I9IS-] 



SURFACES OF TELEPHONIC DIAPHRAGMS. 



Ill 



r becomes 328 dynes per cm. per sec. The precision in measuring 

 r by this method is relatively low, owing to the difficulty in measur- 

 ing the successive amplitudes with accuracy, on a curve of such 

 small dimensions. 



Since, as is shown in Appendix II., a circular diaphragm, in its 

 fundamental mode of motion, ordinarily develops a circular graph 

 of velocity, at varying impressed frequency, with constant vibro- 



motive force, the plan has suggested itself, in the course of this 

 research, to use the circle-velocity diagram of a diaphragm for com- 

 paring the vibro-motive forces (vmf.'s) of different organ-pipes. 

 In this connection, the vmf. of a pipe at the exploring diaghragm, 

 may be defined as its harmonically varying pressure / = Fe "^' 

 (dynes) produced, at the diaphragm, by the pipe, under the geo- 

 metrical conditions of the system, including acoustic reflections from 

 walls, or other objects in the room, on both surfaces of the explor- 

 ing diaphragm. In the simplest, or standard, geometrical condition, 

 the standard vmf., which is proportional to the square root of the 

 sound intensity^ at the diaphragm, would be observed in free space, 

 with the orifice of the pipe facing the diaphragm at a definite dis- 

 tance, and with the diaphragm perpendicular to the line joining 

 them. It is our understanding that there is, as yet, no simple pub- 

 lished method of measuring the vmf. of organ pipes, of different 



° Bibliography 6, Barton, "Text Book of Sound," p. 21 r, par. 146. Mac- 

 millan Co., 1908. 



