OPTIMUM REVERBERATION TIME FOR AUDITORIUMS 391 



quency /. The values of A; chosen from this figure are given by the 

 next, Fig. 2. This approximation simpHfies our calculations very 

 much and introduces errors which are not intolerable. 



Referring back to Fig. 1, if we wish to adjust the absorption of the 

 room so that the loudness of all pure tones will decay at the same rate, 

 say for the moment 60 units per second, it is seen that the sensation 

 level must drop 60 db per second for frequencies between 700 and 



10 20 30 40 50 60 70 



SENSATION LEVEL 



80 



90 



100 



Fig. 1. — Loudness of pure tones. 



4000 cycles and for other frequencies the sensation level must drop 

 60/.(4/ db per second; or in other words, the reverberation time for 

 frequencies between 700 and 4000 cycles should be one second and 

 outside of this band it should be Aj seconds. Fig. 2, then, which is a 

 plot oi A} vs. frequency now becomes also an illustration of the shape 

 of the reverberation time vs. frequency curve which a room should 

 have in order that the loudness of pure tones of all frequencies shall 

 decay at the same rate. 



