Laseraudiometer

Vibration of the ear drum in response to acoustic stimulation is measured with a laser Doppler vibrometer. The laser light derives from a helium-neon laser (wavelength 633 nm), which is coupled into a standard operating 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.