Optical measurements of eardrum vibrations and sound propagation in the ear canal for the fitting of active middle ear implants

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BACKGROUND: Middle ear implants (MEI) are for the medical rehabilitation of the hearing function in case of sound conduction hearing losses as well as cochlear hearing losses and their combinations. OBJECTIVES: An objective tool to reach the best fitting of the external worn sound processors is essential for patients who do not want or cannot participate in the fitting process. METHODS: In addition to Laser-Doppler-Vibrometry (LDV) measurement, the sound pressure was measured distant to the eardrum to attain additional information for comparison. Three groups of patients with different middle ear characteristics were examined. RESULTS: Because of the large spreading of measuring results even within a patient group with similar eardrum and middle ear conditions it is difficult to develop characteristic diagrams which represent the mean values of eardrum displacements with different sound processor adjustments being the base for normative data courses. CONCLUSIONS AND SIGNIFICANCE: The LDV measurements can be used as a tool for fitting sound processors by finding individual maximum eardrum velocities in the frequency range 125 Hz to 8 kHz. In comparison to acoustical measurements the optical measurements have advantages concerning lower variations of measurement values, higher spectral resolution, and robustness against disturbing acoustic noise, especially at low frequencies.

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