HANDBOOK OF ACOUSTICS - online book

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

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58
HAND-BOOK OF ACOUSTICS.
vibrations in equal times, whether their amplitudes be great or small, so the weight performs its swings (as long as they are not too violent) in equal times, whether their range be small or great. Again, the conditions of success are the same in both cases; for, in the first place, the impulses in the second experiment must be exactly timed, that is, they must be repeated at an interval of time which is identical with the time taken by the weight to perform a complete swing. In other words, the hand which pulls the fibre must move in perfect unison with the weight. If this were not so, the impulses would destroy one another's effects. Just so with the forks; they must be in the most rigorous unison, in order that the effects of the impulses may accumulate. Again, in each experiment a certain lapse of time is necessary to allow the effects of the suc­cessive impulses to accumulate.
The complete explanation of the experiment with the two tuning-forks is as follows. Let the prong A of the fork in the right hand be supposed to be advancing in the direction of the non-vibrating fork B ; the air between A and B will be compressed, and thus the pressure on this side of the prong B will be greater than that on the other; the latter prong will therefore move through an infinitesimal space away from A. Now suppose the prong A has reached its extreme position and is returning; then, as both forks execute their vibrations in exactly equal times, whether these be of large or small extent, it follows that B must be returning also ; but as A moves through a greater space than B, the air between the two will become rarefied, and thus the pressure on this side of B will be less than that on the other; B will therefore receive another impulse, which will slightly increase its amplitude. On its return, it will receive another slight impulse, and thus, by these minute successive additions, the amplitude is soon sufficiently increased to produce an audible sound.
In the preceding experiment, the exciting fork communicates a small portion of its motion to the air between the forks, and then this latter gives up part of its motion to the other fork. Now, as the density or weight of air is so exceedingly small in comparison with that of the steel fork, the amplitude of the vibrations thus set up in the latter must necessarily be always very small, that is, its sound will be very faint. By using a medium of greater elasti­city, the sound may be obtained of sufficient intensity to be heard by several persons at once. Thus, let one fork be struck Bharply, and the end be immediately applied to a sounding board,