The technique used by bats, dolphins and porpoises to locate objects by using reflected sound is called:

Echolocation, also called bio-sonar, is the method used by some animals to find their way and to locate objects, including prey, by sending out sound waves and listening to the echoes that bounce back from objects around them. By judging the time taken for an echo to return and its direction and quality, the animal can tell how far away an object is, its size, shape, texture and even its movement.

Animals that use echolocation:

Bats: Most bats (especially microbats) emit high-frequency ultrasonic sounds (above 20,000 Hz), usually through their mouths or noses, while flying in the dark. This allows them to hunt insects and avoid obstacles at night. The Italian scientist Lazzaro Spallanzani showed in the 1790s that bats navigate using their ears, and the American scientist Donald Griffin described echolocation in bats in the 1940s and coined the term.

Dolphins, porpoises and toothed whales: they produce clicks, using special structures in their heads, and receive the echoes through their lower jaws. Echolocation helps them hunt fish in murky water.

Some birds, such as oilbirds and swiftlets, and some shrews also use simple forms of echolocation.

Humans use the same principle in technology:

SONAR (Sound Navigation and Ranging): uses ultrasonic waves to detect underwater objects such as submarines, shipwrecks and shoals of fish, and to measure the depth of the sea (echo sounding). The depth is calculated as d = (v × t)/2, where v is the speed of sound in water and t is the time taken for the echo to return.

Ultrasound scanning (sonography) in medicine: used to see images of internal organs and of the foetus during pregnancy.

RADAR works on a similar principle but uses radio waves instead of sound.

An echo is the reflection of sound. For a distinct echo to be heard by humans, the reflected sound must reach the ear at least 0.1 second after the original sound; in air, the reflecting surface must be at least about 17.2 metres away.

Why not the other options?

• ✗Refraction is the bending of waves when they pass from one medium to another.
• ✗Resonance is when an object vibrates with large amplitude at its natural frequency.
• ✗Interference is the combination of two or more waves.