Air-Bone Gap and Conductive Component Calculator
Air-Bone Gap and Conductive Component Calculator
The air-bone gap at one frequency, with the published criteria for a significant gap, the bone vibrator’s output ceiling, the Carhart notch and why a negative gap is an artefact. A gap localises a loss; it does not grade one.
Air-bone gap
Air minus bone at one frequencyAir conduction 45 dB HL, bone conduction 15 dB HL, both at 2000 Hz in the same ear
Formula
- same ear, same frequency
- both halves are load-bearing. A gap computed across frequencies, or against the other ear’s bone threshold, is not a gap
- significant gap
- the Australian DVA Statement of Principles for conductive hearing loss requires, at 500, 1000, 2000, 3000, 4000 or 6000 Hz, either “an air-bone gap in the affected ear of at least 10 dB at three of those frequencies” or “a gap of at least 15 dB at any one of those frequencies”. BSA’s masking Rule 2 fires at the same 10 dB
- what it localises
- a gap places part of the loss in the conductive pathway — canal, tympanic membrane, middle ear, ossicular chain — because bone conduction bypasses that pathway and air conduction does not. A loss with no gap is sensorineural in that sense; a loss with a gap and a raised bone threshold is mixed
- what it does NOT do
- grade anything. The gap is not a severity, and the published hearing-loss grades are applied to the air-conduction average, not to the gap. A gap also does not identify the lesion: a 30 dB gap is equally consistent with wax, a perforation, an effusion, ossicular discontinuity and stapedial fixation
- negative gap
- an artefact. Bone conduction cannot genuinely be poorer than air conduction in the same ear, and BSA puts the uncertainty of any threshold at “at least plus or minus 5 dB”, so this page reports gaps down to −5 dB and refuses below that rather than printing a number that cannot be a finding
- the ceiling
- bone vibrators have a limited maximum output — about 45 dB HL at 250 Hz and 70 to 75 dB HL through the middle frequencies on the datasheet quoted here, within an overall range of −10 to 80 dB HL. Where the bone threshold is at the ceiling the measured gap is a lower bound on the true gap, and a large mixed loss cannot be fully characterised
- a false gap
- collapsing ear canals under supra-aural earphones “may lead to measurement of a false air-bone gap” in BSA’s own words, and insert earphones avoid it. Above 4000 Hz a reference equivalent threshold force level correction of about 14 dB is needed for the same reason
Worked example
Air conduction 45 dB HL, bone conduction 15 dB HL, both at 2000 Hz in the same ear
45 − 15 = 30 dB air-bone gap
30 dB is 15 dB or more, so this meets the single-frequency limb of the DVA criterion at this frequency on its own
The orientation is not interchangeable. Bone minus air would give −30, and a calculator checked only where air and bone are equal cannot tell the two versions apart — at ac 40, bc 40 both give 0
Change bone conduction to 35 dB and the gap becomes 10 dB: the three-frequency limb, which needs 10 dB at three of the six frequencies before it counts, so one frequency is no longer enough
Change bone conduction to 40 dB and the gap becomes 5 dB: inside measurement uncertainty, and no conductive component is indicated
Change bone conduction to 50 dB and the result is withheld. The gap would be −5 dB... in fact −5 dB is still reported; at bone conduction 51 dB the gap is −6 dB and the page refuses, because bone conduction cannot genuinely be poorer than air conduction in the same ear
The Carhart correction, at this frequency. If this is stapedial fixation, Carhart's mean 15 dB elevation of the bone threshold at 2000 Hz means the true bone threshold may be nearer 0 dB and the true gap nearer 45 dB. The measured gap understates the conductive component at 2000 Hz, and understates it most at exactly the frequency clinicians look at
The ceiling. Set air conduction to 100 and bone conduction to 75 — the vibrator's recommended maximum at 2000 Hz — and the gap reads 25 dB, but the true bone threshold is unknown and the true gap is 25 dB or less. The subtraction has run out of measurement
What a gap at one frequency does and does not establish
| Gap at this frequency | Published criterion | What it means |
|---|---|---|
| Under 10 dB | Neither limb met; below BSA’s masking trigger | Air and bone agree within measurement resolution at this frequency |
| 10 to 14 dB | Three-frequency limb: needs 10 dB at three of 500, 1000, 2000, 3000, 4000, 6000 Hz | Counts only alongside two more such frequencies; masked bone conduction required |
| 15 dB or more | Single-frequency limb met at this frequency | A conductive component at this frequency, localised and not graded |
| Negative | No criterion; not a finding | A masking, placement or calibration artefact — bone cannot genuinely be poorer than air |
| Any size, with bone conduction at the vibrator ceiling | Criteria still apply to the measured value | The measured gap is a floor on the true gap; a large mixed loss cannot be fully characterised |
The bone vibrator’s ceiling, and the Carhart notch
| Frequency | Maximum recommended bone output (dB HL) | Carhart’s mean bone-threshold elevation (dB) |
|---|---|---|
| 250 Hz | 45 | not reported |
| 500 Hz | 65 | 5 |
| 1000 Hz | 75 | 10 |
| 2000 Hz | 75 | 15 |
| 3000 Hz | 75 | not reported |
| 4000 Hz | 70 | 5 |
A gap localises, it does not grade
The air-bone gap is the simplest arithmetic in audiology and the most misread. Air conduction minus bone conduction, same ear, same frequency: air conduction travels through the canal, the drum and the ossicular chain to the cochlea, bone conduction goes more or less straight to the cochlea, so the difference between them is what the conductive pathway is costing. That is all it is. It says where part of a loss sits; it says nothing about how big the loss is, and the published grades of hearing loss are applied to the air-conduction average rather than to the gap.
