The Technology of the Tympanometer

Probe assembly, the pressure pump, the probe tone, and how admittance is actually recorded.

A tympanometer's probe seals the ear canal and contains three elements: a loudspeaker delivering a constant probe tone, a microphone monitoring the sound pressure level in the sealed canal, and a pump that varies the canal air pressure. [Onusko E 2004]

How admittance is inferred

The instrument holds the probe tone at a constant voltage. If the middle ear becomes less mobile, more sound energy stays in the canal and the microphone records a higher sound pressure level; if the ear becomes more mobile, more energy is absorbed and the recorded level falls. The instrument converts this into an admittance value, expressed in millimho or, equivalently, in cm³ of equivalent volume. [American Speech-Language-Hearing Association 1988]

Why the pressure sweep matters

Mobility is greatest when the pressure on both sides of the drum is equal. By sweeping ear-canal pressure from positive through to negative, the instrument finds the pressure at which the drum is most mobile — the peak — and that peak pressure estimates middle-ear pressure. At the extremes of the sweep the drum is stiffened by the pressure differential, and the residual admittance there represents the ear-canal volume alone. [Terkildsen K 1959][Fowler CG 2002]

Probe-tone frequency

Low-frequency probe tones (226 Hz) give single-peaked tympanograms in most ears and are the clinical standard for adults and older children. Higher probe frequencies produce more complex, sometimes notched shapes, and are used for infants and for specialised assessment of the middle ear's mass and resonance. [American Speech-Language-Hearing Association 1988][Hunter LL 2013]

How the tympanogram is traced

Watch the pressure sweep build the curve point by point — the drum stiffens at the extremes and is most mobile near the middle-ear pressure.