Pure Tone and Impedance Audiometry
On this page
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.