Echolocation and sonar
Finding things by sending out a sound and timing the echo, as bats, dolphins, orcas and ships' sonar all do.
Echolocation means sending out a sound and listening for the echoes that bounce back from objects around you. The delay tells you how far away something is, because the sound has to travel there and back; the direction and character of the echo reveal where it is and something about its shape.
Many bats and the toothed whales, including dolphins and orcas, hunt this way. Bat calls range from about 11 kHz to 212 kHz, mostly far above what humans can hear. The American zoologist Donald Griffin, who with Robert Galambos showed how bats do it, had coined the word echolocation by 1944.
Ships do the same with sonar and the echo sounder. A pulse of sound is sent down, and the time until the echo returns from the seabed is halved and multiplied by the speed of sound in seawater, about 1,500 metres per second. An echo that takes two seconds therefore means water about 1,500 metres deep. Thousands of such soundings, plotted as profiles of the sea floor, let oceanographers such as Marie Tharp map the ridges and rift valleys of the ocean bed.
The usual slip is to forget the round trip: the echo's delay covers twice the distance to the target, so it must be divided by two.
Coming up in future stories
- The Killer Whales That Hunted With Humans 1840s–1930 · Eden, New South Wales, Australia
- The Map Her Boss Called 'Girl Talk' 1952-1977 · Lamont Geological Observatory, New York
- The Shrimp That Jammed the Navy's Sonar 1940s · Pacific Ocean, US Navy