The effects of positive and negative pressure in the middle ear on tympanic membrane (TM) vibrations were studied in twenty canine temporal bones by holographic interferometry. The displacement of the TM was measured by moire topography.
1) When pressure was applied to the tympanic cavity, the curvature of the TM became small under negative pressure and large under positive pressure, with the displacement being greater under positive pressure.
2) Without pressure load, the vibration pattern below 2kHz was simple and there were peak displacement regions in the posterior and anterior parts of the membrane and the peaks occurred approximately halfway along the manubrium. The TM vibrations showed sectional patterns, above 3kHz in the posterior and above 4kHz in the anterior.
The amplitude of the anterior peak was larger than that of the mallar tip, but smaller than that of the posterior.
3) At frequencies below 2kHz, the vibration pattern was not affected by negative pressure load. At frequencies of 3kHz or higher, the sectional patterns changed into the simpler patterns and the sectional vibrations diminished as the pressure increased.
4) Below 2kHz, the TM amplitude decreased with increasing negative pressure. Above 3 kH, the amplitude showed an initial increase but decreased at higher negative pressure loading.
With the amplitude of the mallar tip, the same tendency was observed. The resonance frequency shifted to a higher frequency range with pressure loading.
5) Under positive pressure, the vibration pattern remained unchanged below 2kHz, and above 3 kH, sectional vibrations changed to the simple vibrations. The vibration amplitude showed the same tendency as observed under negative pressure.
6) When either negative or positive pressure was applied to the tympanic cavity, the tympanic membrane was displaced inwardly or outwardly together with a shift of the ossicular chain. Due to this displacement, a change of the curvature of the TM and also an increase or decrease in the volume in the tympanic cavity occur. An increase in the stiffness of the TM was considered to have been the main cause of the change of vibrations.
