304 



AUDITORY BIOPHYSICS 



o. 



10 



20 



30 mm 



maximal displacements at positions on its surface governed by the fre- 

 quency of the pulses traveling up the canal. Pressure gradients can 

 be produced across the membrane, the locations of which are also deter- 

 mined by the frequency. Reboul showed 

 that if the compressional wave starting at 

 the stapes had a steep wave front, such as is 

 associated with high-frequency excitation, then 

 the greatest pressure distortions of the mem- 

 brane took place in the basal turn of the 

 cochlea. At low frequencies it took place in 

 the apical turn. Samples of the calculated 

 pressure distribution in these wave forms are 

 shown in Fig. VII-24. 



An important item in the analysis is the 

 speed of propagation of the pressure pulse dis- 

 torting the elastic and highly damped floorplate. 

 The calculated speed of propagation was found 

 to be approximately 50 meters per second. 

 This low velocity is possible because of the 

 wall effects which enter the calculations in 

 the form of frictional factors introduced by 

 the rapidly decreasing dimensions of the tube 

 and the drag of the elastic membranes, which 

 become progressively thicker and wider as the 

 apex of the cochlea is approached. 



That such a low velocity can exist is sup- 

 ported by experimental evidence. The exist- 

 ence of a slow-traveling cochlear wave was veri- 

 fied by Bekesy [1933]. From his data one may conclude that a wave 

 pulse introduced at the basal end of the cochlea travels to the apical 

 end with a speed of 20 to 30 meters per second. The speed is larger 

 at the stapes and smaller at the helicotrema. The experimental time 

 interval between the arrival of a sound at the eardrum and its arrival 

 at the round window was found to be not greater than 0.1 millisecond. 

 If the impulse had traveled via the helicotrema with a speed of 25 

 meters per second, it would have taken 2.4 milliseconds to pass through 

 the 60-mm path of fluid. Since it took only 0.1 millisecond, it must 

 have short-circuited across the aural membrane at a point lying 0.12 mm 

 from the basilar end. This evidence indicates that the pressure gradient 

 of the hydraulic impulse short-circuits across the aural membrane to 

 reach the round window. 



In this way a response area due to an 800-cycle frequency was located 



Fig. VII-24. Reboul's 

 [1938] calculated curves 

 showing the positions of 

 maximum pressure gradi- 

 ent in the cochlea. The 

 positions on the aural 

 membrane of the maxi- 

 mum pressure gradients 

 are indicated by the 

 dotted lines. 



