How do noise-canceling headphones block out sound?

Noise-canceling headphones use a technique called active noise control. Tiny microphones on the headphones continuously pick up ambient sound from the environment. The headphone's processor then generates a sound wave that is the exact mirror image of the incoming noise, meaning it has the same amplitude but opposite phase. When this anti-noise wave combines with the original sound, the two waves cancel each other out through destructive interference, significantly reducing what you hear. This works best on consistent, low-frequency sounds like airplane engine hum or air conditioning. Higher-pitched or sudden sounds are harder to cancel in real time, so most headphones also use passive noise isolation, where the physical ear cups simply block sound through physical barriers like foam or padding.