The sound we can’t hear

By Oliver Morton

Untrue as it is, there is something wonderful about the notion that you can hear the ocean when you hold a conch to your ear—a synecdoche of shell for sea, a beguiling fantasy that if you listen hard enough you can hear the womb-sound of the world. The systems used to study infrasound are less poetic; the ones I’ve seen look like little tarmac car parks with sawn-off drainpipes sticking up out of them. But they make the seashell’s promise real. They let you hear far-off oceans.

Infrasound is, like sound (and, for that matter, ultrasound), a matter of vibration. The difference is that infrasound vibrations are slower, which makes the pitch of the sound too deep for the human ear to make out—and the distance the sound can travel much longer. Nine-tenths of the energy in a sound wave at a frequency of 1,000HZ (the C above middle C, more or less) is absorbed by the air within seven kilometres. Absorbing that much of the energy in an infrasound signal at 1HZ takes 3,000 kilometres.