Question

what is an echo?what is the minimum distance of obstacles from the source of sound for hearing distance echoes​

Answer 1

It is the phenomenon of repeation of sound due to its reflection from the surface of a larger object optical for hearing distinct sound in the time interval between the original sound and the reflected one must be reach in 0.1 sec.

Answer 2

An echo is a sound that is repeated because the sound waves are reflected back. ... Echoes can be heard in small spaces with hard walls, like wells, or where there are lots of hard surfaces all around.

minimum distance required is 17 m.