MINIMUM AUDIBLE INTENSITY OF SOUND. 



433 



Combining these expressions, we have 



o 3 o o o 



YAe At 



1 - 



2-rrna 



In order to find the energy per square centimeter per second, it is 

 necessary only to multiply the value of i' by the velocity of sound v, 

 thus giving the expression 



E' = i'v = 



O 1 *> *> 



VAe At 



vJi 



■iwna 



1 - 



2irna 



from which the results of this investigation were computed. In order 

 to determine A, it is evidently necessary to measure the time of decay 

 of the sound for at least two different amplitudes of the fork. 



Apparatus. 



The experiments to be described were made in the Constant Temper- 

 ature Room of the Jefferson Physical Laboratory of Harvard Univer- 

 sity. This room has a volume of 76 cubic meters and is constructed 

 with heavy double brick walls situated in the sub-basement of the 

 tower of the Laboratory and free from structural connection with the 

 surrounding building. It is situated at some distance from the street 

 and its attendant noises. All observations were made at night under 

 conditions of absolute quiet. 



The source of sound consisted of a circular aluminum diaphragm, 

 free at the edges, vibrating in a circular aperture in the door of the 

 room, the diaphragm being attached to one prong of a tuning fork F 

 outside of the door. 



Symmetrical conditions in respect to weight of the fork were secured 

 by attaching an exactly similar diaphragm to the other tyne, idle of 

 course as far as producing sound in the room was concerned, since it 

 was located outside of the room. Symmetrical damping conditions 

 were secured by surrounding the second diaphragm with a guard-ring. 

 The air damping of the diaphragms was reduced by tuning to reso- 

 nance the space between them. This was accomplished by means of a 

 plunger resonator. This device also served the even more important 

 purpose of causing the air in the very small annulus between the 



