An air-bone gap is the difference, measured in decibels, between how well you hear sound delivered through the air (via headphones or speakers) and how well you hear sound delivered through vibration of the skull bone (via a vibrating device pressed against your head). When bone conduction hearing is noticeably better than air conduction hearing, the gap between those two numbers points to something blocking or dampening sound on its way through the outer or middle ear. It is one of the most informative measurements in a standard hearing test, because it separates problems in the sound-conducting machinery of the ear from problems in the inner ear or auditory nerve.
How the Two Pathways Work
Sound normally reaches your inner ear by traveling through the ear canal, vibrating the eardrum, and passing through the three tiny bones of the middle ear (the ossicles) into the fluid-filled cochlea. That route is air conduction. Bone conduction bypasses most of that chain: a vibrating device on the skull sends energy through bone directly to the cochlea and surrounding structures. Several mechanisms contribute to bone-conducted hearing, including inertia of the ossicles and cochlear fluid, and at higher frequencies, sound radiated into the ear canal itself plays a role.1PubMed Central. Comparison of umbo velocity in air- and bone-conduction
In a healthy ear, air conduction and bone conduction thresholds are close together because the entire mechanical chain is working smoothly. The air-bone gap is essentially zero, or very small. When something interferes with the outer or middle ear, air conduction thresholds rise (meaning you need louder sounds to hear them through headphones), but bone conduction thresholds stay roughly the same, because the vibrating skull still delivers energy to the cochlea. The result is a measurable gap. A gap of 15 dB or more at a given frequency is generally considered clinically meaningful.2PubMed Central. Aging and the 4-kHz air-bone gap
What the Gap Tells a Clinician
The air-bone gap is the audiologist’s or ear surgeon’s primary tool for sorting out the type of hearing loss you have. Hearing loss falls into three broad categories based on audiometry:
- Conductive: The problem is in the outer or middle ear. Air conduction thresholds are elevated, bone conduction is normal, and the gap is substantial.
- Sensorineural: The problem is in the cochlea or auditory nerve. Both air and bone conduction thresholds are elevated equally, so there is no gap.
- Mixed: Both components are present. Bone conduction is worse than normal, but air conduction is worse still, producing a gap on top of the sensorineural loss.
Knowing which type you have completely changes the treatment plan. A conductive loss with a large air-bone gap often has a surgically correctable cause, while a purely sensorineural loss typically does not. The size and shape of the gap across different frequencies also gives clues about which specific structure is at fault.
Eardrum Perforations
One of the most common causes of an air-bone gap is a hole in the eardrum. A perforation reduces the effective vibrating surface of the membrane and lets sound energy leak out, raising your air conduction thresholds while leaving bone conduction largely unchanged. Research consistently shows that the size of the perforation is the dominant factor in how large the gap becomes: bigger holes produce bigger gaps.3Tikrit Journal of Pharmaceutical Sciences. Effect of the size of tympanic membrane perforation on hearing The gaps tend to be largest at low frequencies and increase as the perforation grows.4PubMed Central. Determinants of Hearing Loss in Perforations of the Tympanic Membrane
Where on the eardrum the hole sits matters less than you might expect. Studies have found no significant difference in air-bone gaps between front-quadrant and back-quadrant perforations at any frequency, though front perforations tend to have slightly smaller gaps at low frequencies.4PubMed Central. Determinants of Hearing Loss in Perforations of the Tympanic Membrane What does drive the gap higher is when the perforation spans both front and back quadrants, touches the bony handle of one of the ossicles, or reduces the air volume behind the eardrum.5PubMed Central. The Effect of Tympanic Membrane Perforation Site, Size and Middle Ear Volume on Hearing Loss
Fluid in the Middle Ear
When the space behind the eardrum fills with fluid, whether from an ear infection, allergies, or Eustachian tube dysfunction, the fluid mass damps the eardrum’s vibration and stiffens the ossicular chain. In experimental models, filling the middle ear with fluid drops the eardrum’s movement by roughly 25 to 30 dB at low frequencies, with even larger losses at high frequencies.6PubMed Central. Restoration of middle-ear input in fluid-filled middle ears by controlled introduction of air or a novel air-filled implant That translates directly into an air-bone gap, because the fluid doesn’t meaningfully change how bone-conducted sound reaches the cochlea.
