Ear-canal impedance and reflection coefficient in human infants and adults

TLDR

The development of the external ear canal and middle ear, which continues until at least 24 months, influences impedance and reflection coefficient responses through changes in canal size, wall resonance, and middle‑ear cavity growth. The study proposes using 2–4 kHz impedance and reflectance measurements as a clinical tool to overcome difficulties in interpreting neonatal tympanograms. The authors measured ear‑canal impedance and reflection coefficient from 125 to 10,700 Hz in adults and infants aged 1–24 months, and recommend 2–4 kHz measurements for clinical use. Infants exhibit lower middle‑ear compliance, higher resistance, and reduced power transfer compared to adults, leading to altered hearing measurements and making neonatal tympanograms difficult to interpret because of ear‑canal wall vibration.

Abstract

The ear-canal impedance and reflection coefficient were measured in an adult group and in groups of infants of age 1, 3, 6, 12, and 24 months over frequency range 125–10 700 Hz. The development of the external ear canal and middle ear strongly affect input impedance and reflection coefficient responses, and this development is not yet complete at age 24 months. Contributing factors include growth of the area and length of the ear canal, a resonance in the ear-canal walls of younger infants, and a probable influence of growth of the middle-ear cavities. The middle-ear compliance is lower in infants than adults, and the middle-ear resistance is higher. The power transfer into the middle ear of the infant is much less than into that of the adult. Such differences in power transfer directly influence both behavioral and physiological measurements of hearing. The difficulties of interpretation of neonatal tympanograms are shown to be a consequence of ear-canal wall vibration. Impedance and reflectance measurements in the 2–4-kHz range are recommended as a potentially useful clinical tool for circumventing these difficulties.