Question

What is an echo..? what is the minimum distance needed to hear a echo?

Answer 1

Echo is the reflection of sound due to a surface medium.

The minimum distance for an echo should be 17.2m between the source of sound and the surface.

The time game is minimum of 0.1 seconds.

Answer 2

The sound heard after reflection from a distant obstacle after the original sound has ceased is called echo.
To hear an echo distance should be 17 metre