2.3 Air and bone conduction

The audiogram up to now has been a single set of thresholds per ear: the levels at which the patient hears tones presented through the earphone. Those are air-conduction thresholds, abbreviated AC: the sound travels through the air in the ear canal, mechanically drives the eardrum, the ossicular chain, and the cochlear fluids in turn, and stimulates the inner ear. A pathology anywhere along that chain — a perforated eardrum, a fluid-filled middle ear, a fixed stapes — can elevate the AC threshold.

The audiogram has a second measurement that bypasses much of the chain: bone conduction, BC. A small mechanical oscillator pressed against the mastoid bone (just behind the ear) vibrates the skull directly, and the cochlea responds to that vibration without the outer or middle ear’s involvement. If the inner ear works, the BC threshold will be near 0 dB HL regardless of any outer- or middle-ear pathology. Comparing AC and BC thresholds is what lets the audiogram distinguish between conductive and sensorineural hearing loss.

This lesson develops the physics of bone conduction, the air-bone gap as a diagnostic indicator, and the resulting taxonomy of audiogram types.

How bone conduction works

A bone-conduction oscillator is a small electromechanical transducer that converts an electrical drive signal into mechanical vibration. Pressed against the mastoid with a calibrated headband (typically 5–6 N of static force), the oscillator drives the skull, and via the skull, the cochlea. The cochlea responds to vibration of its surrounding bone in three mechanisms, all simultaneously:

1. Distortional bone conduction — high-frequency vibration distorts the cochlear capsule itself. The geometry of the cochlea changes slightly with each oscillation, and the change couples into displacement of the basilar membrane.
2. Inertial bone conduction — at mid frequencies, the ossicles’ inertia means they lag behind the rest of the skull’s motion, producing a relative displacement of the stapes within the oval window — effectively driving the cochlea through its normal stapes input.
3. Osseotympanic bone conduction — at low frequencies, vibration of the skull radiates sound into the external ear canal, which then drives the eardrum normally. This is the most “air-conduction-like” of the three routes.

For the audiologist, the clinical consequence of all three mechanisms together is straightforward: the BC threshold measures the sensorineural sensitivity of the cochlea, with the outer and middle ear essentially bypassed.

The air-bone gap

If both AC and BC reach the same cochlea, why might their thresholds differ?

Because AC depends on the outer and middle ear’s normal mechanical impedance-matching, while BC does not. If anything obstructs the AC pathway — wax, perforation, effusion, otosclerosis — AC thresholds rise while BC stays normal. The difference between them, called the air-bone gap (ABG), is the audiometric signature of conductive hearing loss.

Formally: $\text{ABG}(f) = \text{AC}(f) - \text{BC}(f)$ in dB HL at each frequency.

The standard categories:

The 15-dB cutoff for “significant” ABG is conventional; smaller gaps may be within test-retest variability of the audiometer (which is approximately ±5 dB even for cooperative adults).

Common conductive pathologies

The differential diagnosis for a conductive loss usually reduces to one of:

• Cerumen impaction — wax blocking the ear canal. Trivially treatable; usually resolves the loss completely on cleaning.
• Tympanic membrane perforation — a hole in the eardrum, often from a ruptured infection or trauma. The hole reduces the acoustic driver area at the cochlea; the resulting ABG depends on hole size and location.
• Middle-ear effusion — fluid behind an intact eardrum, typically from otitis media. Mass-loads the middle-ear system; ABG is greatest at low frequencies (the mass of the fluid resists motion most at low frequencies; see Sound 5.4 — Specific acoustic impedance for the impedance picture).
• Otosclerosis — abnormal bone growth fixing the stapes in the oval window. ABG worst at low frequencies, classically with a “Carhart notch” at 2 kHz on the BC trace (a confound where the stiffness change affects the BC measurement itself). Surgically correctable.
• Ossicular discontinuity — disarticulation of the ossicular chain, usually from trauma. Produces a large flat ABG (often > 30 dB) and a hyper-mobile tympanogram (see Ch 4).
• Cholesteatoma — a keratin-filled cyst eroding through the middle ear. Late-stage produces conductive or mixed loss; the audiogram alone doesn’t make the diagnosis but flags the workup.

These are all addressed by Hearing Ch 3 — the middle ear and by Ch 4 — tympanometry and the middle ear on the workup side. The conductive audiogram is the screen; further tests refine the diagnosis.

Common sensorineural pathologies

A sensorineural loss has normal middle-ear mechanics but impaired cochlear or neural function. Most common etiologies:

• Presbycusis — age-related decline in cochlear function. Classically a downward-sloping high-frequency loss; bilateral, symmetric, gradually progressive.
• Noise-induced hearing loss — damage to outer hair cells from chronic loud exposure. Classic 4-kHz notch on the audiogram; bilateral if exposure was symmetric.
• Ménière’s disease — fluctuating low-frequency loss with episodes of vertigo and tinnitus. Audiogram shows a rising loss in early stages, evolving toward flat or sloping as the disease progresses.
• Sudden sensorineural hearing loss — abrupt onset, often viral or vascular etiology. Audiogram shape highly variable; usually unilateral; medical emergency requiring prompt treatment.
• Ototoxicity — drug-induced (cisplatin, aminoglycosides, loop diuretics, salicylates at high doses). Often starts at high frequencies and progresses downward.
• Genetic / congenital — many varieties, mostly identified in childhood. Audiogram configurations range from flat profound to cookie-bite mid-frequency.
• Acoustic neuroma / vestibular schwannoma — benign tumour on cranial nerve VIII. Usually unilateral asymmetric high-frequency sensorineural loss with disproportionately poor word recognition. Requires MRI for definitive diagnosis.

Mixed loss

A patient with both a middle-ear pathology and a cochlear pathology has a mixed loss: AC is elevated relative to BC (the conductive component), and BC is itself elevated (the sensorineural component). The audiogram shows two parallel curves with a visible ABG, both displaced downward.

A common scenario: an older patient with presbycusis (sensorineural baseline) who develops cerumen impaction (added conductive component) presents with a worsening mixed loss. Cleaning the cerumen restores the BC-equivalent AC and reveals the underlying sensorineural baseline.

The half-octave bone-conduction frequencies

Bone-conduction thresholds are typically measured at 500, 1000, 2000, and 4000 Hz — not 250 or 8000. The reasons:

• 250 Hz is hard to measure by bone conduction without significant cross-contamination from vibratory artefact perceived as touch.
• 8000 Hz is essentially impossible to deliver via bone conduction at usable levels — the oscillator’s high-frequency response and the skull’s transmission together drop off.

For these “edge” frequencies, audiologists rely on AC alone for threshold and infer the underlying sensorineural status from the trend across the testable BC frequencies. In practice this is rarely a problem; the diagnostic categories are clear even with the four standard BC frequencies.

What’s next

The next lesson, 2.4 — Masking and audiogram configurations, addresses the procedural problem the audiometer must solve: when the test ear and the non-test ear have very different thresholds, the test tone presented to the test ear can “cross-hear” through the non-test ear, giving a false reading. Masking is the technique that corrects for it. The lesson closes with the canonical audiogram configurations and their interpretive shorthand.