Acoustics & Sound For Musicians - Online Book

The Theory Of Sound Which Constitutes The Physical Basis Of The Art Of Music.

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VIII. § 79.]        CAUSE OF DISSONANCE.
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for individual recognition. Exactly the same thing may be observed in the roll of a side drum, which no one is in danger of mistaking for a continuous sound. Rapid beats produce a decidedly harsh and grating effect on the ear; and this is quite what the analogy of our other senses would lead us to expect. The disagreeable impressions excited in the organs of sight by a flickering unsteady light, and in those of touch by tickling or scratching, are familiar to every one. The effect of rapid beats is, in fact, identical with the sensation to which we commonly attach the name of dissonance. Let us examine, in somewhat greater detail, the conditions necessary for its production between two simple tones. If we take a pair of middle-C forks, and gradually throw them more and more out of unison with each other in the way already described, the roughness due to their beats reaches its maximum when the interval between them is about a half-Tone: for a whole Tone it is decidedly less marked, and when the interval amounts to a Minor Third, scarcely a trace of it re­mains. Hence, in order that dissonance may arise between two simple tones, they must form with each other a narrower interval than a Minor Third. If we call this interval the beating-distance for two such tones, we may express the above condition thus. Dissonance can arise directly between two simple
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