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. In measurements on the tympanic membrane, the prevailing S/N ratio made it convenient routinely to 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. In all experiments, repeatability was excellent, and between-animal variations were low. The resulting 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 before. The phase curve was much more complex than earlier ones and cannot be approximated by the assumption of simple network models. [This work was supported by grants from the National Institutes of Health.]