Summary of: Keefe, D. H., Bulen, J. C., Arehart, K. H., & Burns, E. M. (1993). Ear-canal impedance and reflection coefficient in human infants and adults. The Journal of the Acoustical Society of America, 94(5), 2617–2638. https://doi.org/10.1121/1.407347
Energy reflectance is the ratio of reflected energy to incident energy. Absorbance represents the amount of energy absorbed by the ear canal and middle ear and equals 1 – reflectance. These metrics are useful for describing the response of the ear canal (and middle ear) to acoustic inputs such as speech. Although impedance at a single frequency is useful for this endeavor, a wideband measurement provides additional detail not captured in discrete frequencies. In addition, development of the ear canal and middle ear are shown to have an impact on input impedance and the reflection coefficient response. Thus, the current study aims to measure ear-canal impedance and energy reflectance from 125 to 10700 Hz in adults and in infants 1, 3, 6, 12, and 24 months of age. Results indicate that middle-ear compliance is lower and middle-ear resistance is higher in infants compared to adults. As such, power transfer via the middle ear is reduced in infants relative to adults and may partially account for differences in measurement of behavioral threshold sensitivity. For all age groups, energy absorbance is low at frequencies below 1 kHz and above 6 to 8 kHz. Absorbance is highest for 1 month olds up to 2 kHz. In general, energy transmission into the middle ear is most efficient, as indicated by absorbance, in the frequency region most important for speech (i.e., 1 to 4 kHz). With increases in age, absorbance decreases in the 4 to 8 kHz range and the frequency boundary where absorbance dips below 0.5 decreases. In sum, interpretation of neonatal tympanograms are difficult to interpret in part because of the vibration of the ear-canal walls. Impedance and absorbance measurements between 2 and 4 kHz may help alleviate such difficulties.