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II. § 25.] LOUDNESS AND EXTENT OF VIBRATION 53 |
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tides composing the conveying medium. A sound-producing instrument can be readily observed to be in a state of rapid tremor. The vibrations of a tuning-fork are recognisable by the eye in the fuzzy, half-transparent, rim which surrounds its prongs after it has been struck; and by the touch, if we place a finger gently against one of the prongs. The harder we hit the fork the louder is its sound, and the larger, estimated by both the above modes of observation, are its vibrations. The experiment may be tried equally well on any pianoforte whose construction allows the wires to be uncovered. It is natural to infer that a vibration on the part of a sound-producing instrument communicates to the particles of the air in contact with it a corresponding movement. Thus a sound of given loudness is conveyed by vibrations of given extent, and, if the sound increases or diminishes in intensity, the extent of the vibrations which carry it must increase or diminish correspondingly.
We conclude, then, that the loudness of a musical sound depends solely on the extent of excursion of the particles which constitute the conveying medium in the neighbourhood of our ears. This limitation is clearly essential, since a sound grows more and more feeble the greater our distance from the point where it is produced. This diminution of intensity |
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