A METHOD FOR DETERMINING 3-DIMENSIONAL VIBRATION IN THE EAR

In the classical concept of the middle ear function the malleus rotate s around a fixed axis which implies that at small amplitudes of vibrat ion its displacement is essentially one dimensional. As a consequence malleus vibrations have been measured previously along a single viewin g axis. As a first step in the study of the complete malleus motion we determined the three dimensional components at a single point (umbo) of the manubrium. To define 3-D motion it is in principle necessary to measure the vibrations from widely different observation angles. The viewing angles are limited however in our case by the ear canal geomet ry to about +/-15 degrees. In order to resolve the 3-D components unde r these conditions it is necessary to measure the vibration components with high accuracy. Amplitude and phase of the umbo vibrations were m easured with a heterodyne interferometer over a wide frequency range ( 100 Hz to 20 kHz). The system included a two axis goniometer with the axes of rotation positioned at the focal plane of the interferometer o bjective lens. It was therefore possible to change the viewing angle i n small increments around two orthogonal axes while keeping the same p oint in focus. From a redundant set of measurements the three orthogon al components of vibration were calculated by least squares fitting. T he vector sum of the three components gives the three dimensional moti on of the observed point. The vibration of the point on the umbo was f ound not to follow a straight line but an elliptical path instead. The shape of the ellipse and the inclination of the plane of the ellips w ith respect to the stationary malleus position changed with frequency. These observations are consistent with our earlier findings that the mode of malleus vibration changes with frequency [Decraemer et al. (19 91) Hear. Res. 54, 305-318].