What Is an Echo?

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Sound spreads out in all directions.

A noise sends pressure waves outward everywhere at once, not in a beam. Some of them will hit whatever is around you.

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Hard flat surfaces reflect it.

Sound bounces off rock, brick and water rather than passing through. Soft surfaces absorb instead, which is why curtains kill echoes.

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The return trip takes time.

Sound travels about 340 metres a second. A wall 170 metres away means a round trip of 340 metres — one full second of delay.

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You need enough delay to hear it separately.

Below roughly a tenth of a second, your brain merges the reflection with the original. That is why close walls do not make echoes.

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Animals use it to see.

Bats and dolphins emit sound and time the return to build a picture of their surroundings. Ships and hospitals use the same principle.

Some blind people use tongue clicks to navigate, and brain scans show them processing the returns in visual areas.