The tympanic membrane (TM) (or eardrum) is the initial structure in the middle-ear's acoustic-mechanical transformation of environmental sounds to sound within the inner ear. While there are many hypotheses of how the TM couples sound to the rest of the ear, there is little data to test these hypotheses. In the past three years we have used newly developed laser holography techniques to measure the magnitude and phase of sound-induced motions of the TM surface in a number of mammalian species. The results of those measurements suggest that TM motions can be well described by a summation of a few types of motion, including: relatively long-wavelength modal (2D standing-wave) motions and relatively short wavelength traveling waves. This grant will first test the generality of this simple description across a broad range of sound frequencies and middle-ear types, and use spatial variations in the velocity of the traveling waves to compare the mechanical properties of different anatomically identifiable locations of the TM in multiple mammalian species. Next, these data will be compared to simultaneously gathered laser-vibrometer measurements of sound-induced ossicular motion in order to test the popular theory that delays associated with wave-travel on the TM contribute to delays in sound-conduction through the middle ear. Additional TM surface and ossicular motion measurements made with modified and artificial ear canals test a second theory by determining whether the location and orientation of the TM within the ear canal contribute to sound- induced surface waves on the TM and delays in ossicular motion. Several other theories of how perforations affect TM motion and middle-ear sound transfer will be tested by measurements of the sound-induced motion of the TM and ossicles before and after controlled perforations and slits in the TM. Finally, we apply our techniques to assess how a common middle-ear reconstruction technique, the use of thin cartilage sheets on the TM, affects both TM and ossicular motion. A surprising preliminary result that requires further investigation is that the placement of cartilage sheets can greatly reduce traveling waves on the TM while producing little change in ossicular motion. If generally true, this result implies that the traveling waves we see on the TM are not relevant to sound transfer through the middle ear. Such a result would suggest that long-wave-length modal displacements of the TM, which are less affected by the cartilage, determine TM function, and refute theories that complex interactions of multiple short-wave-length responses drive the TM's response to higher frequency sounds.
Disorders of the eardrum and middle ear are some of the most common causes of hearing loss, and procedures to eliminate middle-ear disease and reconstruct middle-ear function are the most common surgeries performed by otologists. The work proposed will better define the function of the normal and pathologic eardrum and can lead to improvements in reconstructing the eardrum.
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