HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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142
HAND-BOOK FO ACOUSTICS.
angles to the first mentioned; and as we have just seen, the waves that issue from between the prongs are in the opposite phase, to those that proceed at right angles to them ; that is, whenever there are condensations at c and d, there are rarefactions at a and b, and vice versa. Now, each of these four sets of waves, in passing outwards from its source, will of course spread in all directions; and therefore, the adjacent waves will meet along four planes, represented by the dotted lines in the figure. Along these lines, therefore, the interference must be total; that is, any air particle in any one of them, which is urged in any direction by the waves in c or d, will be urged in the opposite direction, with precisely equal force, by the waves from a or b; that is, it will remain at rest. Consequently the dotted lines are lines of silence, the maximum of sound being midway between any two of them. If the vibrating fork were large enough, and a person were to walk round it in a circle, starting from one of these points of maximum intensity, he would find, that the sound gradually diminished as he approached the dotted line, where it would be nil. After passing this point, the sound would increase to the maximum, then diminish again, and so on; four points of maximum, and four of minimum intensity occurring during the circuit.
To verify all this, strike a tuning-fork, and then hold it with the prongs vertical, and with the back of one of them parallel to the ear. Note the intensity of the sound, and then quickly revolve the fork half-way or a quarter-way round: the intensity is unaltered. Now strike the fork again, and after holding it as at first, turn it one-eighth round, so that it is presented corner-wise to the ear; the sound will be all but extinguished. Again strike the fork, and holding it to the ear as at first, revolve it slowly: the four positions of greatest intensity and the four interference positions are readily perceived. To vary the experiment, again strike the fork, and rotate it rapidly before the ear : the effect is very similar to the beats, to be studied presently.
These experiments are much more effective, and the results can be demonstrated to several persons at once, when a resonator is used. For an ordinary 01 tuning-fork, a glass cylinder closed at one end, about inch in diameter, and between six and seven inches long, is very convenient. If not of the exact length to resound to the fork, a little water maybe gradually poured in, as described in Chap. VII. When the vibrating fork is held with the back of one prong parallel to the top of the resonator, or at right angles to this portion, the sound of the fork is much intensified; but when held