Impedance Measurements of the Human Basilar Membrane
* Presenting author
The cochlea is a mechanical frequency analyzer, owing its characteristics to the impedance of the basilar membrane (BM). In humans, the acoustic impedance of the BM has never been measured and the stiffness (or elasticity) of the human BM has not been revised since von Bekesy’s experiments. We measured intracochlear pressures in scala vestibuli and velocities of the BM 1.2 mm from the base of the cochlea. By taking the ratio of the pressure and velocity measurements, the specific acoustic impedance (Z) is calculated. At low frequencies, where the BM impedance is stiffness-dominated, the stiffness is extracted by multiplying the imaginary part of Z by the angular frequency. Our results show that the specific acoustic impedance of the BM is decreasing by 6 dB per octave at frequencies between 100 – 10,000 Hz, with a phase close to -90 degrees. The real part of the impedance is positive and slightly increasing at low frequencies. The imaginary part is negative and dominating Z at low frequencies. The specific acoustic stiffness at the measurement location amounts to 0.85 GPa/m ±0.3 GPa/m, which is about one order of magnitude higher than von Bekesy’s estimates of the static stiffness.