This explains our loss of hearing sensitivity when we change altitude quickly, either ascending or descending. If the pressure within the middle ear is lower than that in the external ear canal, the tympanic membrane will bulge inward and sound sensitivity will also decrease. If the pressure within the middle ear is higher than that in the external ear canal, the tympanic membrane will bulge outward and sensitivity to sound will decrease. The position of the tympanic membrane sets the position of the ossicles and therefore sets the transfer of vibration from air to cochlear fluid. If there is no dissipation of energy in the bones themselves, then the sound energy can be concentrated some 15-fold where the stapes contacts the oval window. The effective area of the tympanic membrane is about 0.4–0.6 cm 2 the area of the oval window is 0.03 cm 2. The tympanic membrane and bones of the middle ear provide an impedance matching device to transfer sound energy to the cochlear fluid.
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Because of these differences, sound usually reflects off the air/water interface rather than being conducted into the water. Compressional waves in water require a completely different pressure because of the different inertia (the density) and different elastic properties (the bulk modulus: see Appendix 4.7.A1). This vibration must be transferred to the fluid in the cochlea of the inner ear. Compressional waves in air cause the tympanic membrane to vibrate. The fluid in the external ear canal is air. They transfer vibration of the tympanic membrane to vibration of the fluid in the cochlea. The three bones are the malleus ( hammer), incus ( anvil), and stapes ( stirrup). The oval window leads to a fluid-filled chamber which coils around in a structure called the cochlea. The middle ear contains three ossicles, tiny bones that connect the tympanic membrane to the membrane covering the oval window on the inner ear. The middle ear is an air-filled cavity between the tympanic membrane on one side and the promontory of the temporal bone on the other. Joseph Feher, in Quantitative Human Physiology (Second Edition), 2012 The Middle Ear Transforms Air Pressure Waves to Fluid Pressure Waves Signals pass into the cochlear nucleus, the trapezoid body, the inferior colliculus, the medial geniculate nucleus, and the auditory cortex. Several centers of the brain process signals from the inner ear. The auditory nerve courses through the modiolus into the internal auditory canal, and thence to the brain. The cell bodies of the auditory nerve reside in Rosenthal’s Canal.
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A cochlear electrode array is placed within the scala tympani, where it drives the peripheral processes of the auditory nerve cells. The hair cells reside on top of the basilar membrane within the scala media. The organ that converts auditory signals into the signals that drive the central nervous system.
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The stirrup (stapes), hammer (malleus), and anvil (incus) provide a mechanical advantage so that low-pressure, large-displacement vibrations of the tympanic membrane can be converted into high-pressure, small-displacement signals at the oval window of the cochlea. The membrane vibrates in response to acoustic stimulation. The inner boundary of the external ear canal. This work was supported in part by Grants DC005531, NS37944, and DC04614 from the National Institutes of Health.