106 Neural Mechanisms of Hearing /6 : I 



strings at the high end to thick, long strings at the low end. Accordingly, 

 the resonator theory postulated that the basilar membrane was made up 

 of resonant fibers held under tension as piano wires. These fibers were 

 very sharply tuned and resulted in a mechanical analysis of incoming 

 sounds in much the same fashion as the Helmholtz resonators. Each 

 fiber of the basilar membrane was supposed to activate a nerve fiber. 

 Thus, pitch would be detected by the particular fiber most strongly 

 activated, loudness (or sound pressure level) by the amplitude of the 

 fiber motion, and quality by the relative amplitudes of various fibers. 



The resonator theory can be disproved in a number of ways. One 

 objection, not too serious, is that a sharply tuned resonator is hard to 

 excite; also it continues to vibrate long after the excitation has ceased. 

 It is impossible to design mechanical resonators whose sharpness would 

 permit the pitch discrimination possessed by many people and which 

 would also permit the time resolution necessary to understand speech. 

 If pitch discrimination is partly a function of the nervous system, then 

 the resonators need not be so sharp. However, this inclusion of the 

 central nervous system destroys the beautiful simplicity of the resonator 

 theory. 



The most direct tests of the resonator theory were carried out by 

 Bekesy. He measured the width of the basilar membrane of human ears 

 and found it changed only by a factor of a hundredfold, whereas the 

 thickness varied by a factor much less than 100. If this membrane were 

 made up of resonant fibers similar to piano wires, the frequency of 

 resonance f T should be given by the expression 



where T is the tension, p x is the mass per unit length, and L is the 

 length. Because audible frequencies vary from 30 to 20,000 cps, that is, 

 by a factor of almost 10 3 , Tjp-^L 2 would have to vary by 5 x 10 5 . 

 Bekesy's measurements of tensions showed that 5 x 10 5 was at least a 

 factor of 20 too great. His measurements depended on modern tech- 

 nology and could not have been made at a much earlier date. 



With this knowledge that the resonator theory is clearly wrong, it 

 is possible to find other pieces of information also tending to contradict 

 the resonator idea. For instance, no one has ever actually found fibers 

 in the basilar membrane which were independent of and ran directly 

 across the membrane. 



An alternative hypothesis of hearing was the telephone theory. 

 Rayleigh and many other scientists of his day were very impressed with 

 the telephone, which acted as a transducer changing sound energy into 

 electrical energy and then back to sound energy at another point. They 



