6Immittance — tympanometry
Tympanometry is the quiet workhorse of the battery: a few seconds, no cooperation required, and a great deal learned. By varying the air pressure in a sealed ear canal and measuring how readily the eardrum accepts sound, it images the state of the middle ear — taut or floppy, air-filled or fluid-filled, normally pressurised or pulled in by a blocked Eustachian tube. Its trace falls into a handful of recognisable shapes, and a single extra number, the ear-canal volume, settles the one ambiguity those shapes leave. It is usually the first thing run in a session, because it tells the audiologist whether a conductive component is in play before a single tone is presented — and because, in the very young, it needs one crucial change of setting to work at all.
FImaging the middle ear
Tympanometry measures the ear's acoustic immittance as ear-canal pressure is swept, plotting compliance against pressure. The result images middle-ear status — a healthy air-filled space peaks near 0 daPa; fluid, fixation or negative pressure each distort the trace in a characteristic way.[2020]
TThe Jerger types
The classic shapes: Type A normal; As shallow (a stiff system — fixation/otosclerosis); Ad deep (hypermobile — ossicular discontinuity or a flaccid drum); Type B flat (effusion or perforation); Type C negative peak (Eustachian-tube dysfunction). Run at the start of the session, immittance flags a conductive component early and predicts the acoustic reflex (a flat trace usually means an absent reflex on that side).
CEar-canal volume
A flat (Type B) tympanogram is ambiguous until you read the ear-canal volume: a normal volume with a flat trace means effusion (the drum cannot move), while a large volume means the probe is seeing past the drum — a perforation or a patent ventilation tube. One number resolves two very different clinical pictures.
CThe infant probe tone
The standard 226 Hz probe tone is valid for older children and adults but unreliable in infants under about 4–6 months, whose small compliant canals give flat or doubly-peaked traces. From birth to ~6 months, tympanometry (and reflex testing) must use a 1000 Hz probe tone, which restores a clean single-peak response.
How do their middle-ear states differ?
Which tympanogram type indicates middle-ear effusion?
Why must infants under ~6 months be tested with a 1000 Hz probe tone?