428 swan. 



research using a special air damped telephone receiver pressed to the 

 ear, with an alternating current from a vacuum tube oscillator, re- 

 duced by an attenuator until the sound was just audible. Their 

 results cover a range of pitch from 60 to 4000 cycles. 



Table 1 gives a summary of the results of the principal investiga- 

 tions already mentioned, and shows their manifest discrepancies. The 

 figures give the energy in ergs per square centimeter per second. 



Aside from the criticisms which each investigator has made of the 

 researches which have preceded his own, the criticism offered from the 

 standpoint of this paper is on the one hand the impossibility of getting 

 conditions for testing minimum audibility out-of-doors, and on the 

 other, the neglect in indoor experiments of the intensifying effect of 

 reverberation and the complication arising from interference. In this 

 research a new method has been adopted which eliminates the one and 

 takes account of the other. 



Method. 



Some work in architectural acoustics by Sabine 18 suggested a new 

 method of approaching the problem. Sabine shows that the time of 

 decay of sound in a closed room after the source has ceased is a function 

 of the initial intensity in the room and of the minimum audible in- 

 tensity. Hence if the first two of these quantities can be measured 

 experimentally, the last can be calculated. In order to compute the 

 initial intensity in a room, however, it is necessary not only to know 

 certain constants of the room, but also the rate of emission of energy 

 from the sounding source. That form of sound emission which lends 

 itself most readily to theoretical computation is a circular diaphragm, 

 free at the edges, vibrating in an infinite plane wall. In the present 

 investigation, a near approximation to such a source was employed by 

 a circular diaphragm attached to one prong of v a tuning fork vibrating 

 in an aperture in a door flush with one wall of a room. By means of 

 electrical connections to be described presently, the sounding source 

 could be damped almost instantly and the time of decay of the residual 

 sound in the room measured by a chronometer. 



Theory. 



The rate of emission of energy from a circular diaphragm, free at 

 the edges, vibrating in an infinite plane wall, may be calculated as 

 follows : 



18 loc. cit. 



4 



