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ILLINOIS STATE ACADEMY OP SCIENCE 



are not produced was evidenced by the fact that devices 

 of tliis character were no more effective than the open 

 horn of the same dimensions. To reflect sound as a mir- 

 ror reflects light, the dimensions of the reflecting surface 

 must be large as compared with the wave length of the 

 sound. When it is recalled that the wave lengths of tones 

 in the middle register are of the order of several feet, it 

 is apparent that any amplifying device based upon such 

 a principle must of necessity be prohibitively large as 

 a portable instrument. Hence there is no virtue in such 

 instruments when small, other than that of their simple 

 action as resonators. 



Figure 10. 

 Logarithmic amplifications of Sound in- 

 tensities produced by artificial aids * 

 to hearing. See Text. 



Telephonic -devices proved to be much more efficient as 

 sound amplifiers than the simpler ear trumpets. In in- 

 struments of this type, however, the maximum amplifica- 

 tions are limited to a small range of tones in the neigh- 

 borhood of the natural frequencies of the transmitter and 

 receiver diaphragms. The logarithmic amplifications pro- 

 duced by the largest of the ear trumpets [curve 1] and 

 by one of the better known telephonic devices [curve 2] 

 are shown in Figure 10. The upper cui-ve shows the 

 amplification necessary to enable the deaf observer to 

 hear sounds as faint as those that can be heard by normal 



