How Do You See With Sound?

Step 1 of 5

Anything that can shout and listen can measure.

An echo comes back late by an amount that depends only on how far it went, so timing it is measuring. Bats do it, dolphins do it, oilbirds and cave swiftlets do it, and some shrews do it. Blind people who click their tongues do it well enough to walk a strange corridor.

Step 2 of 5

Water is by far the better place to do it.

Sound moves about four and a half times faster in water than in air and fades much less as it goes. A dolphin can find a fish buried under sand, which is a good deal more than eyes could manage down there.

Step 3 of 5

To find something small you need a very short sound.

A sound cannot report anything smaller than its own wavelength — a long, low note simply washes around a moth and comes back as though it were not there. The calls are pitched high because high means short, not because high means loud.

Step 4 of 5

The hard part is not deafening yourself.

The shout leaves at deafening volume and the echo comes back millions of times quieter, and the same ear has to survive one and detect the other. A bat clenches the muscles in its middle ear as it calls and releases them in time to listen.

Step 5 of 5

Ships and hospitals do exactly the same arithmetic.

A sonar set pings the sea floor and times the return; an ultrasound scanner pings a baby and times thousands of returns to build a picture. Neither was copied from an animal, and both landed on the same answer, because there is only one answer.

A sperm whale's click is the loudest sound any animal makes — loud enough, close up, to be dangerous to a swimmer.