22 Sound and the Ear /I : 4 



discriminating against vibrations reaching them via the skull. This 

 insensitivity of the ossicles to bone conduction, as well as the symmetry 

 of the vocal cords, restricts most of the hearing of one's own voice to 

 sound transmitted in the air from the mouth around to the ears. (This 

 can be demonstrated by covering one's ears while talking and noting 

 the changes in loudness and quality.) 



The ossicles are believed to have an additional function besides 

 impedance matching. This is to decrease the amount of energy fed into 

 the inner ear at high sound levels. Part of this is thought to be accom- 

 plished by changes in the tension of the tensor tympani and stapedius 

 muscles which hold the ossicles in place. The action may be compared 

 to the automatic volume control in a superheterodyne radio. In both 

 cases, when a large signal enters the system and is detected, the amplifica- 

 tion of an earlier portion of the system is decreased. These are specific 

 examples of so-called "feedback systems" or "automatic control," as 

 this type of phenomenon is called by physicists and engineers. (Physiol- 

 ogists usually call this type of effect a "homeostatic mechanism.") In 

 the case of the middle ear, one may describe this action in teleological 

 terms as trying to maintain a constant sound level incident to the inner 

 ear. Although this response is too slow to protect the ear from damage 

 due to sudden noises, it is of the proper nature to explain the flattening 

 of the equal loudness contours at high intensities. 



High signal transmissions are also limited by a shift in the mode of 

 vibration of the stapes. In one of its two possible modes of vibration, 

 the stapes pushes uniformly on the oval window. In the other it rocks 

 in such a fashion that it causes a negligible net displacement of the oval 

 window. The latter type of motion is believed more important at higher 

 intensities. Both the variable coupling and the two possible modes of 

 vibration are nonlinear effects. Both contribute to harmonic generation 

 as well as to amplitude distortion. 



In physical form the outermost ossicle, the malleus, is pressed against 

 the tympanic membrane. The innermost one, the stapes, pushes against 

 a membrane called the oval window which separates the air-filled middle 

 ear from the liquid-filled channels of the inner ear. The oval window 

 forms one end of one of these channels, the scala vestibuli. Another 

 channel, the scala tympani, also ends in a membrane separating it from 

 the middle ear. This second membrane is called the round window. 



The effective area of the tympanum in a human is about 0.66 cm 2 

 of which perhaps 0.55 cm 2 is in contact with the malleus. The force 

 F m on the malleus, due to the acoustic wave, equals the product of the 

 pressure, p t , on the tympanum times the area of contact. That is, 



F m = 0.55 p t 



