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Tympanometry: tympanogram types, ear canal volume and choice of probe tone
Tympanometry measures the acoustic admittance of the middle ear as a function of canal pressure; the curve shape (A, As, Ad, B, C) together with the equivalent canal volume tells fluid, perforation, a stiff or disarticulated ossicular chain and negative pressure apart — but it does not measure hearing.
Why it matters in the exam
Exam questions give a tympanogram with numbers and ask for the type: a canal volume of 1.6 mL, an admittance of 3.6 mmho and a pressure of minus 15 — type Ad; a flat tympanogram with a volume of 2.8 mL in a girl — a perforation or a patent tube; a flat tympanogram with a normal volume — middle ear fluid; and flat with a very small volume — occluding wax. Type Ad with a maximal air-bone gap points to ossicular discontinuity, and in otosclerosis the tympanogram can be normal despite an air-bone gap.
Other questions test principles: otoscopy is required before tympanometry; the y-axis is admittance in mmho and the x-axis pressure in daPa; canal volume matters in every test; tympanometry is not affected by hearing threshold; in young infants a 1,000 Hz probe tone is chosen instead of 226 Hz; after a Valsalva manoeuvre the peak shifts to a more positive pressure; and with a perforated eardrum Eustachian tube function testing measures change in pressure over time. Middle ear fluid forms as a result of negative pressure, and with a type B tympanogram OAE is not done because the result would mislead.
Key points
- Tympanometry = admittance as a function of pressure.
- A normal; As stiff; Ad flaccid or disarticulated; B flat; C negative pressure.
- With B decide by volume: normal — fluid; large — perforation or tube; small — wax.
- Otoscopy first; 1,000 Hz for young infants.
- After Valsalva the peak shifts to a more positive pressure.
Summary table
| Type | What is seen | Typical interpretation |
|---|---|---|
| A | Peak at pressure around 0 with normal admittance | Normal middle ear (or a sensorineural loss) |
| As | Peak at normal pressure but low | A stiff ear: otosclerosis, scarring; sometimes normal in otosclerosis too |
| Ad | A very high peak | Ossicular discontinuity, flaccid or scarred eardrum |
| B with normal volume | A flat curve with no peak | Middle ear fluid (low admittance) |
| B with large volume | A flat curve, canal volume larger than normal | Eardrum perforation or a patent ventilation tube |
| B with small volume | A flat curve, very small volume | Occluding wax or a probe against the canal wall |
| C | Peak at significant negative pressure | Eustachian tube dysfunction; may lead to fluid |
In the full version for subscribers
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Sources
- Jerger, Archives of Otolaryngology, 1970 — Clinical experience with impedance audiometry (tympanogram types)
- Katz (ed.), Handbook of Clinical Audiology, 7th edition (2015) — Tympanometry in clinical practice
- ASHA Guidelines, 2004 — Guidelines for the Audiologic Assessment of Children from Birth to 5 Years of Age (high-frequency probe tone)
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.