Echo

a sound returned to its source. If a person speaks against a wall or cliff the wave of sound is sent back again, and the speaker hears the sound of his own voice as though someone in the distance were calling to him. Under ordinary conditions sound travels at a rate of 1,080 feet per second. The time elapsing between the call and the return of an echo gives a clue to the distance of the reflecting surface. The reflecting surface must be a concave or, if flat, it must stand squarely across the path of the sound, or else the sound wave will glance off in another direction and fail to return to the speaker. Many caves and mountain passes have celebrated echoes. Some public halls echo to such an extent that it is difficult to address an audience. In such cases cloth hangings are a help.