what is an echo? what are the two conditions required to hear an echo?

Echo : An echo can be defined as the phenomena of the reflection of sound produced by a source placed in a relatively open environment after it hits a distant obstacle like a cliff etc. The sound waves that have bounced back can be heard at the source a few seconds after it was produced.

(1) The echo must reach our ears atleast after 1/10 seconds we hear the original sound, otherwise we will not be able to distinguish it from the original sound as a sound stays ( persists) in our brain for about 1/10 of a second ( persistence of sound).

(2) The source of sound and the reflecting surface should be atleast 17 m apart for us to hear the echo.