How Noise-Canceling Headphones Work

Noise-canceling headphones don't simply block outside sound the way a thick pair of earmuffs would. Instead, they actively fight the noise with more noise, using a principle called destructive interference. A tiny microphone built into the headphones constantly listens to the sound arriving from the outside world, particularly steady, repetitive sounds like an airplane engine's hum or a car's road noise. A processor inside the headphones analyzes that incoming sound wave and, in a fraction of a second, generates a new sound wave that's an exact mirror image of it -- identical in shape but flipped upside down. When the two waves reach your ear at the same moment, the peak of one wave lines up with the dip of the other, and they cancel each other out almost completely, leaving near silence where the noise used to be. This technique works best on low, constant sounds, since their steady, predictable pattern is easy for the processor to mirror quickly and accurately. Sudden or highly variable sounds, like a dog barking or someone talking, change too fast for the system to keep up, which is why noise-canceling headphones are far more effective at muting an engine's drone than a conversation happening nearby. Some higher-end models also use microphones on the inside of the ear cup, allowing the system to check how well the cancellation actually worked and adjust the mirrored wave in real time.