Chapter 4 — Tympanometry and the middle ear

Acoustic admittance, tympanograms, acoustic reflex.

The pure-tone audiogram and the speech tests of Chapters 2 and 3 are behavioural — they require the patient to attend, respond, and follow instructions. They tell us what the whole hearing system can do, end to end, but they do not tell us where a loss lives. A 30 dB conductive loss looks the same to a behavioural test whether the cause is a perforated tympanic membrane, a stiff stapes, or a middle-ear effusion.

This chapter introduces the first objective test in the audiologist’s toolkit: tympanometry. We place a small probe in the ear canal, present a continuous probe tone, sweep the ear-canal pressure from positive to negative, and measure how the eardrum’s acoustic admittance changes. The result — a tympanogram — is a one-minute test that anatomically localises middle-ear pathology with remarkable precision. We then extend the same hardware to measure the acoustic reflex, a brainstem-mediated contraction of the stapedius muscle whose four-condition pattern (ipsilateral / contralateral, right / left) localises lesions across the entire central auditory pathway from cochlea to brainstem.

Four lessons:

• 4.1 Acoustic immittance: impedance, admittance, the probe — what the probe actually measures, and why audiologists work in admittance (Y) rather than impedance (Z). Cross-links to Sound 5.4 and Hearing 3.
• 4.2 The tympanogram and the Jerger types — the Y-vs-pressure curve, Jerger types A / As / Ad / B / C, the static-admittance and TPP numbers, ear-canal volume.
• 4.3 The acoustic reflex and what it localises — the stapedius reflex arc, the four-condition pattern, conductive vs cochlear vs retrocochlear vs brainstem.
• 4.4 Eustachian tube dysfunction and the C-tympanogram — middle-ear pressure regulation, why the peak shifts negative in colds and altitude.