302 AUDITORY BIOPHYSICS 



Stevens and Newman attribute the change in even harmonic content to 

 changes in the tension of the muscles of the middle ear, and they sug- 

 gested that this change in tension does not appreciably change the 

 magnitude of the odd harmonics. They found that, when the tendinous 

 attachment of the tympanic tensor muscle of a cat was cut at a point 

 near the eardrum, the second-harmonic content was increased without 

 an appreciable change in the third-harmonic content. 



Since the two interaural muscles control the degree of coupling of the 

 malleus-incus joint, it should follow that when the tensor tympanic 

 attachment is severed the resulting malleus-incus coupling depends ex- 

 clusively on the tension of the stapedius tensor. This tension produces 

 a loose connection between the malleus and incus, resulting in large 

 coupling which would allow the coupling mechanism to pass any gener- 

 ated surges. Among these the low-frequency harmonics (the second 

 harmonic) are therefore enhanced. 



Beat Frequencies are Aural Harmonics 



The sum and difference frequencies are often called beat frequencies. 

 Thus, if frequencies of 2000 and 3000 cycles simultaneously excite the 

 eardrum and then pass through the non-linear aural mechanism, there 

 will be present in the output not only the two original frequencies but 

 also frequencies of 1000 (3000 - 2000) and 5000 (3000 + 2000) cycles. 



The presence of these tones is a necessary consequence of the type of 

 characteristic operating curve attributed to the ear by Stevens and 

 Newman (Fig. VII-12) and subsequently verified experimentally by 

 Stuhlman (Fig. VII-11). 



The Response Characteristics of the Aural Membrane 



The aural membrane was defined as the elastic structure separating 

 the scala media above from the scala tympani below the basilar mem- 

 brane. It is a highly damped structure in which the acoustic nerves 

 terminate. Its upper structure is composed of the tectorial membrane 

 resting on the organ of Corti, which in turn is supported by the basilar 

 membrane lined on its lower side with the epithelium of the tympanic 

 mucosa. 



An important aspect of the aural membrane as a resonating structure 

 is its damping factor. This membrane is closely coupled to the fluids 

 which cover its anterior and posterior surface. Since the damping factor 

 of the overall auditory mechanism is about 50 milliseconds, the ampli- 

 tude of a wave traveling through the ear as a whole, after the cessation 

 of the incident vibration, falls to. 1/e = 1/2.718 of its initial value in 



