HANDBOOK OF ACOUSTICS - online book

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RESONANCE.
59
on which the end of the non-vibrating fork is already resting; after the lapse of a second or so, the latter will be heard giving forth a sound of considerable intensity, the motion in this case having been transmitted through the board.
The following experiments illustrate the phenomenon of resonance or co-vibration in the case of stretched strings. Press down the loud pedal of a pianoforte, so as to raise the dampers from the strings. Each sound on the pianoforte is generally pro­duced by the vibration of two or three wires tuned in unison. Set one of these vibrating, by plucking it with the finger. After the lapse of a second or so, damp it, and the other wire will be heard vibrating. Again, having raised the dampers of a pianoforte, sing loudly any note of the piano, near and towards the sound-board. On ceasing, the piano will be heard sending back the sound sung into it. The full meaning of this experiment will be explained hereafter.
The resonance of strings may be visibly demonstrated to an audience in the following manner. Tune two strings on the mono-chord or any sound-board, in perfect unison, and upon one of them place a rider of thin cardboard or paper. On bowing the other string very gently, the rider will be violently agitated, and on in­creasing the force of the bowing, will be thrown off.
In these experiments the sound-board plays an important, or rather an essential part. Thus, in the above experiment with the piano, the sound waves from the larynx of the singer strike the sound-board of the piano, setting up vibrations in it, which are communicated through the bridges to the wires. It will be found that in these experiments with stretched strings, such perfection of unison as was necessary with the forks is not absolutely essential, and the reason is obvious; for in the first place, the medium by which the vibrations are communicated in the former, viz., the wood, is much more elastic than in the latter; and secondly, the light string or wire is much more easily set in vibration than the heavy steel of the fork. A smaller number of impulses is therefore sufficient to excite the string, and consequently such a rigorous unison is not absolutely essential. As we might expect, however, the more exact the unison, the louder is the sound produced.
In consequence of their small density, masses of enclosed air are very readily thrown into powerful co-vibration. Strike a C tuning-fork, and hold the vibrating prongs over the end of an open tube, about 13 inches long and about an inch in diameter. The sound of the fork, which before was very faint, swells out with considerable