ABSTRACT
The ear drum is caused to vibrate in sympathy with the air in the auditory canal when excited by a sound wave, and these vibrations are transferred via the bones of the middle ear to the inner ear, being subject to a multiplication of force of the order of 15:1 by the lever arrangement of the bones. The lever arrangement, coupled with the difference in area between the tympanic membrane and the oval window, helps to match the impedances of the outer and inner ears so as to ensure optimum transfer of energy. Vibrations are thus transferred to the fl uid in the inner ear in which pressure waves are set up. The basilar membrane is not uniformly stiff along its length (it is narrow and stiff at the oval window end and wider and more fl exible at the far end), and the fl uid is relatively incompressible; thus a high-speed pressure wave travels through the fl uid and a pressure difference is created across the basilar membrane.
