28 



ILLINOIS STATE ACADEMY OF SCIENCE 



factors upon Avl^icli it depends. Clearly this time will 

 decrease as the area of the absorbing material is in- 

 creased. Making a virtue of the defects of the room to 

 be studied and corrected, Professor Sabine determined 

 the general relation between the time of reverberation 

 and the absorbing area. Cushions from Sanders Theater 

 were brought into the room in varying amounts, and the 

 time of reverberation from a selected organ i^ipe ar- 

 ranged to speak with uniform power was determined in 

 each case by means of a chronograph. Figure 1 shows 

 the reduction of this time from 5.5 seconds in the empty 

 room to 2.14 seconds after 220 meters of the cushions 

 were introduced. An analytical expression for this curve 



Length of cushions in metres. 



Carve showing the relation of the duration of the residual sound 

 to the added absorbing material. 



Figure 1. 



appears immediately vrhen the data are plotted in a 

 somewhat different way as shown in Figure 2. Here the 

 ordinates are the reciprocals of the times. The upper 

 scale represented the actual length of cushions intro- 

 duced. The points so determined lie very close to a 



straight line so that we may Avrite 

 1 



— = [Lr-hLc] 1/k 

 t 



Clearly Lr is the length of cushions that has an absorb-. 

 ing power equal to that of the empty room. If now, the 

 origin of co-ordinates be shifted to the left by an amount 



