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AUDITORY BIOPHYSICS 



wave is altered in neither amplitude nor shape. If, however, the slope 

 is greater or less than 45°, the transmitted amplitude is magnified or 

 suppressed but the wave shape is not distorted. 



If the characteristic becomes non-linear but remains symmetrical 

 with respect to the point of inflection A , which functions as the oper- 

 ating point, then the impressed wave form will be delivered as a sym- 

 metrically distorted wave whose 

 amplitude is enlarged or reduced 

 according to the slope of the charac- 

 teristic. For very small amplitudes 

 this characteristic reproduces the 

 wave form as if it had the properties 

 of a linear characteristic because of 

 the small changes in slope just above 

 and below the operating point A . 

 On the other hand, if the operating 

 point lies on the lower or more pro- 

 nounced non-linear part of the curve 

 so that the point of inflection A has 

 been moved up, as shown in Fig. 

 VII-13D, then the transmitted wave 

 form is unsymmetrical both in shape 

 and amplitude. 



That contraction of the interaural 

 muscles changes the efficiency of 

 transmission, i.e., changes the ampli- 

 tude of the output which is com- 

 parable to producing a change in slope 

 of the malleus-incus characteristic, 

 is supported by the experiments of 

 Wiggers [1937]. Wiggers investigated 

 the effect of contraction of the inter- 

 aural muscles on sound transmission 

 in anesthetized guinea pigs and found 

 that in the frequency region 250 to 1000 cycles the efficiency was re- 

 duced when the muscles were under tension. In the range 1000 to 

 1200 cycles no measurable change in intensity took place, and the fre- 

 quency range 1300 to 1800 contained a peak of maximum efficiency at 

 1500 cycles where contraction augmented the efficiency in the transmis- 

 sion mechanism. 



For a working hypothesis, the change in slope of an interaural charac- 

 teristic may be attributed to a change in intensity, and the shift of the 



Fig. VII-13. A and B show a 

 linear characteristic of a coupled 

 mechanism. Input wave form plot- 

 ted on y axis. Output wave form 

 on x axis. A shows amplification, 

 no distortion, angle less than 45°. 

 B shows an input-output 1 to 1 rela- 

 tion, equal to 45°. C and D charac- 

 teristics are non-linear. C is non- 

 linear but symmetrical. The output 

 wave form is symmetrically distorted. 

 D is non-linear and asymmetrical. 

 The output wave form is unsym- 

 metrically distorted. 



