Perforations of the tympanic membrane and their effects upon middle-ear transmission.

Abstract

This series of experiments concerns the basis for losses in low-frequency sensitivity accompanying a perforation in the tympanic membrane. 1. A series of experiments with a model of the tympanic membrane and middleear cavity was performed. These experiments apply to the general function of a vibrating membrane facing a cavity. They were used as an aid in drawing up experimental procedures for the animal experiments. 2. When the middle-ear cavity is in its normal unopened condition, then perforation losses for the cat are shown to increase inversely with frequency below about 1,000 Hz. The slope of the loss function is −10.2 dB per octave. 3. It is suggested that this graded low-frequency loss is caused by direct admittance of sound pressures into the middle-ear cavity, through the aperture at the site of perforation. In support of this conclusion, the presence of a perforation has been shown to affect the normal ratio of sound pressures in front and in back of the tympanic membrane, during stimulation by air-conducted tones. In addition, it is demonstrated that admittance of sound to this cavity through an opening in its outer wall also causes a graded low-frequency loss. 4. Data relevant to the effects of location of a perforation have also been presented.