Echo

Echo (Gr., 'sound'). Sound is produced by waves of compression and rarefaction in the air; when such a wave comes against a wall or other opposing surface, it is reflected, and proceeds in another direction, and the sound caused by this reflected wave is called an echo. Even the surface of a cloud suffices to reflect sound, as may be observed during thunder and the discharge of cannon. That the echo of a sound may return to the point where the sound originated, the reflecting surface must be at right angles to a line drawn to it from that point. Oblique walls send the echoes of a person's voice off in another direction, so that they may be heard by others, though not by him. In order to echo words distinctly, the reflecting surface must on the whole be plane, or so curved as to resemble a concave mirror. A curved form is necessary for returning a distinct sound when the distance is considerable. Great evenness of surface, however, is not essential, as it is no uncommon thing for the edge of a wood to return an echo. The distance of the reflecting surface must also be such as to allow a sufficient time to elapse between the sound and the return of the echo for the ear to distinguish them; when they succeed too closely, they merge into one. An interval of about $\backslash frac\{1\}\{2\}$ of a second is necessary to discriminate two successive sounds; so that if we assume 1125 feet as the distance traversed by sound in a second, $\backslash frac\{1\}\{2\}$ of 1125, or 62 feet, will be the least distance at which an echo can be heard, as the sound will go that distance and return in $\backslash frac\{1\}\{2\}$ of a second. If the distance is less, the echo only clouds the original sound, and is not heard distinct. It is these indistinct echoes that interfere with hearing in churches and other large buildings (see ACOUSTICS); hence anything that breaks the evenness and continuity of the reflecting surfaces is an improvement in this respect. The number of syllables that any particular echo will repeat, depends upon how many can be uttered in the time that the sound takes to go to and return from the reflecting surface. The echo at the tomb of Metella, in the Campagna, near Rome, of which Gassendi speaks as repeating a hexameter line requiring $2\backslash frac\{1\}\{2\}$ seconds to utter it, must therefore come from a distance of about 1500 feet. Such echoes are rare, as the various conditions are seldom all fulfilled. When there happen to be several reflecting surfaces at different distances in the direction of the sound, with a sufficient interval between them, each gives a separate and distinct echo. A similar effect is produced when two surfaces are inclined to each other in such a way as to give repeated reflections of the sound from the one to the other like the mirrors of a kaleidoscope, thus producing echoes of echoes. To this multiple and repeating class belong the famous echoes of Killarney, and that produced between the wings of the castle of Simonetta, near Milan, which repeats the report of a pistol 60 times.