38 



ILLINOIS STATE ACADEMY OF SCIENCE 



pure alDsorption process, and makes it possible to express 

 the insulating efficiency in terms of the thickness and two 

 experimentally determinable coefficients. The extension 

 of the study to a large number of materials of the same 

 character leads to the general conclusion that their in- 

 sulating efficiencies follow the order of their densities. 



Another, somewhat different application of the Sound 

 Chamber method has been an experimental study of the 



Figure 8. 

 Reduction of Intensity of Sound trans- 

 mitted by Hair Felt of varying thick- 

 ness, plotted as a function of the 

 pitch. 



sound amplifications produced by various hearing de- 

 vices.' The actual effectiveness of artificial aids to de- 

 fective hearing has never been the subject of any more 

 precise determination than the opinion of their users. 

 Accordingly, an investigation was conducted to determine 

 the efficiencies of a number of the more commonly used 

 devices for the aid of the deaf. The eleven different in- 



