Laser--Doppler velocity meter applied to tympanic membrane vibrations in cat

This paper describes a simple interferometer that can be used for the measurement of extremely small vibrations (down to 0.01 nm) in biological preparations. Instead of amplitudes it measures velocities by detecting the Doppler--shift in the frequency of light diffusely scattered by a vibrating object. It has been applied to the measurement of malleus vibrations in cat. The malleus itself scatters sufficient light for the interferometer to operate so that no mirror is needed that might load the malleus. Since the instrument measures velocities it is very well suited for this application because the velocity of the malleus vibrations is much less dependent on the signal frequency than is its amplitude. The method is very insensitive to background noises or vibrations so that no special precautions for isolation need to be taken. The vibrations of the malleus measured with this instrument turn out to be in excellent agreement with data by previous workers. The effect of underpressure in middle ear cavities on the transfer function was measured and appears to be rather severe (11 dB sensitivity decrease) for frequencies below 1500 Hz only. The linearity of the malleus vibration ws checked for sound pressures of 30 dB SPL up to 110 dB SPL.