HANDBOOK OF ACOUSTICS - online book

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ON INTERFERENCE.                            143
with the corner of the prong towards the resonator, the sound dies out. The effects produced when the fork is revolved, are precisely the same as those above mentioned, but much intensified by the resonance of the tube.
These experiments with resonators may be varied in many ways. Thus, first hold the vibrating fork with the edge of one of the prongs towards the resonator; no sound is heard. Now, keeping the fork still in this position, move it along horizontally for a short distance, so that only the lower prong is over the resonator; the sound will now burst forth, for the side of the resonator cuts off the waves issuing from between the prongs, which before interfered with those from the outside of the lower prong.
Again, tune two such resonators as the above to any tuning-fork, and arrange them at right angles to one another, as represented in fig. 75. Now hold the vibrating tuning-fork in such a position,
Fig. 75.
that, while the back of one of the prongs is presented to one resonator, the space between them is presented to the other. Under these circumstances, very little sound will be heard, for, from what has been already said, it will be seen, that the waves proceeding from the two resonators will always be in opposite phase, and thus will neutralize one another's effects. If, however, while the fork is vibrating, we slide a card over the mouth of one of the resonators, the other resonator will produce its due effect, and the sound will burst forth.
It is found, that two similar organ pipes placed together on the same wind chest, interfere with one another; the motion of the air in the two pipes taking place in such a manner, that as the wave streams out of one, it streams into the other and hence an observer at a distance hears no tone, but at most the rustling of the air. For this reason, no reinforcement of tone can be effected in an