Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

254. Sound not Transmitted through a Vacuum.—As sound is a vibration of some substance it can not be transmitted through absolute space. This can be proved by an experiment with the air-pump, as represented in Fig. 185. The apparatus in the receiver is so arranged that the bell, a, can be struck by pressing down a sliding rod, h. If it be struck before the air is exhausted the sound is heard through the glass. But the more you exhaust the air the fainter will be the sound; and at length, if you keep on pumping, it can not be heard at all. The same experiment can be tried with a music-box. It is from the thinness of the air on high mountains, and at the great heights reached by balloons, that all sounds are so faint. The report of a pistol fired off on top of Mont Blanc is a mere crack compared with its report when fired in the valley below.

255. Motion of the Heavenly Bodies without Noise.—Sound is often heard at a very great distance on the earth. The sound of an eruption of a volcano has been heard in one case at the distance of 970 miles. But suppose that the same sound should occur at the same distance from the earth, that is, over 900 miles beyond the atmosphere that enrobes the earth, no inhabitant of our world could hear it, for the same reason that you do not hear the bell ringing in an exhausted receiver. If, therefore, any sound, however loud, should be given forth by any of the heavenly bodies we could not hear it. The course of these bodies in their orbits is noiseless, because they meet with no resistance from any substance. Bodies passing rapidly through our atmosphere cause sound, from the resistance which the air gives to their passage. The whizzing of a ball is an example of this. It is the passage of the electric fluid through the air which produces the thunder. But the heavenly bodies, having no such resistance, make no sound in their course, though their velocity be so immense. In the expressive language of the Bible, "their voice is not heard."

256. Velocity of Sound.—The velocity of sound varies in different media. Thus it passes through water four times as rapidly as it does through air. Dr. Franklin, with his head under water, heard distinctly the sound of two stones struck together in the water at the distance of more than half a mile. Sound passes through solids much more easily, and therefore more rapidly, than through liquids. Thus its velocity through copper is twelve times and through glass seventeen times greater than through air. If you place your ear against a long brick wall at one end, and let some one strike upon the other end, you will hear two reports, the first through the wall and the second through the air. Indians are in the habit of ascertaining the approach of their enemies by putting the ear to the ground. When the eruption of a volcano is heard at a great distance the sound comes through the solid earth rather than through the air. The ready transmission of sound through solids furnishes us with a very valuable means of examining diseases of the lungs and heart. The sounds occasioned by the movement of the air in the lungs and by the action of the heart are very distinctly heard through the solid walls of the chest.

257. Measurement of Distances by Sound.—It makes no difference with the velocity of sound whether it be loud or not. Thus the sounds of a band of music at a distance all reach your ear at the same time, the sounds of the instruments that can scarcely be heard keeping exact pace in the air with the sounds of the loudest. So, also, the velocity of sound is uniform throughout its whole course, being just as rapid when it is about to die away as it was when it began. It is from this uniformity in the velocity of sound that we can estimate the distance of the object by which any sound is made. We do it by a comparison between light and sound. Sound moves at the rate of 1120 feet in a second. Now light moves 192,000 miles a second, and therefore, for all ordinary distances on the earth, we need make no allowance of time for light in comparison with sound. If we see, then, the operation by which a sound is produced we can estimate its distance from us by the length of time which elapses between what we see and what we hear. In this way we can estimate very accurately the distance of a cannon that we see fired, or the distance of a flash of lightning.

258. Loudness of Sound.—The loudness of sound depends upon the width of the vibrations producing it. The harder you strike the end of the tuning-fork, Fig. 182, the farther will it vibrate the one way and the other, and the louder will be the sound. The same thing is true of the strings of a piano. A round bell, when it is struck, tends in its vibration to take an oval form, and the extent of its vibration back and forth as it does this governs the loudness of the sound. As sound passes from the sounding body the vibration gradually lessens, and at length dies away. It is like the successive vibrations or waves of water produced by dropping a stone in it. The louder the sound is the larger are the first vibrations, and the farther will the vibrations extend, as in water a large stone dropped into it will produce larger waves than a small one, and the waves will extend over a greater space.

259. Diffusion of Sound.—When there is no hindrance sound spreads equally in all directions. It is in this respect with the vibrations or waves of air as it is with the waves of water when a stone is dropped into it. Light is also diffused in the same manner, as you will see in another chapter.

Fig. 186.

260. Reflection of Sound.—As waves of water striking against any object bound off, so it is with the vibrations or waves of sound. And the same is true of this as of all motion, as stated in § 206, that the angle of incidence is equal to the angle of reflection. The reflection of sound is the cause of echoes. In order that an echo be perfect the sound must be reflected back to the ear from some plane surface of some size. Sometimes when there are successive plane surfaces of rocks along a river there are successive echoes. Thus in Fig. 186 (p. 200) is represented a locality on the Rhine where a sound is reflected at successive places, 1, 2, 3, 4. The rolling of thunder, though sometimes caused by the different distances of parts of the same flash of lightning, is commonly owing to reflections of the sound among the clouds. From this cause the report of a cannon is more apt to be a rolling sound when there are clouds above than when the sky is clear. Sound is continually reflected in every variety of direction from obstacles with which it meets. Thus in a room it is reflected from the walls and from all the objects in the room; and the more varied are the surfaces the more varied and confused are the reflections. You know that a voice has a very different sound in a room when it is empty from what it has when the room is filled with an audience. Indeed, a blind speaker can estimate very nearly the size of his audience by the sound of his own voice. The explanation is, that with a full audience the surfaces for reflection are vastly multiplied, and so deprive the sound of the sharp and ringing character which is given to it by reflection from comparatively few surfaces which are plane and firm. The effect produced by an audience upon the voice of the speaker is quite analogous to that of muffling upon the sound of a drum.