Echo

From Encyclopediak

Echo, Ek o, in Greek mythology, a mountain nymph who, having offended Juno, was deprived of the power of speech. All she could do was to repeat the sounds that she heard. Echo loved Narcissus, but her love was slighted and she pined away until nothing was left but her voice.

Echo, a sound so distinctly reflected that it seems to come from another source. Broad, flat surfaces, such as walls of buildings, ledges of rock, etc., often produce this effect. To hear the echo of one's own voice, a person must stand in a line perpendicular to the echoing surface, but to hear the echo of another's voice the two must take positions in lines making equal angles with a perpendicular to the reflecting surface. This is because sound waves are reflected in such a manner that the angle made by the original sound wave with a line perpendicular to the reflecting surface is equal to the angle made by the reflected wave and the perpendicular. An echo is distinct only when the distance of the sounding body from the reflecting surface is so great that the sound is complete before the echo returns. Distances may be judged by the length of time between a sound and the return of the echo. At ordinary temperatures an echo which returns in two seconds is reflected from a body about one-fifth of a mile distant. Echoes are simple if returned by a single reflecting surface and complex when the reflection is from two or more surfaces. A sound produced between parallel walls 100 ft. apart has been known to return 30 or 40 times before dying away. Among the mountains of Switzerland such echoes are common, and the yodel of the mountain guide, repeated again and again by the walls of rock, dies away in faintly reechoing musical notes.