SABINE. — ARCHITECTURAL ACOUSTICS. 83 



over, the expense of plastering and replastering a room — and this 

 process, to arrive at anything like a general solution of the problem, 

 would have to be done a great many times — would be very great, and 

 is at the present moment prohibitive. A little data along some of 

 these lines have been secured, but not at all in final form. The work 

 in the past has been largely of an analytical nature. Could the in- 

 vestigation take the form of constructive research, and lead to new 

 methods and greater possibilities, it would be taking its more interest- 

 ing form. 



The above discussion has been solely with reference to the deter- 

 mination of the coefficient of absorption of sound. It is now proposed 

 to discuss the question of the application of these coefficients to the 

 calculation of reverberation. In the first series of papers, reverbera- 

 tion was defined with reference to C4 512 as the continuation of the 

 sound in a room after the source had ceased, the initial intensity of 

 the sound being one million times minimum audible intensity. It is 

 debatable whether or not this definition should be extended without 

 alteration to reverberation for other notes than C4 512. There is a 

 good deal to be said both for and against its retention. The whole, 

 however, hinges on the outcome of a physiological or psychological 

 inquiry not yet in such shape as to lead to a final decision. The 

 question is therefore held in abeyance, and for the time the definition 

 is retained. 



Retaining the definition, the reverberation for any pitch can be 

 calculated by the formula 



KV 



T = 



a 



where V is the volume of the room, ^ is a constant depending on the 

 initial intensity, and a is the total absorbing power of the walls and 

 the contained material. K and V are the same for all pitch fi-equen- 

 cies. K is .164 for an initial intensity 10® timeis minimum audible 

 intensity. The only factor that varies with the pitch is a, which can 

 be determined from the data given above. 



In illustration, the curves in the accompanying Figure 14 give the 

 reverberation in the large lecture room of the Jefferson Physical 

 Laboratory. The upper curve defines the reverberation in the room 

 when entirely empty ; the lower curve defines this reverberation in 

 the same room with an audience two thirds filling the room. The 

 upper curve represents a condition which would be entirely impractical 

 for speaking purposes ; the lower curve represents a fairly satisfactory 

 condition. 



