I : 4/ Sound and the Ear 23 



Models indicate that the ossicles have a theoretical mechanical advan- 

 tage of 1.3. Therefore, the force on the stapes F s would be given by 



F = 1 3F 



if friction were absent. Likewise, the pressure p w , exerted by the stapes 

 on the oval window, which it contacts for 0.032 cm 2 , can be computed 

 from 



p w = FJ0.032 



Solving for the pressure amplification, 



A=^ 



Pt 



one finds a theoretical value, in the absence of friction, of twenty- two-fold. 

 Actual measurements carried out by von Bekesy have shown that the 

 correct value is 



A = \lx 



The latter number is a 25 db gain in acoustic pressure. This value is 

 believed valid throughout most of the auditory range although it is 

 based on extrapolations from low frequencies and high sound pressures. 



Since the middle ear is filled with air, any difference in pressure on 

 the two sides of the tympanic membrane will tend to displace the 

 membrane. Small differences in pressure at frequencies to which the 

 cochlea responds cause the vibrations of the tympanic membrane during 

 normal hearing. In contrast, large slow changes in pressure, due to 

 atmospheric variations or altitude changes, could distort the shape and 

 position of the tympanic membrane. To avoid this distortion, a con- 

 nection is necessary between the middle ear and the ambient air; but 

 this connection must be unable to transmit changes that take place in 

 less than a tenth of a second. A small narrow tube will do exactly this. 

 Such a tube does connect the middle ear with the pharynx; it is called 

 the eustachian tube. 



The soft walls of the eustachian tube are easily collapsed by an excess 

 pressure outside the tube. This leads to a very unpleasant feeling often 

 experienced when descending in an airplane. Swallowing, chewing 

 gum, or attempting to blow with the mouth and nose held shut, all open 

 the eustachian tube permitting the equalization of the pressure outside 

 and within the middle ear. 



The outer and the middle ear together produce a maximum pressure 

 amplification of about 35 db. They tend to reduce the hearing of 

 sounds that are conducted through the bones, to make one insensitive 

 to one's own voice except inasmuch as it is heard through air conduction 

 outside the head, and also to act as an automatic control unit. None of 



