Maturation of the Middle and External Ears: Acoustic Power-Based Responses and Reflectance Tympanometry

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Abstract
The maturation of the external and middle ear in the human infant has significant effects on the interpretation of measured ear-canal responses to acoustic stimuli. A tutorial section is presented of power-based response functions, accompanied by a hierarchy of stimulus specifications contrasting pressure-based and power-based responses. An experimental section follows on reflectance tympanometry, the aims of which are to introduce and assess the feasibility of the technique and to discuss implications for tests of hearing development. A tympanometric measurement of admittance is used with an estimate of ear-canal area to calculate a so-called reflectance tympanogram as a function of frequency and static pressure in the ear canal. Selected results on 226 Hz reflectance tympanograms are reported for normal-hearing adults and for infants of age 3 to 6 mo with both normal and flat 226 Hz admittance tympanograms. A multifrequency reflectance tympanogram is reported for an adult. Measured at ambient ear-canal pressure, the acoustic external- and middle-ear responses of infants of age 1 to 6 mo are compared with those of adults. The admittance level is influenced by the ear-canal area, the interplay of compliant- and inertance-controlled effects in the middle ear, and the presence of losses. Ear-canal area is a major factor in distinguishing infant from adult responses. Energy reflectance provides a measure of middle-ear power transmission that is approximately independent of probe placement in the ear canal and that varies with maturation. These power-based responses, measured at ambient pressure, are contrasted with tympanometric measurements. Reflectance tympanometry is defined and easily measured in infants and adults. Some infants with flat 226 Hz tympanograms have energy reflectance in the normal range at higher frequencies (2 to 4 kHz). Acoustic measurements of power-based responses in the ear canal-reflectance, admittance, and impedance-provide insight into the maturation of the external and middle ear. Reflectance tympanometry tests the relative accuracy underlying the tympanometric measurement of compensated eardrum admittance and may have clinical utility.

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