Measurement of the human cadaver ossicle vibration amplitude by fiber-optic interferometry

In this paper we present the results obtained by contactless measurement of vibration of the incus inside the middle ear of a human cadaver, exposed to an external calibrated sound source. The measurement configuration was a Michelson fiber-optic interferometer based on a 3x3 fiber optic coupler, with a VCSEL source. The interferometric signal fading was solved using two quasiquadrature signals and specialized signal processing. The ossicle reflection was augmented by a very light piece of retroreflective material attached to the bone. We measured the amplitude of vibration at different sound pressure levels, independently monitored by a decibel meter placed in the proximity of the cadaver ear. The accuracy of the measurement technique was proved by using a calibrated piezo transducer exposed to vibrate with amplitude of 7nm at 1kHz. We performed the experiments on 8 cadavers in total. The measured amplitudes were found to be in the range between 1 pm and 1 nm, under the sound excitation of 40-90 dB SPL @ 1kHz, respectively.