io6 



SCIENCE PROGRESS 



effectiveness of the cushions being practically identical wherever 

 they were placed.^ 



Cushions present Time of Decay 



(metres). (sees.). 



5-61 



8 



17 



63 

 128 



162 



213 



5-33 

 4-49 



3-94 

 3-00 

 2-64 



2-33 



The absorbing powers of various materials in terms of metres 

 of these particular cushions were then determined for the same 

 note, and yielded the following results : 



The cushions of a particular lecture theatre are clearly not 

 suitable units in which to express absorption coefficients, 

 and Sabine next proceeded to determine absolute absorption 

 coefficients, i.e. the ratio of the absorption of a given area of 



the surface considered 

 to the same area of 

 complete absorption. 

 The nearest approach 

 to complete absorption 

 is the open window, and 

 it was first shown that 

 the absorbing power of 

 a window was propor- 

 tional to the area of 

 the opening. The ab- 

 sorbing power of the 

 cushions was then de- 

 termined in terms of 

 open window. 



The curve in Fig. i 

 is found to have the 

 equation {a + x) t = k, 

 i.e. it is a rectangular hyperbola with its origin displaced ; 

 a is clearly the absorbing power of the walls of the room in 

 metres of cushions. In this case a = 146 and ^ == 813 and 

 the calculated durations agree with those observed on the 

 average within -02 of a second. 



1 This is only true when the intensity in the room is uniform. 



10 



w 9 

 o 



CD 



J2 8 



37 



o 

 cs 



l5 



CO 



Cc 4 



O 

 •■^ _ 



a 



Ql 



20 40 60 80 100 120 140 160 180 200 220 



IfJetrea of CushJona 

 Fig. I. 



