HANDBOOK OF ACOUSTICS - online book

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

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60                      HAND-BOOK OF ACOUSTICS.
intensity. The material of the tube is without influence 'on the result. A sheet of paper rolled up so as to form a tube answers very well. On experimenting with tubes of different lengths, it will be found that the sound of the C fork is most powerfully re­inforced by a tube of a certain length, viz., about 13 inches. A slightly shorter or longer tube will resound to a smaller extent, but little resonance will be obtained, if it differs much from the length given. Before reading the following explanation of this phenomenon, the student should read over again the account given in Chapter II of the propagation of sound.
Let A B, fig. 32, represent the tube, with the vibrating tuning-fork above it. As the lower prong of the latter descends, it will press upon the air particles beneath it, giving rise to a condensation, AC. The particles in A C being thus crowded together, press upon those below, giving rise to a condensa­tion C D; the particles in which, in their turn, press upon those beneath, thus transmitting the wave of condensation to D E. In this way, the condensation passes through the tube, and at length reaches the end, E B. The crowded particles in E B will now press outwards in all directions, and overshooting the mark, will leave the remainder farther apart than they originally were ; that is, a rarefaction will be formed in EB. But as there is now less pressure in EB than in DE, the particles of air in the latter space will tend to move towards E B, and they themselves will be left wider apart than before; that is, the rarefaction will be trans­mitted from BE to ED, and in like manner will pass up the tube till it reaches A C. On arriving here, as the pressure in A C will be less than the pressure outside the tube, the air particles will P ' „„          crowd in from the exterior and give rise to a con-
densation. Thus, to recapitulate; the downward movement of the prong gives rise to a slight condensation in the tube below; this travels down the tube to B, where it is reflected as a pulse of rarefaction; this, rushing back, on reaching A is changed again to a pulse of condensation. Now if, while this has been going on, the fork has just made one complete vibration, the lower prong will now be coming down again as at first, and thus will cause an increase in the degree of condensation. The same cycle of change will take place as before, and will recur again and again, the degree