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Publication

Middle ear mechanics and eardrum material properties in mammals and lizards

Book - Dissertation

Subtitle:an experimental and modeling approach
In the first step of the hearing process, the eardrum captures sound vibrations that are transmitted by three ossicles to the fluid-filled cochlea. The combined system of eardrum and ossicles is called the middle ear and acts as an acoustic-mechanical transformer. Computer modeling plays a vital role in understanding how the middle ear transports sound energy to the cochlea. Moreover, these simulations allow us to vary the material properties, such as mass distribution and stiffness, of this system to investigate which properties play an important role and at which frequencies. Such variations are difficult or impossible to achieve experimentally, so simulations lead to valuable insights. However, such models depend on well-defined input parameters to obtain valid results. First, the middle ear of the anolis lizard (Anolis sagrei) was examined using finite element modeling. The computer simulations showed that these animals use internally coupled middle ears to localize sounds. It was found that simulating the effect of the fluids of the inner ear improved the accuracy of the simulations and it is important to include in future studies. Next, the displacement of the eardrum of the gecko (Gekko gecko) was measured when slowly varying pressure was applied. The largest displacements to date were observed, showing that this middle ear is highly elastic. Next, the stiffness of the human eardrum was calculated using a new method. A human eardrum was made to vibrate under different sound frequencies. From this vibration data, the displacement and deformation of the eardrum were calculated. Then the so-called virtual-field method was developed for these thin vibrating membranes, which led to the identification of the stiffness of the eardrum. When the displacements become large, the virtual-field method must be extended to include the theory of finite deformations. This new method was validated on rubber membranes, but should certainly be tested on eardrums in the future to find out material values of the eardrum in this case as well. Finally, the presence of so-called preload was investigated. When the eardrum is at rest, tensile forces from the middle ear can still create internal tension on the eardrum. By making several small incisions across the rabbit's eardrum, it was shown that prestress was definitely present. Future work should investigate whether this effect also occurs in the human eardrum.
Number of pages: 246
Publication year:2022
Keywords:Doctoral thesis
Accessibility:Open