Abstract
The eardrum or tympanic membrane (TM) transforms acoustic energy at the ear canal into mechanical motions of the ossicles. The acousto-mechanical transformer behavior of the TM is determined by its shape, three-dimensional (3-D) motion, and mechanical properties. We have developed an optoelectronic holographic system to measure the shape and 3-D sound-induced displacements of the TM. The shape of the TM is measured with dual-wavelength holographic contouring using a tunable near IR laser source with a central wavelength of 780 nm. 3-D components of sound-induced displacements of the TM are measured with the method of multiple sensitivity vectors using stroboscopic holographic interferometry. To accurately obtain sensitivity vectors, a new technique is developed and used in which the sensitivity vectors are obtained from the images of a specular sphere that is being illuminated from different directions. Shape and 3-D acoustically induced displacement components of cadaveric human TMs at several excitation frequencies are measured at more than one million points on its surface. A numerical rotation matrix is used to rotate the original Euclidean coordinate of the measuring system in order to obtain in-plane and out-of-plane motion components. Results show that in-plane components of motion are much smaller (<20%) than the out-of-plane motions’ components.
Highlights
The hearing process involves a series of physical events in which acoustic waves in the outer ear are transduced into mechanical motions of the middle ear, acoustic and mechanical motions in the inner ear, and into chemo-electro-mechanical reactions of the inner ear sensors that are interpreted by the brain.[1]
In our previous works,[10,11,12] we have reported holographic interferometric measurements of sound-induced displacements over a majority of the surface of mammalian tympanic membrane (TM)
An optical lens is used to focus on an object of interest; our techniques are based on lensless digital holography in which reconstructions and focusing of the holograms are numerically obtained by Fresnel–Kirchhoff integrals.[13]
Summary
The hearing process involves a series of physical events in which acoustic waves in the outer ear are transduced into mechanical motions of the middle ear, acoustic and mechanical motions in the inner ear, and into chemo-electro-mechanical reactions of the inner ear sensors that are interpreted by the brain.[1] Air in the ear canal has low mechanical impedance, whereas the mechanical impedance at the center of the eardrum, the umbo, is high. It was not possible to characterize all three-dimensional (3-D) motion components including those tangent (in-plane) and normal (out-of-plane) to the local plane of the TM. Developments of a single holographic system capable of measuring both shape with sub-millimeter
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