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Basilar Membrane Filtering

Once introduced into the cochlea, a remarkable mechanical frequency decomposition of the sound vibration pattern occurs. This process can, perhaps, best be understood by considering the gross mechanical structures involved. From a functional point of view the cochlea can be thought of as a very narrow, very tall, U-shaped tube embedded in the skull. The input port of the fluid-filled tube is located at one tip (the oval window operated by the stapes of the middle-ear bones), and a pressure release membrane (the round window) at the other tip. Actually the two arms of the "U" are so close together that they touch. They are, as shown in figure [1], separated only by an elastic partition - the basilar membrane.

Figure [1]: Schematic Diagram of Cochlea There is thus effectively two fluid-filled tubes, coupled together along one whole side by the flexible basilar membrane (Geisler 1987).

The flow of fluid in the cochlea, induced by the movement of the oval window, causes a standing wave-like displacement of the basilar membrane and the structures attached to it. It is this that is responsible for the simulation of the hair cells, which transform the mechanical movement to neural activity. Different sections of the basilar membrane resonate in response to different input frequencies: high frequencies cause standing waves towards the base of the cochlea, while the higher energy low frequencies cause high amplitude standing waves further along the basilar membrane towards the apex of the cochlea. The basilar membrane thus behaves like a multiple filter bank, which decomposes combined frequency sound waves into separate frequency bands. In models digital filter banks are used to simulate the action of the basilar membrane. There has been a qualitative convergence of physiological results and psychophysical theory to such an extent that the use of these filters in a physiological model can be justified (Meddis & Hewitt 1991b).


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