How Do Noise-Cancelling Headphones Work?

Active noise cancellation is based on the principle of destructive interference: when two sound waves of equal amplitude but opposite phase (one positive where the other is negative) combine, they cancel each other out, producing silence. ANC headphones contain one or more external microphones that continuously sample ambient noise. A digital signal processor (DSP) analyses the incoming sound in real time (within microseconds) and generates an inverted (anti-phase) copy, which is played through the headphone speakers along with your music. Modern ANC systems also use internal microphones ("feedforward" and "feedback" configurations) to monitor and correct the cancellation in real time. ANC works best on steady, low-frequency sounds (airplane cabin noise, air conditioning hum) because their longer wavelengths are easier to predict and invert. High-frequency, irregular sounds (speech, sudden noises) are harder to cancel and are typically reduced by passive isolation (physical ear cup sealing). The technology was pioneered by Amar Bose in the 1980s.