What are active noise-cancelling headphones?

Active noise-canceling headphones both play sound and block ambient noise. The headphones combine a noise-blocking structure with a system that erases lower-frequency sound waves by generating a sound wave identical to the incoming sound, but flipped 180 degrees out of phase. The system may reduce noise by 20 decibels.

To generate an out-of-phase sound wave, active noise-canceling headphones are constructed with an external microphone, noise-canceling circuitry, a speaker and a rechargeable battery. The microphone detects the incoming external sound, then the circuitry generates the new sound wave and plays it through the speaker. All of the components are powered by the battery.