What is an echo ?write the …

The phenomenon of repetition of a sound due to its reflection from the surface of a large obstacle is termed as echo.
For example, when a loud sound is produced in an empty hall, the same sound is heard again after some time. 
To produce an echo, the conditions required a are: 
(i) There should be sufficient time gap between original and reflected sounds. So, if we want to hear distinct echo, the reflected sound must reach the ear 0.1 s after the direct original sound.
(ii) The distance between the source of sound and the obstacle should be sufficient. An echo will be heard distinctly if the minimum distance between the source of sound and the obstacle is 34.4/2 = 17.2 m.
(iii) Nature of the obstacle plays an important role. For the formation of echo, the reflecting surface must be rigid such as a building, hill or a cliff.
(iv) Size of the obstacle should be quite large for the echoes to be heard.