13.4 Reflection of Sound (Echo)
When we clap or shout near a tall building or mountain, we hear the sound return after a brief delay. This repeated sound is called an echo, caused by the reflection of sound waves from a surface.
lightbulbWhen sound waves hit a surface and bounce back into the same medium, the phenomenon is called an echo or reflection of sound.
The human brain retains sound for about 0.1 seconds, so to hear a distinct echo, the reflected sound must reach us after at least 0.1 seconds. Given the speed of sound in air (340 m s⁻¹), the minimum distance for an echo to form is 34 metres (total distance travelled by sound), meaning the reflecting surface must be at least 17 metres away. Echoes can also repeat multiple times due to successive reflections.