What is an echo?

Shout in a big empty hall or near a hill and you hear your voice come back. That is an echo.

Sound travels through the air as invisible waves. When the waves hit a hard flat surface — a wall, a cliff — they bounce back, like a ball thrown at a wall.

If the wall is close, the sound comes back so fast that it mixes with your voice and you cannot hear it separately. If the wall is far — about 17 metres or more — the sound takes long enough to return that you hear it as a clear echo.

Bats and dolphins use echoes to see in the dark: they make sounds and listen for the bounce. Ships use the same trick, called sonar, to measure how deep the sea is.