Question

state two conditions for the formation of an echo.

Answer 1

 To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 s.
(ii) For hearing distinct echoes, the minimum distance of the obstacie from the source of sound must be 17.2 m.
Echo is a repetition of sound due to the reflection of original sound by a broad and hard obstacle.

Answer 2

Intensity of sound should be sufficient