General Science, Bertha May Clark [miss read books TXT] 📗
- Author: Bertha May Clark
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Sound waves may be said to consist of a series of condensations and rarefactions, and the distance between two consecutive condensations and rarefactions may be defined as the wave length.
257. How One Sounding Body produces Sound in Another Body. In Section 255 we saw that any object when disturbed vibrates in a manner peculiar to itself,—its natural period,—a long-roped hammock vibrating slowly and a short-roped hammock vibrating rapidly. From observation we learn that it requires but little force to cause a body to vibrate in its natural period. If a sounding body is near a body which has the same period as itself, the pulses of air produced by the sounding body will, although very small, set the second body into motion and cause it to make a faint sound. When a piano is being played, we are often startled to find that a window pane or an ornament responds to some note of the piano. If two tuning forks of exactly identical periods (that is, of the same frequency) are placed on a table as in Figure 174, and one is struck so as to give forth a clear sound, the second fork will likewise vibrate, even though the two forks may be separated by several feet of air. We can readily see that the second fork is in motion, although it has not been struck, because it will set in motion a pith ball suspended beside it; at first the pith ball does not move, then it moves slightly, and finally bounces rapidly back and forth. If the periods of the two forks are not identical, but differ in the slightest degree, the second fork will not respond to the first fork, no matter how long or how loud the sound of the first fork. If we suppose that the fork vibrates 256 times each second, then 256 gentle pulses of air are produced each second, and these, traveling outward through the air, reach the silent fork and tend to set it in motion. A single pulse of air could not move the solid, heavy prongs, but the accumulated action of 256 vibrations per second soon makes itself felt, and the second fork begins to vibrate, at first gently, then gradually stronger, and finally an audible tone is given forth.
The cumulative power of feeble forces acting frequently at definite intervals is seen in many ways in everyday life. A small boy can easily swing a much larger boy, provided he gives the swing a gentle push in the right direction every time it passes him. But he must be careful to push at the proper instant, since otherwise his effort does not count for much; if he pushes forward when the swing is moving backward, he really hinders the motion; if he waits until the swing has moved considerably forward, his push counts for little. He must push at the proper instant; that is, the way in which his hand moves in giving the push must correspond exactly with the way in which the swing would naturally vibrate. A very striking experiment can be made by suspending from the ceiling a heavy weight and striking this weight gently at regular, properly timed intervals with a small cork hammer. Soon the pendulum, or weight, will be set swinging.
258. Borrowed Sound. Picture frames and ornaments sometimes buzz and give forth faint murmurs when a piano or organ is played. The waves sent out by a sounding body fall upon all surrounding objects and by their repeated action tend to throw these bodies into vibration. If the period of any one of the objects corresponds with the period of the sounding body, the gentle but frequent impulses affect the object, which responds by emitting a sound. If, however, the periods do not correspond, the action of the sound waves is not sufficiently powerful to throw the object into vibration, and no sound is heard. Bodies which respond in this way are said to be sympathetic and the response produced is called resonance. Seashells when held to the ear seem to contain the roar of the sea; this is because the air within the shell is set into sympathetic vibrations by some external tone. If the seashell were held to the ear in an absolutely quiet room, no sound would be heard, because there would be no external forces to set into vibration the air within the shell.
Tuning forks do not produce strong tones unless mounted on hollow wooden boxes (Fig. 175), whose size and shape are so adjusted that resonance occurs and strengthens the sound. When a human being talks or sings, the air within the mouth cavity is thrown into sympathetic vibration and strengthens the otherwise feeble tone of the speaker.
259. Echo. If one shouts in a forest, the sound is sometimes heard a second time a second or two later. This is because sound is reflected when it strikes a large obstructing surface. If the sound waves resulting from the shout meet a cliff or a mountain, they are reflected back, and on reaching the ear produce a later sensation of sound.
By observation it has been found that the ear cannot distinguish sounds which are less than one tenth of a second apart; that is, if two sounds follow each other at an interval less than one tenth of a second, the ear recognizes not two sounds, but one. This explains why a speaker can be heard better indoors than in the open air. In the average building, the walls are so close that the reflected waves have but a short distance to travel, and hence reach the ear at practically the same time as those which come directly from the speaker. In the open, there are no reflecting walls or surfaces, and the original sound has no reënforcement from reflection.
