General Science - Bertha May Clark (children's books read aloud TXT) 📗
- Author: Bertha May Clark
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Matter of any kind can transmit sound to the ear. The following experiments will show that matter is necessary for transmission. Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber tube and pass the rod through one of two openings in a rubber cork. Insert the cork in a strong flask containing a small quantity of water and shake the bell, noting the sound produced. Then heat the flask, allowing the water to boil briskly, and after the boiling has continued for a few minutes remove the flame and instantly close up the second opening by inserting a glass stopper. Now shake the flask and note that the sound is very much fainter than at first. As the flask was warmed, air was rapidly expelled; so that when the flask was shaken the second time, less air was present to transmit the sound. If the glass stopper is removed and the air is allowed to reenter the flask, the loudness of the sound immediately increases.
Since the sound of the bell grows fainter as air is removed, we infer that there would be no sound if all the air were removed from the flask; that is to say, sound cannot be transmitted through empty space or a vacuum. If sound is to reach our ears, it must be through the agency of matter, such as wood, water, or air, etc.
252. How Sound is transmitted through Air. We saw in Section 250 that sound can always be traced to the motion or vibration of matter. It is impossible to conceive of an object being set into sudden and continued motion without disturbing the air immediately surrounding it. A sounding body always disturbs and throws into vibration the air around it, and the air particles which receive motion from a sounding body transmit their motion to neighboring particles, these in turn to the next adjacent particles, and so on until the motion has traveled to very great distances. The manner in which vibratory motion is transmitted by the atmosphere must be unusual in character, since no motion of the air is apparent, and since in the stillness of night when "not a breath of air" is stirring, the shriek of a railroad whistle miles distant may be heard with perfect clearness. Moreover, the most delicate notes of a violin can be heard in the remotest corners of a concert hall, when not the slightest motion of the air can be seen or felt.
In our study of the atmosphere we saw that air can be compressed and rarefied; in other words, we saw that air is very elastic. It can be shown experimentally that whenever an elastic body in motion comes in contact with a body at rest, the moving body transfers its motion to the second body and then comes to rest itself. Let two billiard balls be suspended in the manner indicated in Figure 167. If one of the balls is drawn aside and is then allowed to fall against the other, the second ball is driven outward to practically the height from which the first ball fell and the first ball comes to rest.
If a number of balls are arranged in line as in Figure 168 or Figure 169, and the end ball is raised and then allowed to fall, or if A is pushed against C, the last ball B will move outward alone, with a force nearly equal to that originally possessed by A and to a distance nearly equal to that through which A moved. But there will be no visible motion of the intervening balls. The force of the moving ball A is given to the second ball, and the second ball in turn gives the motion to the third, and so on throughout the entire number, until B is reached. But B has no ball to give its motion to, hence B itself moves outward, and moves with a force nearly equal to that originally imparted by A and to a distance nearly equal to that through which A fell. Motion at A is transmitted to B without any perceptible motion of the balls lying between these points. Similarly the particles of air set into motion by a sounding body impart their motion to each other, the motion being transmitted onward without any perceptible motion of the air itself. When this motion reaches the ear, it sets the drum of the ear into vibration, and these vibrations are in turn transmitted to the auditory nerves, which interpret the motion as sound.
FIG. 169.—Elastic balls transmit motion.
FIG. 170.—When a ball meets more than one ball, it divides its motion.
