Silence made from sound

The wall that fails

Inside a jet, the engines roar at about eighty decibels, for hours on end. So you put on headphones. The obvious fix is a wall: a hard shell and thick foam between the roar and your ear. And the wall does stop the hiss. Short, fast waves die in the foam. But the deep rumble has waves metres long. They shove the whole cup back and forth, and pass straight through.

The mirror

So these headphones do something that sounds backwards. They add more sound. Sound is air pressure, rising and falling. Here is the rumble arriving at your eardrum: push, pull, push, pull. The speaker is about to play the same wave upside down: a pull for every push.

Now it plays. Every push meets an equal pull, and the air at your eardrum barely moves. No movement, no sound. That stillness is the quiet you hear. But it has to be exact. If the mirror arrives even a little late, the pushes and pulls stop lining up, and the rumble leaks back.

Making it work

So the headphones race the sound. A microphone on the outside hears the rumble first. A chip flips it, and the speaker plays the mirror before the rumble is through the shell. A slow, steady wave is easy to match. That is why the engine drone vanishes. A voice or a clatter changes too fast to mirror. Those are left to the foam, so a little still gets through. The quiet in your headphones isn't less sound. It is two sounds, perfectly opposed.