Acoustics & Sound For Musicians - Online Book

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

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	CHAPTER VII. ON THE INTERFERENCE OF SOUND, AND ON 'BEATS'. 74. In § 71 we examined the principle on which the general problem of the composition of vibrations is solved. We now approach certain very import­ant particular cases of that problem, which it will be worth while to solve both independently and as instances of the method repeatedly applied in § 72. Suppose that a particle of air is vibrating be­tween the extreme positions A and B while convey- ing a sustained simple tone produced by a tuning-fork, or stopped flue-pipe. Now let a second instrument of the same kind be caused to emit a tone exactly in unison with the first. We will assume that, when the waves of the second tone reach the particle, it is just on the point of starting from A towards B. Two extreme cases are now possible, depending on the movement which the particle would have exe­cuted in virtue of the later-impressed vibration alone.