253. Why Sound dies away with Distance. Since the last ball B is driven outward with a force nearly equal to that possessed by A, it would seem that the effect on the ear drum should be independent of distance and that a sound should be heard as distinctly when remote as when near. But we know from experience that this is not true, because the more distant the source of sound, the fainter the impression; and finally, if the distance between the source of sound and the hearer becomes too great, the sound disappears entirely and nothing is heard. The explanation of this well-known fact is found in a further study of the elastic balls (Fig. 170). If A hits two balls instead of one, the energy possessed by A is given in part to one ball, and in part to the other, so that neither obtains the full amount. These balls, having each received less than the original energy, have less to transmit; each of these balls in turn meets with others, and hence the motion becomes more and more distributed, and distant balls receive less and less impetus. The energy finally given becomes too slight to affect neighboring balls, and the system comes to rest. This is what occurs in the atmosphere; a moving air particle meets not one but many adjacent air particles, and each of these receives a portion of the original energy and transmits a portion. When the original disturbance becomes scattered over a large number of air particles, the energy given to any one air particle becomes correspondingly small, and finally the energy becomes so small that further particles are not affected; beyond this limit the sound cannot be heard.
If an air particle transmitted motion only to those air particles directly in line with it, we should not be able to detect sound unless the ear were in direct line with the source. The fact that an air particle divides its motion among all particles which it touches, that is, among those on the sides as well as those in front, makes it possible to hear sound in all directions. A good speaker is heard not only by those directly in front of him, but by those on the side, and even behind him.
254. Velocity of Sound. The transmission of motion from particle to particle does not occur instantaneously, but requires time. If the distance is short, so that few air particles are involved, the time required for transmission is very brief, and the sound is heard at practically the instant it is made. Ordinarily we are not conscious that it requires time for sound to travel from its source to our ears, because the distance involved is too short. At other times we recognize that there is a delay; for example, thunder reaches our ears after the lightning which caused the thunder has completely disappeared. If the storm is near, the interval of time between the lightning and the thunder is brief, because the sound does not have far to travel; if the storm is distant, the interval is much longer, corresponding to the greater distance through which the sound travels. Sound does not move instantaneously, but requires time for its transmission. The report of a distant cannon is heard after the flash and smoke are seen; the report of a near cannon is heard the instant the flash is seen.
The speed with which sounds travels through the air, or its velocity, was first measured by noting the interval (54.6 seconds) which elapsed between the flash of a cannon and the sound of the report. The distance of the cannon from the observer was measured and found to be 61,045 feet, and by dividing this distance by the number of seconds, we find that the distance traveled by sound in one second is approximately 1118 feet.
High notes and low notes, soft notes and shrill notes, all travel at the same rate. If bass notes traveled faster or slower than soprano notes, or if the delicate tones of the violin traveled faster or slower than the tones of a drum, music would be practically impossible, because at a distance from the source of sound the various tones which should be in unison would be out of time—some arriving late, some early.
255. Sound Waves. Practically everyone knows that a hammock hung with long ropes swings or vibrates more slowly than one hung with short ropes, and that a stone suspended by a long string swings more slowly than one suspended by a short string. No two rocking chairs vibrate in the same way unless they are exactly alike in shape, size, and material. An object when disturbed vibrates in a manner peculiar to itself, the vibration being slow, as in the case of the long-roped swing, or quick, as in the case of the short-roped swing. The time required for a single swing or vibration is called the period of the body, and everything that can vibrate has a characteristic period. Size and shape determine to a large degree the period of a body; for example, a short, thick tuning fork vibrates more rapidly than a tall slender fork.
Some tuning forks when struck vibrate so rapidly that the prongs move back and forth more than 5000 times per second, while other tuning forks vibrate so slowly that the vibrations do not exceed 50 per second. In either case the distance through which the prongs move is very small and the period is very short, so that the eye can seldom detect the movement itself. That the prongs are in motion, however, is seen by the action of a pith ball when brought in contact with the prongs (see Section 250).
The disturbance created by a vibrating body is called a wave.
256. Waves. While the disturbance which travels out from a sounding body is commonly called a wave, it is by no means like the type of wave best known to us, namely, the water wave.
If a closely coiled heavy wire is suspended as in Figure 173 and the weight is drawn down and then released, the coil will assume the appearance shown; there is clearly an overcrowding or condensation in some places, and a spreading out or rarefaction in other places. The pulse of condensation and
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