HANDBOOK OF ACOUSTICS - online book

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ON THE QUALITY OF MUSICAL SOUNDS. 81
such a pendulum to be at the highest point of its swing to the left; as it swings to the right, its rate of motion becomes more and more rapid till it reaches its lowest position; during the latter half of its swing it gets slower and slower, till it reaches its extreme position on the right, when after a momentary rest, it begins its journey back. The first half of the journey is the exact counterpart of the second, the motion being accelerated in the first half at exactly the same rate that it is retarded in the second. It is easy to construct a pendulum, that shall write a record of its own motion, and thus to obtain a pictorial representation of pendular vibration. Fig. 42 shows a form of the instrument, which the student will have no difficulty in making for himself. The funnel below which rests in a ring of lead, is filled with sand. As it swings backwards and forwards, the sand escapes, leaving a straight ridge of sand on the board below, as seen at (ab). If, however, the board be at the same time uniformly moved along from A to A1, the sand will be deposited along the wavy track seen in the figure. Such a tracing of a pendular vibration is seen on a larger scale in fig. 43. On comparing this tracing with that made
Fig. 43.
by a tuning-fork as described in Chapter IV it is found that they are of the same character : that is, a tuning-fork executes pendular vibrations. But a tuning-fork, as we have seen, gives simple tones. It seems, therefore, that simple tones are produced by pendular vibrations. Further experiment and observation confirm this, and we may take it as proved, that simple tones are always the result of pendular vibrations.
Now a compound tone is made up of partials : and partials are simple tones. Further, simple tones are due to pendular vibrations. It follows, therefore, that compound tones are due to combinations of pendular vibrations.
How are these pendular vibrations simultaneously conveyed through the air ? Throw a stone into a piece of still water, and while the waves to which it gives rise are travelling outwards, throw another stone into the water. One series of waves will be
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