Submicroscopic Displacement Amplitudes of the Tympanic Membrane (Cat) Measured by a Laser Interferometer

Abstract

A laser interferometer, designed and constructed in this laboratory, was evaluated with respect to its applicability to measurements of amplitudes and phases in the ear. The instrument measures continuously up to a maximal amplitude of 0.92×10−5 cm, and higher at selected points. With respect to vibrations of the tympanic membrane, measured in an open sound field, the vibration isolations of the instrument was ample. The prevailing S/N ratio made it convenient to routinely measure amplitudes of 1 × 10−7 cm in living animals. At low frequencies, this corresponds to SPL's of approximately 60 dB. In fresh cadavers, however, while there was no immediate change in the magnitude of the responses, readings were extended to 1 × 10−9 cm, owing to the absence of pulse and breathing, giving a dynamic range of 80 dB. In all experiments, repeatability was excellent, and between-animal variations were low. The resulting transfer characteristics obtained in living animals, with the tympanic bulla closed, was generally similar to those obtained earlier, with some noted exceptions. The amplitude curve failed to show the slight hump around 1 kHz found earlier. The phase curve was much more complex than earlier ones and cannot be approximated by the assumption of simple network models.