MIDDLE-EAR MECHANISM 267 



was estimated that the order of magnitude of these pressure changes was 

 10~~ 5 dyne/cm 2 . It has been found experimentally that the ear mechan- 

 ism responds to as little as 10 -2 dyne/cm 2 at 3000 cycles (Geffcken 

 [1934]), which limit places the thermal noises just out of the response 

 range. 



Overall Response of the Acoustic Mechanism 



The natural overall frequency of the human ear lies between 800 

 and 1500 cycles. By mounting miniature mirrors on the tympanic mem- 

 brane, Bekesy [1936] was able to show by means of a reflected light beam 

 that explosive noises containing many intense harmonics were communi- 

 cated to the malleus in a very suppressed form. When the overtones 

 of the membrane were not excited, the natural response frequency of the 

 membrane was highly damped. The logical conclusion is that this is due 

 to the retarding force of the air, the internal friction of the diaphragm, 

 and the frictional resistance introduced by the attached malleus and its 

 suspensory ligaments. 



In a specific example the natural frequency was found to be about 

 1400 cycles. There was a complete cessation of vibration after about 

 0.004 second. We may conclude that the overall vibration of the aural 

 mechanism is highly but not quite critically damped. 



Middle-Ear Mechanism 



The mechanical function of the middle ear is to transfer the energy 

 incident on the eardrum to the cochlear fluid of the inner ear. It is 

 accomplished by means of a small pressure spread over the comparatively 

 large area of the tympanic membrane, which converts it into a large 

 pressure spread over a corresponding small area in the form of the 

 footplate of the stapes where it makes contact with the cochlear fluid. 

 If only a transfer of vibrations were desired, a much simpler mechanism 

 would have sufficed, such as is found in reptiles and birds, in which the 

 fundamental plan is a bone (columella) connecting a membranous 

 window in the skull to the oval window set in the bony casing of the 

 labyrinth (Beatty [1932]). 



In discussing the biophysical transformer properties of the mechanism 

 of the middle ear the tympanic membrane with its malleus and attached 

 tympanic tensor will be treated as the first unit; the incus with its 

 attached stapes and tensor stapedius as the second unit; and the foot- 

 plate of the stapes inserted in the flexible ring of the oval window as 

 the third unit. 



