To aid in the establishment of optimum conditions for postoperative function of the tympanic membrane, a model of the middle ear was experimentally prepared, the cross-sectioned area of aditus ad antrum was altered and the effects on the tympanic membrane vibrational characteristics were observed using the holographic interference method. The following results were obtained.
1) At the low frequencies below 2 kHz, the vibration mode of tympanic membrane was concentric on the whole, but the vibration pattern became fractionated at 4 kHz and above. The frequency of onset of the fractionated vibration pattern was changed only by the conditions of the membrane itself and was not affected much by the sound pressure, the volume of the middle ear cavity and the cross-sectioned area of aditus ad antrum.
2) There were resonance and anti-resonance points in the tympanic vibration pattern. These resonance and anti-resonance frequencies became higher as the diameter of aditus increased until eventually a constant value was reached. At diameters in excess of this constant value, the resonance frequency tended to fall and the anti-resonance frequency tended to rise.
3) The tympanic vibration in the middle ear cavity was not influenced very much by the diameter of aditus at 250Hz. Vibration decreased when aditus was blocked at 500Hz and the opening diameter had a major influence at 1kHz. Vibration was maximum at a diameter of about 2mm and a minimum when aditus was blocked. At 2 to 3kHz, the effects on vibration increased when aditus was blocked.
4) When aditus ad antrum was blocked, the vibration frequency characteristics of tympanic membrane deteriorated at under 1kHz and improved at 2 to 3kHz. When a comparison was made between the case with opened aditus, (which was considered to be equivalent to the closed method of tympano - plasty), and the case with closed aditus, (considered to be equivalent to the open method), the vibration frequency characteristics of the tympanic membrane improved in the low sound region up to 2kHz when the diameter was 4mm.