REBOUL'S HYDRODYNAMIC THEORY 303 



50 milliseconds. As pointed out previously, the transmission mechanism 

 possesses inertia and resistance. Measurement of the damping factor 

 of the aural membrane itself is, of course, practically impossible since 

 the membrane is closely coupled to the organ of Corti, the attached 

 tectorial membrane, and the tissues lining the scala tympani. If the 

 aural membrane is considered as a whole, the viscosity of the adjoining 

 fluids must produce a high degree of damping, and evidence from the 

 cochlear microphonic measurements of Saul and Davis [1932] leads to 

 the belief that it is very highly damped. 



To produce a sharp resonance of the aural membrane it is necessary 

 that the resistance of the membrane be small and the damping negligible. 

 If the resonance is poor because of a large resistance, the system is said 

 to be highly damped, so that the peak of resonance is less sharp. In 

 consequence, this poor resonance does not lead to a high degree of selec- 

 tivity on the part of the resonance mechanism. On the other hand, with 

 small damping and its accompanying high degree of selectivity, time is 

 required for a resonance system to come to a steady state in response 

 to an applied force. 



A person is not conscious of appreciable sensations of sound which 

 persist after the external wave has ceased to function. The fact that 

 a discontinued sound is heard to last only about 50 milliseconds is proof 

 of the great damping undergone. This leads one to discount the popu- 

 lar idea sometimes attributed to Helmholtz that the aural membrane 

 acts like a set of tuned resonators strung out in series on the basilar 

 membrane. 



Reboul's Hydrodynamic Theory 



If the cochlea is treated as a hydraulic tunnel in which a strip of 

 longitudinal wall is elastic, simulating the aural membrane, then the 

 necessary conditions for a selective response mechanism with a large 

 damping factor, are present. 



Reboul [1938] suggested that the response of such a hydrodynamic 

 system could be assumed to follow the mathematical analysis of a hydrau- 

 lic system contained in a cylindrical vessel with elastic walls along which 

 a pressure disturbance is propagated. 



The response mechanism of the cochlea tube can then be pictured as 

 follows. The compressional wave starting at the oval window, caused 

 by an inward movement of the stapes, travels up the scala vestibuli 

 towards the helicotrema. The scala media and its aural membrane may 

 be considered as an elastic floorplate having a high damping factor. 



According to Reboul's analysis, which contains reasonable assumptions 

 about the property of the liquid, the elastic membrane can undergo 



