On ear canal measurement techniques applied to hearing development

Few data exist on the functional development of the human external and middle ear, yet constraints imposed by these structures limit the sound power transmitted to the cochlea. One measure of hearing development is the middle ear reflectance, the fraction of incident sound power reflected at the eardrum. Single and double microphone techniques measure the impedance at the tip of an assembly snugly inserted into the ear canal. This impedance is of limited usefulness, since it depends not only upon the middle ear reflectance, but also upon insertion depth, variations in cross-sectional ear canal area, and ear canal curvature. A method has been constructed to assess the joint influence of the ear canal area function and curvature with a spatial resolution along the ear canal of 4 mm (cT/2, for phase velocity c and sample period T). The reflectance at the assembly tip is transformed to that at an ear canal location just outside the eardrum. The method is also relevant for constructing the output impedance of the ear, as seen by a source on the eardrum. This technique makes no use of invasive probe microphones, has a frequency resolution of 40 Hz, and is well suited for use with infants. Preliminary data on tubes of known geometry will be discussed. [Work supported by NIH.]