# Pure Tone and Impedance Audiometry

> Pure tone and impedance audiometry in MBBS ENT: air-bone gaps, degrees of loss, tympanogram types and acoustic reflex interpretation.

- Canonical URL: https://prepelephant.com/topics/mbbs/ent/audiometry-pure-tone-and-impedance
- Exam / course: MBBS · Subject: ENT
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Pure Tone and Impedance Audiometry", PrepElephant, https://prepelephant.com/topics/mbbs/ent/audiometry-pure-tone-and-impedance

## Direct answer

Pure-tone audiometry maps hearing thresholds across 250-8000 Hz for air conduction and 250-4000 Hz for bone, and its two lines carry the conductive-versus-sensorineural discrimination: an air-bone gap means conductive loss — middle ear disease — while air and bone conduction depressed together with no gap means sensorineural loss — cochlea or nerve. Impedance audiometry then interrogates the middle ear mechanically: tympanometry plots compliance against ear-canal pressure, classifying types A, As, Ad, B and C, and the acoustic reflex arc adds a stapedius-response test that disappears early in otosclerosis and decays abnormally in retrocochlear lesions. Together, read with speech audiometry, they convert a tuning-fork impression into a typed, graded, side-localised diagnosis.

## What you must remember

- **Threshold grading to memorise:** normal 0-25 dB; mild 26-40; moderate 41-55; moderately severe 56-70; severe 71-90; profound above 90 dB HL — the classification that decides hearing-aid versus implantation pathways.
- **Conductive signatures:** air-bone gap with bone conduction at normal levels — otitis media with effusion, perforation, ossicular discontinuity; otosclerosis adds the Carhart notch, a bone-conduction peak loss around 2 kHz that disappears after successful stapes surgery.
- **Sensorineural signatures:** sloping high-frequency loss in presbycusis and noise exposure, a 4 kHz notch in acoustic trauma, low-frequency rising losses in early Meniere's disease, and flat severe losses in drug ototoxicity.
- **Speech audiometry pair:** speech reception threshold should agree with the pure-tone average of 500, 1000 and 2000 Hz within about 10 dB — disagreement suggests functional hearing loss; the speech discrimination score falling disproportionately (or rolling over at higher intensities) points retrocochlear, towards a vestibular schwannoma.
- **Tympanogram types with their disease:** A — normal; As — low compliance, stiff system: otosclerosis, tympanosclerosis; Ad — high compliance, flaccid system: ossicular discontinuity, healed perforation; B — flat, no compliance peak: middle-ear effusion; C — negative peak pressure: eustachian tube dysfunction.
- **Acoustic reflex logic:** reflex absent at 500-4000 Hz when conductive loss exceeds about 20-30 dB (either ear's middle ear must conduct both the stimulus and the response); reflex present but with abnormal decay — falling by half within 10 seconds — flags retrocochlear pathology on the stimulus side.
- **Masking principle examiners test:** whenever the interaural gap is large enough for cross-hearing (about 40 dB for air conduction), the non-test ear must be masked with noise, or the shadow curve mislabels a dead ear — a favourite viva on test validity.

## Reading one audiogram and one tympanogram together

A 45-year-old reports slowly progressive right hearing loss; Rinne is negative on the right with Weber lateralising right. His audiogram shows air conduction around 55 dB with bone conduction normal except a 2 kHz dip — an air-bone gap with a Carhart notch, the signature of stapes fixation. Tympanometry shows an As curve with absent acoustic reflexes — the stapedius reflex abolished by fixation at its own origin. The synthesis: right conductive loss, impedance-proven stiffness, consistent with otosclerosis; counsel surgery versus hearing aid.

Now replace his plot with a seven-year-old's: air conduction 35 dB flat, bone conduction 10 dB, type B tympanogram and absent reflexes — effusion, conductive, medical and possibly tubes. Replace it again with a 60-year-old's sloping bilateral high-frequency loss, type A tympanograms, preserved reflexes — sensorineural presbycusis, rehabilitative. One template — thresholds, gap, compliance, reflex — resolves three different clinics in under ten minutes, which is precisely the skill the practical examination tests at the audiometry station.

## Where examiners dig

The graph-interpretation station favours three discriminations. First, air-bone gap direction: bone better than air is conductive, and candidates reversing the convention lose the whole answer. Second, functional hearing loss: speech reception threshold markedly worse than the pure-tone average, absent reflex discrepancy patterns, or consistent 0 dB responses — inconsistency across tests is the tell. Third, tympanogram B versus C: effusion versus retraction with retained aeration, the distinction that decides observation from intervention in children's clinics. A final standing question asks which single pattern suggests a vestibular schwannoma: asymmetric sensorineural loss with poor speech discrimination, rollover, abnormal reflex decay — the audiometric triad that sends the patient to magnetic resonance imaging.

## Frequently asked questions

### How is conductive hearing loss identified on pure-tone audiometry?

Air-conduction thresholds are depressed while bone-conduction thresholds remain near normal, creating an air-bone gap — the signature of middle-ear pathology.

### What are the degrees of hearing loss by pure-tone threshold?

Normal up to 25 dB, mild 26-40, moderate 41-55, moderately severe 56-70, severe 71-90 and profound above 90 dB hearing level.

### What does a type B tympanogram indicate?

A flat curve with no compliance peak, indicating middle-ear effusion or a perforation — the objective confirmation of glue ear.

### What tympanogram types suggest otosclerosis and ossicular discontinuity?

Otosclerosis gives a stiff, low-compliance As curve with absent acoustic reflexes; discontinuity gives a hypercompliant Ad curve.

### What is acoustic reflex decay and its significance?

The stapedius reflex amplitude falling by more than half within 10 seconds of sustained stimulation, indicating retrocochlear pathology such as a vestibular schwannoma on the stimulated side.