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I. § 23.] MUSICAL AND NON-MUSICAL SOUNDS. 47
original disturbing force has to set a constantly increasing number of air-particles into motion, it can only do so by correspondingly shortening the distances through which the individual particles move, i.e. by diminishing their extent of vibration. Accordingly when Sound-waves spread out freely in all directions, the further any given air-particle is from the point at which the sound originated, the smaller will be the extent of the vibration into which it will be thrown when the waves reach it.
23. Sounds are either musical or non-musical. The vast majority of those ordinarily heard—the roaring of the wind, the din of traffic in a crowded thoroughfare—belong to the second class. Musical sounds are, for the most part, to be heard only from instruments constructed to produce them. The difference between the sensations caused in our ears by these two classes of sounds is extremely well marked, and its nature admits of easy analysis. Let a note be struck and held down on the harmonium, or on any instrument capable of producing a sustained tone. However attentively we may listen, we perceive no change or variation in the sound heard. A perfectly continuous and uniform sensation is experienced as long as the note is held down. If instead of the harmonium we employ the pianoforte, where the sound is loudest directly after the moment |
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