Ultrasound and echolocation

Infrasonic waves (below 20 Hz) travel long distances through air and the ground. Instruments use them to detect earthquakes, volcanic eruptions and severe storms.

Ultrasonic waves (above 20 kHz) have many uses:

• ultrasonography: seeing organs inside the body without surgery;
• breaking up kidney stones;
• welding and cleaning delicate machine parts;
• finding hidden cracks inside metal blocks;
• locating objects.

Echolocation. Most bats fly in the dark by sending out short bursts of ultrasound and listening for the echoes from walls, trees and the insects they hunt. Dolphins, whales and some birds echolocate too.

Sonar (sound navigation and ranging) does the same under water. A ship sends ultrasound down; the echoes tell it the distance, direction and speed of things like submarines or shipwrecks, or how deep the sea is.

The same rule as any echo: halve the time. Ultrasound in sea water (1530 m s⁻¹) returns after 0.90 s. It took 0.45 s to reach the object, so the object is 1530 × 0.45 = 688.5 m away.

Listening for drones. Very sensitive sensors can pick up the low-frequency hum of drones and aircraft engines, to keep watch over the sky.