Phonics or Acoustics. A science which treats of the nature and mode of propagation of sound. Whenever any elastic body is made to vibrate, it produces corresponding vibrations in the air surrounding it; these acting on the ear, cause its internal parts to vibrate and excite in us the sensation of sound. From the necessity of air to the conveyance of sound in ordinary experiments, the phenomena of sound have usually been considered as forming part of the science of Pneumatics; but the entire difference of its results, and its connexion with elastic bodies generally, sufficiently justify its claim to a separate denomination. That sound cannot be conveyed from one part of space to another without some material connexion, is well ascertained, and may, to a certain extent, be proved, by suspending a bell within a glass receiver of the air pump, and exhausting the air; it will then be found that as the air is withdrawn, the sound of the bell becomes more and more feeble, so that, at last, it is scarcely audible. That air is not the only conductor of sound may be shown by various experiments.
If a heavy mass of iron, as a kitchen poker, be suspended by a piece of twine, the two ends of which are pressed against the ears, and the poker be then struck against any metallic substance, as a fender, a sound will be heard of so great intensity as to resemble the tolling of a bell. If two stones are struck against each other under water, the sound may be heard at a great distance by plunging the head beneath the surface of the water: Dr. Franklin affirms that he has heard it in this way at the distance of half a mile. Sounds are also transmitted to great distances through solid bodies. If a slight scratch be made at one end of a long piece of timber, and the ear be applied to the other, a distinct sound will be heard. In this manner minershearthe sounds made of their fellow-workmen, and thus judge of theirdirec-tion. If a person be placed at one end of a series of metallic tubes, the blows of a hammer at one extremity are heard distinctly at the other, two sounds being heard, one conducted by the air, and the other by the metal.
To show that sound is really the effect of vibrations in the bounding body, carried to a certain degree of rapidity, a long string, or wire, may be stretched by a small weight; the vibrations at first will be distinctly seen, and may be counted; but in this case they produce no perceptible sound. If the weight which extends the cord be increased, the vibrations become more rapid, and a sound is heard which becomes more acute as the string is more stretched.
By a number of experiments it has been found that sound travels through the air with a velocity of about 1140 feet in a second, or nearly 13 miles in a minute. The velocity with which sound travels may be easily proved by a simple experiment. Let a gun be fired at a given instant, and let a person placed at a known distance, observe the time elapsed before he hears the report, and this will determine the time the sound has been travelling over the given space. By knowing the velocity with which sound travels, we may ascertain the distance of a thunder-cloud, or of a ship in distress. Suppose the light from a gun fired at a distance, or from a flash of lightning to be observed at a given instant, and that five seconds elapse between seeing the flash and hearing the report; then since the motion of light may be considered as instantaneous, the time of seeing the flash may be taken as the instant at which the sound sets out; and as it travels 1142 feet in a second, the space passed over in five seconds will be 1142 + 5 = 5710 feet, or 1 mile and 430 feet, which will be the distance of the object from which the sound proceeds. According to Dr. Thomas Young, the velocity of sound, on an average, is 1130 feet.
The sound of a gun, or of a hammer, is equally swift in its motion; the softest whisper flies as swiftly as the loudest thunder. The equal velocity of the different tones was beautifully shown by Biot in experiments on the pipes of the aqueducts at Paris, a distance of about 3000 feet; an air was played on a flute at one extremity, and listened to at the other end, and the time was perfectly preserved; and hence the equal velocity of the various notes was demonstrated. Different substances transmit sound with different velocities. If the velocity in air be represented by 1, the velocity in rain-water will be 4 1/2, in sea-water 4 7/10, and in brass 10 1/2. The velocity of sound is uniform. The strength of sounds is greatest in cold and dense air, and least in that which is warm and rarefied. Every point against which the pulses of sound strike becomes a centre, from which a new series of pulses is propagated in every direction. Sounds may be reflected like light, and thus form what is termed an echo. For the most powerful echo the sounding body should be in one focus of the ellipse, which is a section of the echoing spheroid, and the hearer in the other. An echo may, however, be heard in other situations, though not so distinctly.
Thus a person often hears the echo of his own voice; but for this purpose he should stand at least 63 or 64 feet from the reflecting obstacle. At the common rate of speaking, we pronounce about seven syllables in a second; in order, therefore, that the echo may return just as soon as three syllables are expressed, twice the distance of the speaker from the reflecting surface must be equal to 1000 feet; for as sound describes 1142 feet in a second, six-sevenths of that space, that is, 1000 feet nearly, will be described, while six, half, or three whole syllables are pronounced; that is, the speaker must stand nearly 500 feet from the obstacle. In general, the distance of the speaker from the echoing surface for any number of syllables must be equal to the seventh part of the product of 1142 feet multiplied by that number. When the walls of a passage, or of an unfurnished room, are smooth and perfectly parallel, any explosion, or a stamping with the foot, communicates an impression to the air, which is reflected from one wall to the other, and from the second to the ear, by which reverberation the primitive sound is greatly increased in intensity.
Sound, like light, may be reflected from several places, and collected in one point as into a focus, and it will be there more audible than at the place from whence it proceeded. On this principle the whispering gallery is constructed, the form of which must be that of a concave hemisphere. Somewhat similar is the effect of speaking and hearing trumpets, which, by reverberating the sounds uttered through them, increase their intensity. By means of an arrangement of these, a deceptive acoustic experiment was exhibited, an idea of which may be formed by the following sketch. It was pretended that the invisible girl was within the ball d; and whenever a person, by applying his mouth to either of the trumpets e, put a question, an answer was returned which seemed to proceed from the ball, in the centre of which the invisible being was supposed to reside. A reference to the cut will explain the manner in which the illusion was accomplished. The upper part of the frame-work is hollow; and by means of the tube g g, passing through the leg of the apparatus and floor of the room into another chamber, communicates with another individual h, who is to represent the invisible girl.
When a person is desirous of trying the experiment, he applies his mouth to either of the trumpet-mouths e e e e, and puts his question; the sound so uttered is reflected so as to pass through the holesff, and through the pipe gg to h, where it is heard by the person who is then listening. A reply is then given through the tube h gg, which, coming out through the holef, is received in the trumpet-mouths, and reflected to the ear of the inquirer at e. The trumpets being suspended by silken strings, no visible connexion appears between the place whence the sound seems to proceed, and the individual who is the author of it; the illusion is therefore complete.