Short · Sound
How do noise-cancelling headphones work?
Headphones play the opposite of the noise, a wave that squeezes where the noise stretches. Get the timing wrong and the two add up, which is why low, steady hums are the easiest to cancel.
Opens Noise Cancelling Lab. Change the same thing yourself.
Transcript
How do noise-cancelling headphones make things quiet? Sound is a wave of pressure. Squeeze, stretch, squeeze. A tiny microphone listens to the noise outside, and the headphones play the opposite wave. Now slide the second wave into place. Where one squeezes, the other stretches, and they cancel out. Get the timing wrong, and they add up instead, making it louder. That's why it works best on low, steady hums like engines, and less well on voices. Try it yourself at Explainer Tools dot com.