The sound heard following reflection off a distant obstacle (such as a cliff, a hillside, a building wall, etc.) after the initial sound has stopped is termed an echo.
There are two requirements that must be met for an echo to be clearly heard.
(i)The minimum distance between the sound source and the air reflector must be 17 meters.
(ii) The reflector’s size must be large enough in comparison to the sound wave’s wavelength.