388 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Curve 3 of Figure 5 is another curve obtained in the same way with 

 a slate reflector at W and a pipe of slightly higher pitch, and with the 

 Siemens and Halske telephone, which had a much smaller sound col- 

 lecting cone, 10 cm. in diameter. This curve is somewhat more nearly 

 symmetrical in character. 



It should be noted in respect to these curves that there was still con- 

 siderable reflection from the room, in spite of the felt curtains, and 

 that these reflected waves act in a manner to distort the stationary 

 system. 



The curves of Figure 5, although taken under somewhat artificial 

 conditions are in themselves instructive, in showing the marked effect 

 of a reflecting wall on the loudness and quality of sounds. When a 

 speaker or an orchestra is at any given distance in fi'ont of a reflecting 

 wall certain tones will be greatly reduced in intensity while tones of a 

 different pitch will be gi-eatly intensified, thus it may be changing 

 completely the emphasis and quality of the composition. When there 

 is only a single strongly reflecting wall (the other walls being strongly 

 absorbtive) this distortion occurs over practically the whole room, al- 

 though, of course, at different points in the room different notes will be 

 suppressed or emphasized depending on the phase difference between 

 the direct and reflected waves to the auditor. 



Experiment III. Interference of Hound Waves in a Large Lecture 

 Boom. — In order to extend the investigation to the study of the dis- 

 tribution of sound intensity in a room of considerable proportions, an 

 organ-pipe and the telephone receiver were set up in the large lecture 

 room of the Jefferson Physical Laboratory. This room, of which a 

 diagram is shown in Figure 7, is IH.G meters long, 12.7 meters wide, 

 and 7.7 meters high at one end. It contains seats for about 300 stu- 

 dents. These seats are progressively raised toward the back of the 

 room so that the height of the ceiling above the seats in the rear is 

 about 4 meters. The walls of the room are of brick. 



The organ-pipe used as a source of sound, G^. 76S, was placed at the 

 position P in the diagram, and was supplied with wind at a constant 

 pressure from a reservoir, from which the air supply to the pipe was 

 turned on and off by an electro-pneumatic valve operated by a battery 

 and clock work. 



The Siemens and Halske telephone receiver, 466 ohms, with the 

 sound-collecting cone 10 cm. in diameter, was used as a receiver for 

 the sound and was provided with a long double lead so that it could 

 be placed anywhere in the room. 



The first position chosen for the receiver was at the extreme rear of 



