How do noise-canceling headphones block out background sound?

Noise-canceling headphones use a technique called active noise control. Tiny microphones on the outside of the headphones continuously sample the surrounding ambient sound. The headphone's internal processor then generates a sound wave that is the exact mirror image of that incoming noise, meaning it has the same amplitude but an inverted phase. When this anti-noise signal is played through the speakers alongside your audio, 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. Sudden or high-frequency sounds are harder to cancel in real time. The physical earcup design also provides passive noise isolation by simply blocking some sound mechanically.