274 



AUDITORY BIOPHYSICS 



-5-4-3-2-1 1 2 3 4 5 

 Pressure in bars 



Fig. VII-12. Form of transmission 

 characteristic proposed by Stevens and 

 Newman [1936] to account for the bio- 

 electric asymmetrical behavior of the ear. 



inward motion of the eardrum, showed a discontinuity at the point 

 due to the dislocation of the joint at large deflections. The lower 

 branch , because of outward motion, approached the displacement axis 

 asymptotically for very large displacements of the lever arm of the 



malleus. The displacement of the 

 malleus in this direction is limited 

 by the tympanic tensor. The 

 curve A has a point of inflection, 

 A , at the origin. This point will 

 be referred to as the symmetrical 

 operating point. For small deflec- 

 tions the malleus displacement is 

 a linear function of the force 

 transmitted to the lenticular proc- 

 ess of the incus. Under these 

 operating conditions the malleus- 

 incus articulation contributes no 

 distorting component. 



A similar sigmoid operating 

 curve (Fig. VII-12) was proposed 

 by Stevens and Newman [1936] to 

 explain the electrical response of the cochlea with changes in sound 

 pressure on the tympanic membrane. 



If the curve of Fig. VII-11A can be considered as the normal response 

 characteristic of the malleus-incus articulation, then variations intro- 

 duced into the slope of the curve and position of the operating point by 

 changes in coupling of this articulation become functionally important. 

 The linear relation, curve D, between malleus displacement and the 

 force developed at the lenticular process of the incus was obtained when 

 the malleus-incus joint was pressed together with sufficient force so that 

 no slip was possible; this is the locked or close coupling position. Under 

 these circumstances both stapedius and tympanic tensors are slack. 

 As the malleus-incus articulating faces were allowed to slip progressively 

 more freely, with respect to each other, curves C, B, and finally A were 

 developed. In A the malleus-incus joint was loosely coupled. Such a 

 condition can arise only when both tensors are applying sufficient antag- 

 onistic tension to separate the malleus-incus joint to the extreme position 

 allowed by the capsular ligaments of the joint. 



Effects of Coupling on the Characteristic Transmission Curve 



The family of transmission curves shown in Fig. VII-11 shows the 

 results obtained with successively smaller coupling in the order D to A. 



