HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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THE ORIGIN 01 A MUSICAL SOUND.               «
the centre and from it, alternately. The air particles at that part of the tube, where the flame is situated, will therefore be alternately, crowded together and scattered wider apart; that is, the pressure of the air upon the flame will be alternately greater and less than the ordinary atmospheric pressure. The effect of the greater pressure upon the flame will be to force it down, or even extinguish it altogether; the effect of the lesser pressure will be to enlarge it. Thus the flame will rise and fall at every vibration of the air in the tube. These movements of the flame are too rapid, however, to be followed by the eye, and the flame itself will still appear to be at rest. In order to observe them, recourse must be had to a common optical device. First, reduce the tube to silence, by lowering the position of the jet. Having then darkened the room, rotate a mirror (M) on a vertical axis behind the flame. The latter now appears in the mirror as a continuous yellow band of light, for precisely the same reason, that a lighted stick, on being whirled round, presents the appearance of a luminous circle. Now restore the jet to its former position in the tube. The latter begins to sing, and on rotating the mirror we no longer see a continuous band of lighti but a series of distinct flames (of) joined together below by a very thin band of light. This clearly shows, that the flame is alternately large and very small; that is, alternately rising and falling, as described above. Now while the mirror is being rotated at an even rate, notice that the intervals between the flames are all equal, and also that the flames themselves are all of the same size. From what is stated above, it will be seen that this proves our point, namely, that the sound in this case is produced by the periodic or vibratory motion of the particles of air.
By examining in this way into the origin of other sounds, it will be found that all musical tones are caused by the periodic motion of some body. Further, a periodic or vibratory motion will always produce a musical sound, provided, (1), that the vibrations recur with sufficient rapidity; (2), that they do not recur too rapidly; (3), that they are sufficiently extensive, and the moving body large enough. The following experiments will illustrate this.
Fig. 4 represents an ordinary cogwheel (B), having some 80 or 90 teeth, which can be rapidly rotated by means of the multiplying wheel (A). Holding a card (E) so as just to touch the cogs, we slowly turn the handle of the multiplying wheel. The card is