Study On Two-way Fluid-structure Interaction Modeling And Active Amplification Of Human Cochlea

Through thousands of years evolutionary process, hearing of mammals is one of the most extremely complex system. Human ears exhibit remarkable transducer characteristics. It is quite hard for people to understand the auditory mechanisms thoroughly. Especially for the key organ, cochlea, mechanisms of the sound transmission in it are not totally clear yet. Therefore study on the dynamic characteristics of cochlear response to sound excitation will be of great importance. In order to better understand the cochlear activity, macromechanics and micromechanics of the human cochlea are studied via modeling and experiment. This paper includes the following five components:The first part is modeling and analysis of the two dimensional cochlear model. A two dimensional model of human cochlea is built, which includes the two-way fluid-structure interaction between the perilymph and basilar membrane. Response of the basilar membrane to oval window stimuli is simulated, and the results indicate: different position of the basilar membrane is sensitive to different frequency stimuli, this is due to the frequency selectivity of the cochlea. The effects of helicotrema on basilar membrane response are analyzed use the model, and the results have shown that appropriate helicotrema could make basilar membrane vibrate exactly. Response of the basilar membrane under round window stimulus is calculated, the results demonstrated that stimulation on the round window can generate an effective basilar membrane vibration.Based on the two dimensional model, a three dimensional finite element model of human cochlea is established with the two-way fluid structure-interaction method. The model is excited by the oval window displacement, and the responses of basilar membrane, round window, fluid pressure and cochlear impedance are simulated. The validity of the finite element model was confirmed by the comparison of simulation results and experiment results. The research result shows that cochlear impedance changes with the frequency, which varies slowly on the low frequencies and increases rapidly on the high frequencies. Round window vibrates with inverse phase compare with the vibration of oval window, it shows that the round window is a buffer of cochlea. The simulation results also indicate the human cochlea possess frequency selectivity and traveling wave characteristics. This model can not only simulate the response of basilar membrane correctly, but also gain more accurate fluid pressure in cochlea.Micromechanical model of the partial organ of corti in human cochlea is established. The differential equations of motion of the systems are established, and the analytic solutions of the displacement of the basilar membrane, tectorial membrane-reticular lamina composite structure and out hair cell are derived. In order to compare with the experiment results of guinea pig, the parameters of guinea pig are used in the system. Basilar membrane responses of the passive and active cochlea are simulated, and the results show that active cochlea possesses nonlinear compression and frequency shift characteristics. We analyzed the phase relations between each structure in the organ of corti in theory for the first time. Research results indicate phase modulation in the organ of corti influences the active amplification. Further study on the phase modulation found that the equivalent nonlinear damping of the out hair cell is the key factor.On the basis of micromechanical model’s research results, the force on basilar membrane from composite structure is calculated in the passive cochlea. And also the resultant force on basilar membrane in the active cochlea is calculated, proportional relation is gained through dividing resultant force in active cochlea by resultant force in passive cochlea. Applying them to the three dimensional finite element model of human cochlea, the effects of composite structure on basilar membrane and the active cochlear responses are computed. It turned out that composite structure has little effect on basilar membrane; active cochlea can amplify the vibration of basilar membrane, and the position of maximum displacement amplitude is different with the passive cochlea.A experiment platform for testing the vibration of mammalian cochlea is built, which used the laser Doppler velocimeter to gather the vibration of basilar membrane and stapes. It is the first time to test the vibration of guinea pig cochlea under external acoustic stimuli in China. The experiment results show that the displacement amplitude of stapes reaches maximum at 2 kHz, and then decreases slowly. Displacement amplitude of the basilar membrane has only one peak over the whole frequency range under each single frequency excitation. There is a one-to-one correspondence between the peak locations and excited frequencies. For any position on the basilar membrane, the frequency which makes it a peak location is the corresponding best frequency. The phase of basilar membrane displacement decreases quickly at the best frequency, and this is consistent with the traveling wave theory. The cochlear gain is calculated and the results show cochlear gain might be unrelated to intensive intensity. From qualitative comparison between simulation results of the finite element model and the tested data on guinea pig cochlea, calculated results are consistent with the experimental measurements.