HANDBOOK OF ACOUSTICS - online book

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

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57
CHAPTER VII.
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Resonance, Co-vibration, or Sympathy of Tones.
Select two tuning-forks which are exactly in unison. Having taken one in each hand, strike that in the right hand pretty sharply, and immediately hold it with its prongs parallel, and close to the prongs of the other, but without touching it. After the lapse of not less than one second, on damping the fork in the right hand, that in the left will be found to be giving out a feeble tone. To this phenomenon, the names of Eesonance, Co-vibration, and Sympathy of tones have been given, the first being the one most commonly used in English works. The explanation of this effect will be better understood after a consideration of the following analogous experiment.
Let a heavy weight be suspended at the end of a long cord, and to it attach a fibre of silk or cotton. The weight being at rest, pull the fibre gently so as not to break it. The weight will thus be pulled forwards through an exceedingly small, perhaps imper­ceptible distance. Now relax the pull on the fibre, till the weight has swung through its original position, and reached the limit of its backward movement. If another gentle pull be then given, the weight will swing forward a trifle further than at first. The weight then swings backwards as before, and again a properly-timed pull will still further extend its excursion. By proceeding in this way, after a time, the total effect of these accumulated impulses will have been sufficient to impart to the weight a considerable oscillation. On examination it will be found that this experiment is analogous to the last one. The regularly timed impulses in the second experiment, correspond to the regularly vibrating fork in the right hand; the weight to the fork in the left hand, and the fibre to the air between the forks. And here it must be observed, that just as the forks execute their