Middle-ear function with tympanic-membrane perforations. I. Measurements and mechanisms

Sound transmission through ears with tympanic-membrane (TM) perforations is not well understood. Here. measurements on human-cadaver ears are reported that describe sound transmission through the middle ear with experimentall y produced perforations, which range from 0.5 to 5.0 mm in diameter. Three response variables were measured with acoustic stimulation at the TM: stape s velocity, middle-ear cavity sound pressure, and acoustic impedance at the TM. The stapes-velocity measurements show that perforations cause frequenc y-dependent losses; at low frequencies losses are largest and increase as p erforation size increases. Measurements of middle-ear cavity pressure coupl ed with the stapes-velocity measurements indicate that the dominant mechani sm for loss with TM perforations is reduction in pressure difference across the TM; changes in TM-to-ossicular coupling generally contribute less than 5 dB to the loss. Measurements of middle-ear input impedance indicate that for low frequencies. the input impedance with a perforation approximates t he impedance of the middle-ear cavity; as the perforation size increases, t he similarity to the cavity's impedance extends to higher frequencies. The collection of results suggests that the effects of perforations can be repr esented by the path for air-volume flow from the ear canal to the middle-ca r cavity. The quantitative description of perforation-induced losses may he lp clinicians determine, in an ear with a perforation, whether poor hearing results only from the perforation or whether other pathology should be exp ected. (C) 2001 Acoustical Society of America.