ECHO, or ECCHO, a sound reflected or reverberated, from a solid, concave, body, and so repeated to the ear. The word is formed from the Greek ἦχος, son, sound, which comes from the verb ἦξω, son.
The ancients being wholly unacquainted with the true cause of the echo, ascribed it to several causes sufficiently whimsical. The poets, who were not the worst of their philosophers, imagined it to be a person of that name metamorphosed, and that she affected to take up her abode in particular places; for they found by experience, that she was not to be met with in all. (See below, ECHO in fabulous history.) But the moderns, who know sound to consist in a certain tremor or vibration in the sonorous body communicated to the contiguous air, and by that means to the ear, give a more consistent account of echo.
For a tremulous body, striking on another solid body, it is evident, may be repelled without destroying or diminishing its tremor; and consequently a sound may be redoubled by the restitution of the tremulous body, or air.
But a simple reflection of the sonorous air is not enough to solve the echo: for then every plain surface of a solid hard body, as being fit to reflect a voice or sound, would redouble it; which we find does not hold.
To produce an echo, therefore, it should seem that a kind of concavitation or vaulting were necessary, in order to collect, and by collecting to heighten and increase, and afterwards reflect, the sound; as we find is the case in reflecting the rays of light, where a concave mirror is required.
In effect, as often as a sound strikes perpendicularly on a wall, behind which is any thing of a vault or arch, or even another parallel wall, so often will it be reverberated in the same line, or other adjacent ones.
For an echo to be heard, therefore, it is necessary the ear be in the line of reflection: for the person who made the sound to hear its echo, it is necessary he be perpendicular to the place which reflects it: and for a manifold or tautological echo, it is necessary there be a number of walls, and vaults or cavities, either placed behind or fronting each other.
A single arch or concavity, &c. can scarce ever stop and reflect all the sound; but if there be a convenient disposition behind it, part of the sound propagated thither, being collected and reflected as before, will present another echo: or, if there be another concavity, opposed at a due distance to the former, the sound reflected from the one upon the other will be tossed back again by this latter, &c.
Many of the phenomena of echoes are well considered by the bishop of Leighs, &c. who remarks, that any sound, falling either directly or obliquely on any dense body of a smooth, whether plain or arched, superficies, is reflected, or echoes, more or less. The surface, says he, must be smooth; otherwise the air, by reverbera-
tion, will be put out of its regular motion, and the sound thereby broken and extinguished. He adds, that it echoes more or less, to show, that when all things are as before described, there is still an echoing, though it be not always heard, either because the direct sound is too weak to beat quite back again to him that made it; or that it does return to him, but so weak, that it cannot be discerned; or that he stands in a wrong place to receive the reflected sound, which passes over his head, under his feet, or on one side of him; and which therefore may be heard by a man standing in the place where the reflected sound does come, provided no interposed body intercepts it, but not by him that first made it.
Echoes may be produced with different circumstances. For, 1. A plane obstacle reflects the sound back in its due tone and loudness; allowance being made for the proportionable decrease of the sound, according to its distance.
2. A convex obstacle reflects the sound somewhat smaller and somewhat quicker, though weaker, than otherwise it would be.
3. A concave obstacle echoes back the sound, bigger, slower, and also inverted; but never according to the order of words.
Nor does it seem possible to contrive any single echo, that shall invert the sound, and repeat backwards; because, in such case, the word last spoken, that is, which last occurs to the obstacle, must be repelled first; which cannot be. For where in the mean time should the first words hang and be concealed; or how, after such a pause, be revived, and animated again into motion?
From the determinate concavity or archedness of the reflecting bodies, it may happen that some of them shall only echo back one determinate note, and only from one place.
4. The echoing body being removed farther off, it reflects more of the sound than when nearer; which is the reason why some echoes repeat but one syllable, some one word, and some many.
5. Echoing bodies may be so contrived and placed, as that reflecting the sound from one to the other, either directly and mutually, or obliquely and by succession, out of one sound, a multiple echo or many echoes shall arise.
Add, that a multiple echo may be made, by so placing the echoing bodies at unequal distances, that they may reflect all one way, and not one on the other; by which means, a manifold successive sound will be heard; one clap of the hands, like many; one ha, like a laughter; one single word, like many of the same tone and accent; and so one viol, like many of the same kind, imitating each other.
Lastly, Echoing bodies may be so ordered, that from any one sound given, they shall produce many echoes different both as to tone and intention. By which means a musical room may be so contrived, that not only one instrument playing therein shall seem many of the same sort and size, but even a concert of different ones, only by placing certain echoing bodies so, that any note played shall be returned by them in 3ds, 5ths, and 8ths.