170 BELL SYSTEM TECHNICAL JOURNAL 



pected when comparing the loudness of pure tones if the balances are 

 made at very high intensities. No such balances have yet been 

 made. 



What happens to the ear when one becomes deaf? This question, 

 of course, is one for the medical profession to answer, but let us take 

 one or two simple cases and see if they fit into this theory. First 

 assume that the nerve endings are diseased for a short distance away 

 from the base of the cochlea so that they send no impulses to the 

 brain. Under certain assumptions the kind of an audiogram one 

 should obtain can be calculated from the vibrational characteristics 

 determined as mentioned above. Such a calculation shows that an 

 audiogram similar to that shown in Fig. 4, which has a rapid falling 

 off in sensitiveness, can be accounted for, both quantitatively as 

 well as qualitatively. On a pure resonant theory corresponding to 

 that first proposed by Helmholtz, a tone island would exist corre- 

 sponding to the affected region for such a case. Although we have 

 tested a large number of cases, no such islands have ever been found. 

 When the intensity of the tone is raised sufficiently to bring the 

 amplitude of the area containing the healthy nerve cells which are 

 adjacent to the diseased portion to a value above that corresponding 

 to the threshold, the tone will then be perceived. 



Again assume that due to some pathological condition, the tissue 

 around the oval window where the stapes join the cochlea has become 

 hardened. Its elasticity will then be greatly increased so that vibra- 

 tional energy at low frequencies will be greatly discriminated against. 

 For such a case, an audiogram similar to that shown in Fig. 7-B 

 would be obtained. 



A number of things can cause a general lowering of the ear sensi- 

 tivity, such as wax in the ear canal, affections of the ear-drum, fixa- 

 tion of any of the ossicles, thickening of the basilar membrane, affec- 

 tions of the nerve endings or loss in nervous energy being supplied 

 to the membrane, etc. However, one would expect that each type of 

 trouble would discriminate, at least to some extent, against certain 

 frequency regions so as to produce some characteristic in the audio- 

 gram. Ear specialists are beginning to realize the possibility of 

 obtaining considerable aid in the diagnosis of abnormal hearing from 

 such accurate audiograms. 



There are a large number of facts obtained from medical research 

 which necessarily have a bearing upon the theory of hearing, but as 

 far as I know none of them is contrary to the theory of hearing given 

 above. It was seen that there are approximately 300,000 tone units 

 in the auditory-sensation area. According to the anatomists, there are 



