Middle-ear function with tympanic-membrane perforations. II. A simple model

A quantitative model of the human middle ear with a tympanic-membrane (TM) perforation is developed. The model is constrained by several types of acou stic measurements made on human cadaver ears, which indicate that perforati on-induced changes in transmission result primarily from changes in driving pressure across the TM and that perforation-induced change in the structur e of the TM and its coupling to the ossicles contributes a substantially sm aller component. The model represents the effect of a perforation on the pr essure difference across the TM by inclusion of a path for sound coupling t hrough the perforation from the ear canal to the middle-ear cavity. The mod el implies that hearing loss with perforations depends primarily on three q uantities: the perforation diameter, sound frequency, and the volume of air in the middle-car cavity. For the conditions that produce the largest hear ing loss (low frequency and large perforation), the model yields a simple d ependence of loss on frequency, perforation diameter, and middle-ear cavity volume. Predictions from this model may be useful to clinicians in determi ning whether, in particular cases, hearing losses are explainable by the ob served perforations or if additional pathology must be involved. (C) 2001 A coustical Society of America.