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Copy of MECH3200 - Cochlear Implants and how they simulate the Inner Ear.
Transcript of Copy of MECH3200 - Cochlear Implants and how they simulate the Inner Ear.
Nathaniel Chand Mech3200
Dynamics of the Inner Ear, and how Cochlear Implants simulate it. The Model The model corresponds to the equation of motion of the Basilar Membrane Analytical Model Membrane movement simulation: Assumptions
anti-symmetric pressure wave
Only consider one fluid duct (Scala Vestibuli) Fluid simulation: Assumption: force exerted by outer hair cells on cochlear partition is proportional to total force acting on partition. Nonlinear cilia mechanics Linear and Nonlinear components: Calculations and Assumptions BM’s movement is governed by: Fourier transform of the basilar membrane’s motion Summing the Fourier harmonics Amplitude: Fluid Pressure: The force acting on the cilia: The force on the basilar membrane is the sum of the forces exerted by the fluids and the outer cilia: Combining: Overview Conclusion Animation and Observations * Fourier Transform: Nonlinear cilia mechanics System of eqn's, force balance: Cochlear Implant Simplifications and Assumptions The Basilar Membrane is modeled as an orthotropic plate.
Curvature is neglected
Overview External microphone pickup
-> filtering of sound
-> signal to receiver (secured in bone)
-> cable to cochlea (internal)
-> electrodes in organ of corti stimulate auditory nerve
->artificial sound signal to brain Processes random vibration by fast Fourier transforms
Limitation to the number of frequencies, processed.
Two models considered
Long flat plate
Share common dimensions
Length of 35mm
Thickness of 0.02mm
Width varying linearly with length
0.15mm at oval window (near the stapes)
0.56mm at helicotrema Simulating the Membrane Young’s modulus 1MPa
Poisson’s ratio 0.5
Damping ratio 2x10-6
Density 1 g/cm3
Such a small modulus will result in a very small stiffness. Material properties Flat plate natural frequencies Curled plate natural frequencies Approximated the cochlear as a series of flat plates
Initial Design and Development Initially utilized "feature extraction"
recognized by fundamental frequencies and higher frequency format
Intended to emphasize certain speech sounds
From a linguistic point of view...useful?
Todays Cochlear Implants Use Fast Fourier Transform filtering system
artificially excite stereocillia associated with particular frequencies
Still much simpler then natural cochlear