If the reflected waves reach the ear too late to blend with the original sound, that is, come later than one tenth of a second after the first impression, an echo is heard. What we call the rolling of thunder is really the reflection and re-reflection of the original thunder from cloud and cliff.
Some halls are so large that the reflected sounds cause a confusion of echoes, but this difficulty can be lessened by hanging draperies, which break the reflection.
260. Motion does not always produce Sound. While we know that all sound can be traced to motion, we know equally well that motion does not always produce sound. The hammock swinging in the breeze does not give forth a sound; the flag floating in the air does not give forth a sound unless blown violently by the wind; a card moved slowly through the air does not produce sound, but if the card is moved rapidly back and forth, a sound becomes audible.
Motion, in order to produce sound, must be rapid; a ball attached to a string and moved slowly through the air produces no sound, but the same ball, whirled rapidly, produces a distinct buzz, which becomes stronger and stronger the faster the ball is whirled.
261. Noise and Music. When the rapid motions which produce sound are irregular, we hear noise; when the motions are regular and definite, we have a musical tone; the rattling of carriage wheels on stones, the roar of waves, the rustling of leaves are noise, not music. In all these illustrations we have rapid but irregular motion; no two stones strike the wheel in exactly the same way, no two waves produce pulses in the air of exactly the same character, no two leaves rustle in precisely the same way. The disturbances which reach the ear from carriage, waves, and leaves are irregular both in time and strength, and irritate the ear, causing the sensation which we call noise.
The tuning fork is musical. Here we have rapid, regular motion; vibrations follow each other at perfectly definite intervals, and the air disturbance produced by one vibration is exactly like the disturbance produced by a later vibration. The sound waves which reach the ear are regular in time and kind and strength, and we call the sensation music.
To produce noise a body must vibrate in such a way as to give short, quick shocks to the air; to produce music a body must not only impart short, quick shocks to the air, but must impart these shocks with unerring regularity and strength. A flickering light irritates the eye; a flickering sound or noise irritates the ear; both are painful because of the sudden and abrupt changes in effect which they cause, the former on the eye, the latter on the ear.
The only thing essential for the production of a musical sound is that the waves which reach the ear shall be rapid and regular; it is immaterial how these waves are produced. If a toothed wheel is mounted and slowly rotated, and a stiff card is held against the teeth of the wheel, a distinct tap is heard every time the card strikes the wheel. But if the wheel is rotated rapidly, the ear ceases to hear the various taps and recognizes a deep continuous musical tone. The blending of the individual taps, occurring at regular intervals, has produced a sustained musical tone. A similar result is obtained if a card is drawn slowly and then rapidly over the teeth of a comb.
That musical tones are due to a succession of regularly timed impulses is shown most clearly by means of a rotating disk on which are cut two sets of holes, the outer set equally spaced, and the inner set unequally spaced (Fig. 176).
If, while the disk is rotating rapidly, a tube is held over the outside row and air is blown through the tube, a sustained musical tone will be heard. If, however, the tube is held, during the rotation of the disk, over the inner row of unequally spaced holes, the musical tone disappears, and a series of noises take its place. In the first case, the separate puffs of air followed each other regularly and blended into one tone; in the second case, the separate puffs of air followed each other at uncertain and irregular intervals and the result was noise.
Sound possesses a musical quality only when the waves or pulses follow each other at absolutely regular intervals.
262. The Effect of the Rapidity of Motion on the Musical Tone Produced. If the disk is rotated so slowly that less than about 16 puffs are produced in one second, only separate puffs are heard, and a musical tone is lacking; if, on the other hand, the disk is rotated in such a way that 16 puffs or more are produced in one second, the separate puffs will blend together to produce a continuous musical note of very low pitch. If the speed of the disk is increased so that the puffs become more frequent, the pitch of the resulting note rises; and at very high speeds the notes produced become so shrill and piercing as to be disagreeable to the ear. If the speed of the disk is lessened, the pitch falls correspondingly; and if the speed again becomes so low that less than 16 puffs are formed per second, the sustained sound disappears and a series of intermittent noises is
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