PROCEEDING HIE AMERICAN ACADEMY. 



Curve •"■ of Figure 5 ia another curve obtained in the same way with 

 ;i Blate reflector al W and a pipe of slightly higher pitch, and with the 

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

 lecting ' , i (| cm. in diameter. This curve ia somewhat more aearly 



-\ iiiinrtiic.il in character. 



It should !"• uoted in reaped to these curves thai there waa still con- 

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

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



Bystem. 



The curve- of Figure •">, although taken under somewhat artificial 

 conditions arc in themselves instructive, in showing the marked ei 

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

 speaker or an orchestra is at any given distance in trout of a reflecting 

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

 different pitch will be greatly intensified, thus it may he changing 

 completely the emphasis and quality of the composition. When t! 

 is only a Bmgle strongly reflecting wall (the other walls being Btrongly 

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

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

 suppressed or emphasized depending on the phase difference between 

 the direct and reilected waves to the auditor. 



Experiment III. Interferena of Sound Waves in a Largi Lecture 

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

 tribution of sound intensity in a room of considerable proportion-, 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 18.6 meters long, r_'.T 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 



r bo that the height of the ceiling above the seats in the rear is 



about 1 meters. The walls of the room are of brick. 



The organ pipe used as a source of sound, G 41 768, was placed at the 

 position 1' 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 \alve operated by a battery 

 and clock work. 



The Siemens and Halske telephone receiver, 166 ohms, with the 

 sound collecting cone 1<» cm. in diameter, was used as a receiver for 

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

 lie placed anywhere in the room. 



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



