Ranges of static impedance in two "normal" middle ear populations are compared. One population (100 ears) showed no audiometric evidence of middle ear abnormality, and tests showed normal function. History and otologic findings also showed a normal middle ear system. The second group (29 ears) also had no audiometric evidence of conductive abnormality, but did have healed perforations of the tympanic membranes.
Ranges for resistive and reactive measurements with a 220-Hz and 660-Hz probe-tone were computed. Healed perforations markedly lowered measured acoustic impedance and strongly suggested that both low and high frequency tympanograms are contaminated by eardrum abnormality. These results account for previously observed but unexplained overlap between some presumably stiff pathological eardrums and normals and restrict use of static acoustic impedance in differential diagnosis of middle ear abnormality to ears with normal tympanic membranes.