What counts as significant is a published criterion, not a judgement. The Australian DVA Statement of Principles for conductive hearing loss asks for a gap of at least 10 dB at three of 500, 1000, 2000, 3000, 4000 and 6000 Hz, or at least 15 dB at any one of them — a two-limbed test, so a single 12 dB gap does not satisfy it and a single 15 dB gap does. BSA’s masking Rule 2 fires at the same 10 dB, for a different reason: at that point the unmasked bone threshold has to be verified with masking before it can be believed at all.
Three limitations deserve stating plainly, because each of them produces a confident wrong answer. A negative gap is not a finding: bone conduction cannot genuinely be poorer than air conduction in the same ear, so a negative result is measurement noise — BSA puts threshold uncertainty at no better than ±5 dB — or a placement or calibration error. Masking errors produce spurious gaps in the other direction: bone conduction crosses the skull essentially unattenuated, so an unmasked bone threshold may be the opposite cochlea answering, and the gap then belongs to the wrong ear. And the bone vibrator has a ceiling, around 45 dB HL at 250 Hz and 70 to 75 dB HL through the middle frequencies, so in a large mixed loss the bone threshold simply cannot be found and the conductive and sensorineural shares cannot be separated by this subtraction.
The Carhart notch is the one systematic error worth knowing by name. In stapedial fixation the bone-conduction threshold is itself artificially elevated, most at 2000 Hz, by the loss of the middle ear’s inertial contribution to bone conduction near the ossicular chain’s resonance. Because the artefact is in the bone threshold, it narrows the measured gap: the gap at 2000 Hz understates the conductive component, and the raised bone threshold is not cochlear damage. Audiometric results are equipment- and calibration-dependent: the dB HL scale is defined against the reference equivalent threshold levels of the ISO 389 series for a particular transducer and coupler, so the same ear can read differently on two audiometers, and the BSA recommended procedure puts the uncertainty of any threshold measurement at no better than plus or minus 5 dB. Your own audiogram and the calibration standard it was recorded against govern, not this arithmetic. This page computes the number and names the body that publishes the thresholds. It does not decide anything: the grades, action values and criteria quoted here are scheme- and jurisdiction-specific, and whether any of them is met in a particular case, and what should follow, is for the clinician, the audiologist or the responsible person under the applicable regulation to determine.
Frequently asked questions
How do you calculate an air-bone gap?
Subtract the bone-conduction threshold from the air-conduction threshold at the same frequency in the same ear. An air threshold of 45 dB HL with a bone threshold of 15 dB HL is a 30 dB gap. Both thresholds should be masked wherever masking was indicated, or the gap is unverified.
What air-bone gap is significant?
Under the Australian DVA Statement of Principles for conductive hearing loss, either at least 10 dB at three of 500, 1000, 2000, 3000, 4000 and 6000 Hz, or at least 15 dB at any one of them. BSA’s masking Rule 2 also triggers at 10 dB. Other schemes draw the line elsewhere, so use the criterion the determination is being made under.
What does an air-bone gap tell you?
Where part of the loss sits, and nothing else. A gap means a conductive component — canal, drum, middle ear or ossicular chain — because bone conduction bypasses that pathway. It does not grade the loss, does not identify the lesion, and a larger gap is not a more severe hearing loss.
Can the air-bone gap be negative?
Not as a finding. Bone conduction cannot genuinely be poorer than air conduction in the same ear, so a negative gap is measurement uncertainty — BSA puts that at no better than ±5 dB — or a vibrator placement or calibration problem. This page reports gaps to −5 dB and withholds a result below that.
What is the Carhart notch?
An apparent elevation of the bone-conduction threshold in stapedial fixation, largest at 2000 Hz. Carhart’s mean values are about 5 dB at 500 Hz, 10 dB at 1000 Hz, 15 dB at 2000 Hz and 5 dB at 4000 Hz. Because it raises the bone threshold it narrows the measured gap, so the gap understates the conductive component at 2000 Hz. It is a mechanical artefact, may correct after surgery, and is not pathognomonic.
Related calculators
References
- British Society of Audiology. Recommended Procedure: Pure-tone air-conduction and bone-conduction threshold audiometry with and without masking, OD104-32. Reading: BSA, 2018.
- Repatriation Medical Authority. Statement of Principles concerning conductive hearing loss. Australian Government Department of Veterans’ Affairs (CLIK).
- Carhart R. Clinical application of bone conduction audiometry. Arch Otolaryngol. 1950;51:798–808.
- Hall JW. Why does the Carhart notch appear? AudiologyOnline, Ask the Experts 509.
- Quaranta N, Besozzi G, Fallacara RA, Quaranta A. Air and bone conduction change after stapedotomy and partial stapedectomy for otosclerosis. Acta Otorhinolaryngol Ital.
- Auditdata. BC1 audiometric bone conductor, datasheet. Related standards IEC 60645-1:2017, ANSI S3.6:2018 (R2023).
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/