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VII. §76.] INTERFERENCE OF SOUND. 155
something possessing an external substantial existence ; not as consisting merely in a state of motion of certain air-particles, and therefore liable, on the application of suitable forces, to be absolutely anni-liilated.
A single tuning-fork presents an example of this very important phenomenon. Each prong sets up vibrations corresponding to a simple tone, and the two tones so produced are of the same pitch and intensity. If the fork, after being struck, is held between the finger and thumb, and made to revolve slowly about its own axis, four positions of the fork with reference to the ear will be found where the sound goes completely out. These positions are mid-way between the four in which the plane sides of the prongs are held straight before the ear. As the fork revolves from one of these positions of loud sound to that at right-angles to it, the sound gradually wanes, is extinguished in passing the Interference-position, reappears very feebly immediately afterwards, and then continues to gain strength until the quarter of a revolution has been completed.
76. The case of coexistent unisons has now been adequately examined: we proceed to enquire what happens when two simple tones differing slightly in pitch are simultaneously produced. The problem is,