Measurements of three-dimensional shape and sound-induced motion of the chinchilla tympanic membrane

Opto-electronic computer holographic measurements were made of the tympanic membrane (TM) in cadaveric chinchillas. Measurements with two laser wavelengths were used to compute the 3D-shape of the TM. Single laser wavelength measurements locked to eight distinct phases of a tonal stimulus were used to determine the magnitude and the relative phase of the surface displacements. These measurements were made at over 250,000 points on the TM surface. The measured motions contained spatial phase variations consistent with relatively low-order (large spatial frequency) modal motions and smaller magnitude higher-order (smaller spatial frequency) motions that appear to travel, but may also be explained by losses within the membrane. The measurement of shape and thin shell theory allowed us to separate the measured motions into those components orthogonal to the plane of the tympanic ring, and those components within the plane of the tympanic ring based on the 3D-shape. The predicted in-plane motion components are generally smaller than the out-of-plane perpendicular component of motion. Since the derivation of in-plane and out-of plane depended primarily on the membrane shape, the relative sizes of the predicted motion components did not vary with frequency.Summary: A new method for simultaneously measuring the shape and sound-induced motion of the tympanic membrane is utilized to estimate the 3D motion on the membrane surface.This article is part of a Special Issue entitled “MEMRO 2012”.

► A method simultaneously measures shape and motion of the tympanic membrane and estimates the 3D motion of the surface.
► The method reveals sound-induced motions of the chinchilla TM surface similar to those reported in humans.
► The computed in-plane TM motions are of significantly smaller magnitude than the out-of-plane motions.
► The assumptions used in deriving 3D motion lead to all motion components being either in phase or a half-cycle out-of-phase.