Imágenes de páginas
PDF
EPUB

To operate this machine we first neatly coat the cylinder with a sheet of foil; then we bring the point P to bear against this foil, so that, on turning the cylinder, it makes a depressed line or furrow where the foil covers the space between the threads cut on the surface of the cylinder. The mouth is now placed close to the opening in the vulcanite disk B B, and the metal plate is talked to, while the cylinder is revolved with a uniform motion.

The thin iron plate A vibrates to the voice, and the point P indents the foil, impressing in it the varying numbers, amplitudes, and durations of these vibrations. If the vibrations given to the plate A are those of simple sounds, then they are of a uniform regular character, and the point Pindents regular undulating depressions in the foil. If the vibrations are those causing complex and irregular sounds (like those of the voice in speaking), then similarly the depressions made in the foil are complex (like the curve of Fig. 49) and irregular. Thus the yielding and inelastic foil receives and retains the mechanical impressions of these vibrations with all of their minute and subtile characteristics.

Our experiment No. 121 has, however, taught us that the forms of these impressions will change with every change of distance of the place of origin of the compound sound from the vibrating plate A, even when at these various distances the compound sonorous vibrations fall on the plate with precisely the same intensity. Hence the futility of attempting to read sound-writings.

The permanent impressions of the vibrations of the voice are now made. It remains to show how the operation just described may be reversed, and thus to obtain from these impressions the aerial vibrations which made

them. Nothing is simpler. The plate A, with its point P, is moved away from the cylinder by pulling toward you the lever H G. Then the motion of the cylinder is reversed till you have brought opposite to the point P the beginning of the series of impressions which it has made on the foil. Now bring the point up to the cylinder; place against the vulcanite plate B B a large cone of paper or of tin to reënforce the sounds, and then steadily turn the crank D. The elevations and depressions which have been made by the point P now pass under this point, and in doing so they cause it and the thin iron plate to make over again the precise vibrations which animated them when they made these impressions under the action of the voice. The consequence of this is, that the iron plate gives out the vibrations which previously fell upon it, and it talks back to you what you said to it.

CHAPTER XIX.

ON HARMONY AND DISCORD. A SHORT EXPLANATION OF WHY SOME NOTES, WHEN SOUNDED TOGETHER, CAUSE AGREEABLE AND OTHERS DISAGREEABLE SENSATIONS.

IF, toward sunset, you walk on the shady side of a picket-fence, flashes of light will enter your eye every time you come to an opening between the pales. These flashes, coming slowly one after the other, cause a very disagreeable sensation in the eye. Similarly, if flashes or pulses of sound enter the ear, they cause a disagreeable sensation. Such pulses enter the ear when we listen to two sounding organ-pipes, two forks, or two wires on the sonometer which are slightly out of tune with each other. As you already know (see Experiment 71), these flashes or pulses of sound are called beats. You also know that the number of these beats made in a second is equal to the difference in the numbers of vibrations made in one second by the two sounding bodies. Thus, if one sounding body makes 500 and the other 507 vibrations in a second, then 7 beats per second will be heard.

EXPERIMENT 127.-With your toy trumpet and the disk used in Rood's experiment in the reflection of sound, Fig. 42, you can make an excellent experiment, showing the effects of beats on your ear. Sound the trumpet, and

gradually increase the velocity of the turning disk. At first the beats of sound so caused may be separately distinguished by the ear, and, though not pleasant in their effect, yet they can be endured. As the frequency of the beats increases, the harshness of the sensation becomes greater and greater, until the effect on the ear becomes actually painful.

But, if the flashes of light or beats of sound succeed one another so rapidly that the sensation of one flash or beat remains till the next arrives, you will have continuous sensations that are not unpleasant. In other words, continuous sensations are pleasant, but discontinuous or broken sensations are disagreeable.

If two sonorous vibrations reach the ear together and make a disagreeable sensation, then we may be sure that the difference in the numbers of their vibrations gives a number of beats per second which do not follow one another with sufficient rapidity to blend into a smooth, un broken sensation. In other words, these beats are so few in a second that the sensation of one disappears before the next arrives, and so discord is the sensation; but, if the frequency of the beats be sufficiently increased, the sensation of one remains till the next arrives, and the sensation is continuous, and we say that the two sounds are in harmony.

Therefore it at once appears that, if we only can find out the number of beats required in a second to blend sounds from different parts of the musical scale, we shall be able to state beforehand what notes when sounded together will make harmony and what notes will make discord.

By many experiments I have found the number of beats per second that two sounds must make to be in har

mony. In the following table a few of the results of my experiments are given:

[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small]

Column N gives the names of the notes given by the vibrations per second in Column V. The c' in this series is that used by physicists generally, and gives 256 vibrations. In Column B is given the smallest number of beats per second which the corresponding sound must make with another in order that the two may be in harmony, or, as it is generally stated, may make with the other the nearest consonant interval. If 47 beats per second of c', for example, blend, then the sensation of each of these beats remains on the ear of a second. In Column D are given these durations in fractions of a second. As these fractions are the lengths of time that the sensation of sound lingers in the ear after the vibrations of the air near the drum-skin of the ear have ceased, they are very properly called the durations of the residual sonorous sensations.

You observe in the table that this duration becomes shorter as the pitch of the sound rises. Thus, while the residual sensation of C is of a second, that of c'" is only ਨੈਨ•

Let us use the knowledge thus acquired by making it aid us in a few calculations and experiments. The table

« AnteriorContinuar »