We hear an echo due to

An echo is the repetition of sound caused by the reflection of sound waves from a distant surface like a wall, cliff, or building. For an echo to be heard distinctly, the reflecting surface must be at least 17 meters away from the source, because the human ear can distinguish two sounds only if they are separated by at least 0.1 second, and sound travels at about 343 m/s in air. The phenomenon is purely due to reflection, not refraction, diffraction, or resonance.