Laseraudiometer

Vibration of the ear drum in response to acoustic stimulation is measured with a laser Doppler vibrometer. This is the setup that we used in the original publication (Rodriguez Jorge et al., 1997) to demonstrate proof of principle. It is based on a commercially available laserinterferometer. The interferometer together with the head of a standard operating microscope are mounted on a custom-built stand. The stand was so designed that even under the total weight of interferometer and microscope (12 kg), the position of the laser beam on the ear drum could be adjusted with an accuracy of 10 µm. The laser light derives from a helium-neon laser (wavelength 633 nm), which is coupled into the microscope, and focused onto the ear drum to a diameter of about 70 µm. The light is reflected from the ear drum; its frequency is (Doppler) shifted by the motion of the ear drum. A portion of the reflected light returns back through the microscope objective, and the velocity of the ear drum is extracted from the frequency shift of the light. The device functions in the same way, for example, as a radar for determining the speed of a car: the Doppler effect. Mechanical conditions of the middle ear and the cochlea can be determined from the dependence of ear-drum velocity on stimulus frequency and sound pressure.
© 2005 A. W. Gummer. Picture by Dr. J. Rodriguez Jorge.