io8 



SCIENCE PROGRESS 



where a is the total absorbing power of the room and contents . 

 (If there are in the room surfaces Sj, Sj, S3, etc., having respec- 

 tive absorption coefficients aj, a,, ag, etc., then a — ajSj + 

 a,S, + agS, + etc.). This formula, together with the numerous 

 values of absorption coefficients for various substances given 

 in Sabine's tables (5) enables calculation to be made in advance 

 of construction and was responsible for clearing up much of the 

 fog that had surrounded' the subject before the investigation. 



SHORT TABLE OF ABSORPTION COEFFICIENTS 



I sq. metre of open window . 



glass, plaster, or brick 

 heavy rugs 

 hair felt 

 audience 



a 



I'OOO 



•025 

 •25 

 •75 

 •95 



In the case of spacial units it is difficult to express the 

 absorption as an absolute coefficient, and in the following table 

 the absorption of each object is expressed in terms of a square 

 metre of complete absorption. 



The relatively high values for isolated units are due to the 

 larger absorbing area presented when the objects are not in 

 contact. 



Other experimenters confirm the work of Sabine, and from 

 theoretical considerations Franklin (6) deduced the formula, 



/ = ^ — , which agrees with Sabine's experimental formula 



a 



to I per cent. The theory has been extended by Jager (7). 



Jager's theory applies to rooms whose dimensions are small 

 compared with the velocity of sound, the length being not much 

 greater than 20 metres, so that reverberation is sensibly uniform. 

 The whole interior is supposed to be filled with sound-rays of 

 equal intensity going in every direction, and equal volumes 

 contain the same amount of sound energy. The energy 

 density is taken as a measure of the prevailing intensity. 



The absorbing power of a surface is defined as that fraction 

 which, when multiplied by the energy of the sound, will give 

 the average loss in energy suffered by a sound-ray in one 

 reflection. Since for any ray all directions are equally probable, 



