(i) What is an echo?

(i) The sound heard after reflection from a distinct obstacle (such as a cliff, a hillside etc.) after the original sound has ceased is called an echo.

(ii) Conditions for an echo to take place are :

1. The size of the reflector must be large enough as compared to the wavelength of the sound wave.
2. The intensity of sound should be such that the reflected sound reaching the ear is sufficiently loud to be audible.