PIERCE. — A METHOD OF MEASURING THE INTENSITY OF SOUND. 383 



IV. Stationary Sound Waves. Distribution of Intensity. 



In taking the data of Experiment I, the position of the telephone re- 

 ceiver and that of the organ-pipe were left constant. When the tele- 

 phone was removed to different parts of the room, very striking evidence 

 of a stationary-wave system 

 was obtained. This station- 

 ary system was, however, ex- 

 tremely complicated. In 

 some positions, for example, 

 a very slight change of the 

 inclination of the sound-col- 

 lecting cone, without any 

 motion of the receiver as a 

 whole toward or away from 

 the source of sound, would 

 cause several hundred per 

 cent change of the reading of 

 the galvanometer. Professor 

 Sabine has already called at- 

 tention to the existence in 

 this room of a striking inter- 

 ference system. The follow- 

 ing paragraph descriptive of 

 the phenomenon is quoted from his writings on the subject : 



" This room is here described at length because it will be frequently 

 referred to, particularly in this matter of interference of sound. While 

 working in this room with a treble c gemshorn organ-pipe blown by a 

 steady wind pressure, it was observed that the pitch of the pipe appar- 

 ently changed an octave when the observer straightened up in his chair 

 from a position in which he was leaning forward. The explanation is 

 this : The organ-pipe did not give a single pure note, but gave a funda- 

 mental treble c accompanied by several overtones, of which the strong- 

 est was in this case the octave above. Each note in the whole complex 

 sound had its own interference system, which, as long as the sound re- 

 mained constant, remained fixed in position. It so happened that at 

 these two points the region of silence for one note coincided with the 

 region of reinforcement for the other, and vice versa. Thus the ob- 

 server in one position heard the fundamental note, and in the other, 

 the first overtone. The change was exceedingly striking, and as the 

 note remained constant, the experiment could be tried again and 

 again. With a little search it was possible to find other points in the 



.3 .4 



CAPACITY 



.6 .7 



MICROFARAD. 



Figure 3. — Resonance curve. 



