497 



It is remarkable (lial two luaxiiiui of resonance occur in all lliese 

 appai'atus. 1 suppose there is still a t'.iird, very low or very liigli, 

 and therefore lying beyond the lower or the upper limit of the 

 speechzone. If this is the case it would fall in with the fact that 

 such an apparatus is always composed ot two resonating cavities, which 

 invariably yield three maxima, as I i)ointed out elsewhere ^). 



As stated before the same apparatus was tested in 1912 with the 

 aid of microphone and string-galvanometer. The reinforcements tlien 

 detected were of the same onlev and the resonance-maxima were 

 generally found at the same points (only Ideal-Taschen showed 

 considerable deviation). The common average of electric measurements 

 I found to be 14; of non-resonance measurements 10 in an ordinary 

 room and 7 in the camera silenta. 



As to uniformity the reinforcement is far from satisfactory, because 

 the amount of damping of these resonators must not be excessive, 

 of course : considerable damping would destroy the reinforcing faculty. 



§ 3. Bearing-apparatus with foreiga energy. 



The average reinforcement produced by resonating hearing-apparatus 

 cannot be strong even without any damping. For a strong intensi- 

 fication electric energy is required. The sound is made to impact 

 on the microphone, wliicli is in circuit with the telephone; hereby 

 the electric energy is converted into acoustic energy. The gain thus 

 effected is sometimes so great, that by potent damping we are enabled 

 to expand the resonance of the membrane, which we have to take 

 to boot. 



Thus in a perfect way we readily obtain the average reinforcement 

 of Guye's apparatus. The following table shows the results of a 

 comparisorj with the usual control-receiver, whose width is precisely 

 equal to that of the ordinary microphone of aural surgery. The 

 width of the opera-aurophone is 272 times as great, because a funnel 

 is placed before the microphone. 



Far more striking results can be obtained. Then, however, a noise 

 of "boiling" often reveals itself in the microphone, which is imme- 

 diately associated with too extensive amplitudes of the sound-vibrations 

 at the highest resonance of microphone- and telophone-membranes, 

 in tune with each other. During the measurement this disturbance 

 expresses itself in a restless to- and fromotion of the mirror. As 

 yet no damping contrivance has been devised to overcome this disorder. 



^) "Multiple resonance" Ned. Tijds. v. Geueesk. 1913 II p. 647. When trying to 

 delect the 3''' maximum in the pleophone I found it lying rather beyond 63 . 



