248 



Royal Irish Academy: — Dr. Kane on the 



Any body which vibrates as a single mass gives origin at the 

 same moment to two waves, whose motions are in opposite direc- 

 tions, and of which one is rarefied and the other condensed. 



If these two arrive at the tympanum at the same moment and with 

 equal power, perfect neutralization should result, and no sound be 

 heard : hence, where a vibratory body produces upon the ear tbe 

 sensation of sound, it arises from one wave of the two being either 

 totally intercepted, or, at least, diminished in force, and the loudness 

 of the sound is proportional to the difference of the intensity of the 

 two waves when they affect the ear. 



All instruments for increasing sound, and producing resonance, 

 act upon this principle. 



The following facts will illustrate these principles in detail. A 

 tuning fork is a centre of four waves, two + and 

 two — , but unless it be very close to the ear, no 

 sound is heard from it ; because the centre of all 

 the four waves being very close, all act on the 

 ear with equal force, and the difference is (ap- 

 proximately). 



Now, if an open tube, of the same length as a 

 one-phase wave from the fork, be approached to 

 one centre, as A, in the adjoining figure, the air in it commences 

 to vibrate in unison with the fork, from being set in motion by the 

 first wave which passes into it : 

 the vibration of the tube is, 

 however, a phase behind that 

 of the fork, and hence, when 

 a — wave passes from the cen- 

 tre A, it meets a -f- wave from 

 the end of the tube E, and both 

 are destroyed. The —centre, C, 



destroys also a + centre, as D, and there remain only the centres of 

 -f- waves, B from the fork, and F from the tube, and these acting 

 in concert on the tympanum produce the sound that we hear. 



If the tube be closed, and of only one-half the length, the + 

 wave, which emanates from the centre A, passes in, and being re- 

 flected from the bottom, issues again at the moment when the next 

 — wave from A is about to enter ; E and A then destroy each 

 other, and C and D also interfering, 

 there results only the + wave B, 

 which acts unimpeded on the ear. 

 The sound of an open tube is, there- 

 fore, ceteris paribus, much stronger 

 than that of a closed tube, as there 

 are two waves in place of one. 



That the office of closed tubes, when resonant, is to destroy a 

 portion of the sound of the original vibrating body, and of the open 

 tubes to afford, in addition to that, a new centre of a wave of the 

 same phase as that which remains, may be exhibited in many ways. 



+ E 



