HANDBOOK OF ACOUSTICS - online book

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

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64
HAND-BOOK OF ACOUSTICS.
and then tilt up the glass, so that the sand will nearly, but not quite, roll off. Having fixed the glass in this position, sing loudly up and down the scale. On reaching a certain note, the co-vibration of the air in the tumbler will set the paper and sand into violent vibration. By singing a sound of exactly the same pitch as that to which the air in the tumbler resounds, the sand may be moved when the singer is several yards away.
The phenomenon of resonance is taken advantage of, in the con­struction of resonating boxes. These are simply boxes (fig. 36), generally made of wood, with either one or two opposite ends open,
Fig. 36.
and of such dimensions, that the enclosed mass of air will resound to the tuning-fork to be attached to the box. Such boxes greatly strengthen the sound of the fork, by resonance; the vibration being communicated, through the wood of the box, to the air inside. It may be remarked here, that the sound of a fork attached to a resonance box of proper dimensions, does not last so long as it would, if the fork were held in the hand and struck or bowed with equal force; for in the former case it has more work to do, in setting the wood and air in vibration, than in the latter, and there­fore its energy is sooner exhausted.
For forks having the vibration numbers in the first column of the following table, boxes having the internal dimensions given in the 2nd, 3rd, and 4th columns are suitable. The dimensions of the first four are, for boxes open at one end only; those of the last four, for boxes open at both ends. The fork is screwed into the middle of the top of the box. The dimensions are in inches.