On its way to the inner ear a sound wave encounters a resistance at the ear drum, which is the acoustic impedance of the ear and depends on the vibrating characteristics of the ear. This resistance is determined through the amount of energy absorbed and through the phase of the reflected sound wave, so that the numerical value of the impedance becomes a complex entity.
The first systematic study of the acoustic impedance of the human ear was made by Tröger, in 1930. In 1946, Metz introduced such measurements in clinical work employing a mechanical acoustic measuring bridge, by which the phase component as well as the absorption could be determined. He carried out numerous measurements, and others have since corroborated the clinical value of this method of examination.
The scope of the work of Metz was mainly to compare impedances in normal and pathological ears. Wide variations were found,
TERKILDSEN K, NIELSEN SS. An Electroacoustic Impedance Measuring Bridge for Clinical Use. Arch Otolaryngol. 1960;72(3):339–346. doi:10.1001/archotol.1960.00740010347009
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