The hearing process involves a series of physical events in which acoustic waves in the outer ear are transduced into acousto-mechanical motions of the middle ear, and then into chemo-electro-mechanical reactions of the inner ear sensors that are interpreted by the brain. Air in the ear canal has low mechanical impedance, whereas the mechanical impedance at the center of the eardrum, the umbo, is high. The eardrum or Tympanic Membrane (TM) must act as a transformer between these two impedances; otherwise, most of the energy will be reflected rather than transmitted. The acousto-mechanical transformer behavior of the TM is determined by its geometry, internal fibrous structure, and mechanical properties. Therefore, full-field-of-view techniques are required to quantify shape, sound-induced displacements, and mechanical properties of the TM. Shapes of the mammalian TMs are in millimeter ranges, whereas their acoustically-induced motions are in nanometer ranges, therefore, a clinically-applicable system with a measuring range spanning six orders of magnitude needs to be realized. In this Dissertation, several full-field measuring modalities are developed, to incrementally address the questions regarding the geometry, kinematics, and dynamics of the sound-induced energy transfer through the mammalian TMs. First, a digital holographic system with a measuring range spanning several orders of magnitude is developed and shape and 1D sound-induced motions of the TM are measured with dual-wavelength holographic contouring and single sensitivity vector holographic interferometry, respectively. The sound-induced motions of the TMs are hypothesized to be similar to those of thin-shells (with negligible tangential motions) and therefore, 3D sound-induced motions of the TM are estimated by combining measurements of shape and 1D motions. In order to test the applicability of the thin-shell hypothesis, and to obtain further details of complex spatio-temporal response of the TMs, holographic systems with multiple illumination directions are developed and shape and acoustically-induced vibrational patterns of the TMs are quantified in full 3D. Furthermore, to move toward clinical applications and in-vivo measurements, high-speed single-shot multiplexing holographic system are developed and 3D sound-induced motions of the TM are measured simultaneously in one single frame of the camera. Finally, MEMS-based high-resolution force sensing capabilities are integrated with holographic measurements to relate the kinematics and dynamics of the acousto-mechanical energy transfer in the hearing processes. The accuracy and repeatability of the measuring systems are tested and verified using artificial samples with geometries similar to those of human TMs. The systems are then used to measure shape, 3D sound-induced motions, and forces of chinchilla and human cadaveric TM samples at different tonal frequencies (ranging from 400 Hz to 15 kHz) simultaneously at more than 1 million points on its surface. A general conclusion is that the tangential motions are significantly (8-20 dB) smaller than the motions perpendicular to the TM plane, which is consistent with the thin-shell hypothesis of the TM. Force measurements reveal that frequency-dependent forces of the TM, are also spatially dependent so that the maximum magnitudes of the force transfer function of the umbo occurs at frequencies between 1.6 to 2.3 kHz, whereas the maximum values for other points on the TM surface occurs at higher frequency ranges (4.8 to 6.5 kHz). The Dissertation is divided into two Parts, each contains several Chapters. In the first Part, general overviews of the physiology of the human middle ear, along with brief summaries of previous studies are given, and basics of holographic interferometry are described. In the second Part, developments and implementations achieved in completion of this work are described in the form of a series of manuscripts. Finally, conclusions and recommendations for future work are provided.
Worcester Polytechnic Institute
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Khaleghi, M. (2015). Development of Holographic Interferometric Methodologies for Characterization of Shape and Function of the Human Tympanic Membrane. Retrieved from https://digitalcommons.wpi.edu/etd-dissertations/228
tympanic membrane, holographic interferometry, sound-induced motion