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THE mechanics of sound conduction are generally appreciated in clinical otology. The basic principles of impedance matching by tympanic membrane to oval window areal ratio and by the ossicular lever system have been measured with considerable accuracy by investigators using varied techniques. This body of knowledge has stimulated and been stimulated by the development of surgical methods for sound restoration in conductive hearing loss.The otologist, in attempting to correlate abnormalities of the sound conduction system to the degree of functional hearing loss measured audiometrically, tries to rationalize findings through principles of normal physiology. Clinical texts, however, contain only simple empiric statements about hearing losses associated with certain lesions of the conductive mechanism. The physiology literature, as well, contains few studies of the abnormal middle ear. For these reasons, it is desirable to study in detail the hearing changes associated with abnormalities of the sound conduction system. This presentation will describe the hearing losses associated with various carefully documented and distinct lesions of the sound conduction mechanism, including discussion of these findings in relation to known physiologic principles. The presentation will be organized by first discussing the significant physiologic hypotheses of the past and then the present modifications of these ideas. This will be followed by presentation of the collected observations for each specific disease category. Discussion of the specific findings as they relate to the studies of others will then follow within each category of middle-ear lesion. A general discussion of the problem of mechanisms of hearing loss in diseased ears will then be presented, followed by the conclusions of this study. Historic aspectsHelmholtz initiated the period of modern auditory physiology when he defined the principles of impedance matching and how the middle ear served this function. Formulating the problem of matching low impedance air-borne sound to the high impedance of the fluid filled cochlea, Helmholtz conceived of three means by which pressure transformation could take place. The first was through the tympanic membrane which he felt acted as a catenary lever. He stated that the radial fibres of the tympanic membrane acted as stretched cords which, when vibrated in their central *This was presented as a

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Tympanosclerosis is found in more than 30% of patients with chronic ear disease. Clinically significant tympanosclerosis is defined as that affecting the surgical procedure by requiring removal to effect a hearing improvement. This type is seen in less than half of all patients with tympanosclerosis. The incidence and patterns of involvement are documented for 352 of the author's surgical patients subdivided into two groups: one operated on in the early 1960s and another observed more than 15 years later. The surgical techniques employed for hearing restoration are described, as are the hearing results obtained. The findings would indicate that tympanosclerosis is amenable to surgical correction, with a success rate approximately that of nontympanosclerotic ears. The special techniques needed to avoid complications when the disease invades the oval window are emphasized.