How do tympanic-membrane perforations affect human middle-ear sound transmission?

Although tympanic-membrane (TM) perforations are common sequelae of middle- ear disease, the hearing losses they cause have not been accurately determi ned, largely because additional pathological conditions occur in these ears . Our measurements of acoustic transmission before and after making control led perforations in cadaver ears show that perforations cause frequency-dep endent loss that: (1) is largest at low frequencies; (2) increases as perfo ration size increases; and (3) does not depend on perforation location. The dominant loss mechanism is the reduction in sound-pressure difference acro ss the TM. Measurements of middle-ear air-space sound pressures show that t ransmission via direct acoustic stimulation of the oval and round windows i s generally negligible. A quantitative model predicts the influence of midd le-ear air-space volume on loss; with larger volumes, loss is smaller.