Introduction to Impedance Audiometry

What tympanometry measures, where it sits in the test battery, and what it can and cannot tell you.

Impedance audiometry — more commonly called acoustic immittance testing — is an objective, non-invasive way to assess the middle-ear transmission system. It does not require a behavioural response from the patient, which makes it valuable across the age range and in patients who cannot reliably perform pure-tone audiometry.

The test rests on a simple physical idea: when sound strikes the tympanic membrane, some energy is absorbed and the rest is reflected. A stiffer drum reflects more energy; a more compliant system absorbs more. By sealing the ear canal and varying its air pressure, the instrument can chart how the mobility of the tympano-ossicular system changes — and that chart is the tympanogram. [American Speech-Language-Hearing Association 1988]

Where it sits in the battery

Tympanometry is one test among several. A case history, otoscopic examination, air- and bone-conduction thresholds, and acoustic-reflex measurements all contribute information about the middle ear. The tympanogram is most powerful when interpreted together with these, not in isolation. [American Speech-Language-Hearing Association 1988][Fowler CG 2002]

What it measures

• Equivalent ear-canal volume (ECV) — an estimate of the air space between the probe tip and the drum.
• Static (peak) admittance — the maximum mobility of the middle-ear system once the canal contribution is removed.
• Tympanometric peak pressure (TPP) — the ear-canal pressure at which mobility is greatest, reflecting middle-ear pressure.
• Tympanometric width (gradient) — how sharp or broad the peak is.

Each of these is compared against age-appropriate normative data. Tympanometry is not reliable in infants younger than about seven months at low probe frequencies because of the highly compliant infant ear canal — a higher-frequency probe tone is used instead. [Onusko E 2004][Margolis RH 1987]