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

Area of Science:

• Biophysics
• Vibrational Spectroscopy
• Auditory Mechanics

Background:

• Accurate measurement of micro-vibrations in biological tissues is crucial for understanding physiological processes.
• Existing methods for measuring vibrations, such as those in the middle ear, can be limited by loading effects or sensitivity to noise.
• The malleus (ossicle) plays a key role in hearing, and its vibrational characteristics are fundamental to auditory function.

Purpose of the Study:

• To describe a simple, non-loading interferometer for measuring extremely small biological vibrations (down to 0.01 nm).
• To apply this interferometer to measure malleus vibrations in cats.
• To investigate the effect of middle ear underpressure on the malleus transfer function and assess the linearity of malleus vibrations.

Main Methods:

• Development of a simple interferometer utilizing the Doppler shift of diffusely scattered light to measure vibration velocities.
• Application of the interferometer to measure vibrations of the cat malleus without requiring a reflective coating or external mirror.
• Testing the instrument's sensitivity to background noise and its performance under varying sound pressure levels (30-110 dB SPL).

Main Results:

• The interferometer successfully measured malleus vibrations with high accuracy, consistent with previous research.
• The velocity measurement approach proved advantageous due to lower frequency dependence compared to amplitude measurements.
• Middle ear underpressure caused a significant (11 dB) decrease in malleus sensitivity below 1500 Hz.
• Malleus vibration linearity was confirmed across a wide range of sound pressures.

Conclusions:

• The developed interferometer is a sensitive, non-loading tool for measuring micro-vibrations in biological samples.
• The method's robustness to noise and direct velocity measurement make it well-suited for applications like auditory mechanics.
• Middle ear underpressure significantly impacts auditory signal transmission, particularly at lower frequencies.