The primary goal of the proposed research is understanding otoacoustic emissions and their relation to cochlear mechanics in normal and abnormal human ears. Otoacoustic emissions provide a direct, non-invasive measure of outer hair cell function, cochlear nonlinearities and the role of auditory efferents in cochlear function. Thus, they are a potentially powerful non-invasive research and clinical tool for assessing normal and abnormal cochlear mechanics in humans. However, several questions concerning the nature of the emission generator(s) and its relationship to cochlear status must be addressed in order to optimally interpret clinical data. Specific questions to be addressed by the proposed research include: (1) What is the nature of the emission generator ? (2) What are the relationships between transient-evoked, stimulus-frequency and combination-tone emissions ? (3) What is the role of auditory efferents in normal cochlear mechanics ? (4) How are emissions, and thus indirectly cochlear mechanics, affected by temporary threshold shift and permanent hearing loss due to noise exposure and aging in the absence of other cochlear insults ? The proposed research will provide a systematic set of data on the input-output relations among evoking stimuli and evoked emissions in large numbers of normal and abnormal ears. These data will provide a basis for better understanding the nature of cochlear hearing loss and the clinical potential of otoacoustic emissions, and serve as data for evaluating and formulating models of normal and abnormal cochlear mechanics.