Infographysics: Physics of noise cancelling

Noise cancelling headphones combine passive and active techniques to reduce environmental noise and give the user a focused listening experience. With a Fourier transform (1), headphones can access frequency information of the signal and passively remove the low-frequency noise with a high-pass filter (2). The obtained sound can still contain unwanted frequencies disturbing the user's experience, which can be removed by active filters. Modern headphones can distinguish noise from signal based on their temporal behaviour (3) and produce an additional sound wave adjusted to the environmental noise. Overlapping this artificial sound to the environmental noise produces destructive interference, which eliminates all remaing noise.