Acoustics & Sound For Musicians - Online Book

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

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154               INTERFERENCE OF SOUND. [VII. §75.
tively. In Fig. 55, a case of equal and opposite waves is shown. In (1) Fig. 54, the resultant wave is the sum, and in (2) the difference of the compo­nent waves. In (3), we get a wave of intermediate amplitude. These three resultant waves are neces­sarily simple, as otherwise two simple tones in unison would give rise to a composite sound, which would be absurd. In Fig 55 the wave-form degenerates into the initial line, i.e. no effect whatever is pro­duced.
75. Thus, when one simple tone is being heard, we by no means necessarily obtain an increase of loudness by exciting a second simple tone of the same pitch. On the contrary, we may thus weaken the original sound, or even extinguish it entirely. When this occurs we have an instance of a phe­nomenon which goes by the name of Interference. That two sounds should produce absolute silence seems, at first sight, as absurd as that two loaves should be equivalent to no bread. This is, however, only because we are accustomed to think of Sound as
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