cup", so "You can think of feedback ANC as a self-correcting mechanism." Hybrid ANC "combines both feedforward and feedback microphones and processing". ## Best on a steady hum The deadline is also the limit. IEEE Spectrum: "Such a tight computing deadline limits the performance of most active noise-cancellation technology, setting an upper limit on the frequencies it can actively cancel. This requires many headphones to shield the wearer's ears with sound-absorbing materials for higher frequencies." One premium headset in its 2018 report had an "active noise-canceling range of 0 to 1 kHz" and relied on "passive soundproofing material to dampen sounds in the 1- to 4-kHz range". SoundGuys: "Active Noise Cancellation (ANC) systems work best on droning sounds that are constant in frequency and intensity. Because quick, irregular sounds like music sounds and people talking are over with long before your headset can calculate how to cancel them, the only noise attenuation your ANC headset will offer against annoying coworkers/TV noise is simply what sound they can physically block out." BBC Science Focus: "It works best with lower frequencies, like transport noise". ## Near silence, not silence SoundGuys: "you won't ever see 100% cancellation". BBC Science Focus: "while it can't erase unwanted sound entirely". ## Takeaway 1. Noise cancelling is sound against sound: the opposite wave adds up to near silence. 2. It's a race: hear, flip and play in tens of microseconds, before the noise crosses the cup; a mic inside corrects what's left. 3. So it's great on a steady hum and weak on voices and sudden sounds: for those, the fit and the cushions do the work. ## Sources - BBC Science Focus, [How do noise-cancelling headphones cancel sounds?](https://www.sciencefocus.com/science/how-do-noise-cancelling-headphones-cancel-sounds/), 18 Oct 2020 - J. Hsu, IEEE Spectrum, [New Wireless Noise-Canceling Tech Is Faster Than the Speed of Sound](https://spectrum.ieee.org/noisecanceling-techs-edge-is-faster-than-the-speed-of-sound), 5 Sep 2018 - C. Thomas, SoundGuys, [Active noise canceling (ANC) technology types explained](https://www.soundguys.com/noise-canceling-explained-28344/), updated 16 Oct 2025 - The Physics Classroom: [Interference and Beats](https://www.physicsclassroom.com/class/sound/Lesson-3/Interference-and-Beats), [The Speed of Sound](https://www.physicsclassroom.com/class/sound/Lesson-2/The-Speed-of-Sound) The video is animated with code and the script was written with AI assistance; every quote above is checked against its source, and the 58 µs marked "our arithmetic" is ours, over an illustrative 2 cm. Illustrative: the waveforms, the earcup's drawing and its parts, the 2 cm and the headset. The 0–1 kHz and 1–4 kHz ranges are one premium headset reported in 2018 (IEEE Spectrum), not every headset. The video is narrated in English. Made with [Shotloop](https://oliverz.dev/products/shotloop), my free Mac app for code-made explainers, fixed one shot at a time.