Consumer Electronics

ECHOES

Echoes are produced by the reflection of sound from a hard surface such as a wall or cliff.

Let us suppose that a person claps his hands when standing some distance from a high wall and listens for the echo. The time which elapses before the echo arrives will depend on the distance from the wall. In order that the echo may be heard separately from the original clap it must arrive at least 0.1 second later. Since sound travels at about 330 m/s, the reflected wave must have travelled a total distance of at least 33 m. Consequently, the minimum distance of the wall must be about 17m.

When the reflecting surface is less than this distance the echo follows so closely upon direct sound wave that they cannot be distinguished as separate sounds. ...