Middle ear effusion is especially common in children. In kids with cleft palate, who are highly prone to chronic effusion, insertion of ventilation tubes (grommets) leads to significant improvement in the air-bone gap over time, along with better speech development.7PubMed. Long-term audiologic and speech developmental outcomes following ventilation tube insertion in children with otitis media with effusion and cleft palate: a prospective non-randomized study This is a clear example of how treating the conductive problem and closing the gap restores normal hearing input during a critical window for language learning.
Ossicular Chain Problems
The three ossicles (malleus, incus, and stapes) form a mechanical lever system that amplifies sound pressure from the eardrum to the cochlea. If one of these bones is eroded by chronic infection, separated at a joint, or congenitally malformed, the chain’s efficiency drops and an air-bone gap appears. Disruption of the incus, particularly erosion of its long process, is one of the more common patterns seen in chronic ear disease.
Reconstruction of the ossicular chain can dramatically close the gap. In patients with incus defects treated with either bone cement or a partial ossicular replacement prosthesis, preoperative air-bone gaps of about 27 to 29 dB were reduced to roughly 9 to 10 dB after surgery.8PubMed Central. Prognostic factors influencing postoperative air-bone gap in stapes surgery The goal of ossicular surgery is generally to close the air-bone gap to within 10 dB of zero, which returns hearing to a level close to the patient’s bone conduction baseline.
Otosclerosis and the Carhart Notch
Otosclerosis is a condition where abnormal bone growth gradually fixes the stapes footplate in the oval window, preventing it from vibrating freely. The classic audiometric picture is a progressive conductive hearing loss with a widening air-bone gap. But otosclerosis also produces a curious artifact: bone conduction thresholds often dip specifically around 2,000 Hz, creating what’s called the Carhart notch. This dip isn’t a true sensorineural loss. It reflects the fact that bone conduction measurement partly depends on the middle ear’s resonance, which is disrupted when the stapes is immobilized.9PubMed Central. Role of the Carhart Effect and Outcomes from Surgery: A Retrospective Study of 532 Patients with Conductive Hearing Loss Due to Otosclerosis, Otitis Media with Effusion, and Chronic Otitis Media
A widely held assumption is that the Carhart notch is a reliable sign of stapes fixation specifically, but the evidence complicates that idea. The notch appears with similar frequency and depth in stapes fixation, incudostapedial joint separation, and fixation of the malleus or incus, with no statistically significant differences among these groups.10JAMA Otolaryngology–Head & Neck Surgery. Carhart Notch 2-kHz Bone Conduction Threshold Dip: A Nondefinitive Predictor of Stapes Fixation in Conductive Hearing Loss With Normal Tympanic Membrane So while the notch is a useful audiometric clue, it can’t on its own tell the surgeon which ossicle is involved.
The Carhart effect also illustrates something that catches many patients off guard: bone conduction thresholds can improve after middle ear surgery, even though the surgery didn’t touch the inner ear. Correction of a conductive lesion removes the mechanical loading that was artificially depressing bone conduction results, making the bone line look better postoperatively.9PubMed Central. Role of the Carhart Effect and Outcomes from Surgery: A Retrospective Study of 532 Patients with Conductive Hearing Loss Due to Otosclerosis, Otitis Media with Effusion, and Chronic Otitis Media This doesn’t mean the inner ear got better; it means the measurement was influenced by the middle ear all along.
Third Window Lesions and Unusual Low-Frequency Gaps
The inner ear normally has two “windows” connecting it to the middle ear: the oval window (where the stapes sits) and the round window. Certain conditions create a pathological third opening in the bony shell of the inner ear. The most studied example is superior semicircular canal dehiscence, where a thin spot or hole develops in the bone over one of the balance canals. This extra opening acts as an energy shunt, redirecting sound energy away from its normal path. Air-conducted sound loses some of its pressure drive through the cochlea, raising air conduction thresholds, while bone-conducted sound can actually become more sensitive because of the altered pressure dynamics across the cochlear partition.11PubMed Central. A superior semicircular canal dehiscence induced air-bone gap in chinchilla
The result is an air-bone gap that mimics a conductive hearing loss, especially at low frequencies, even though the middle ear is structurally normal. Large vestibular aqueduct syndrome produces a similar pattern through the same third-window mechanism, with air-bone gaps that can be quite large at 250 Hz (averaging around 50 dB in some cases) and taper off at higher frequencies.12PubMed Central. Clinical investigation and mechanism of air-bone gaps in large vestibular aqueduct syndrome These conditions are an important diagnostic trap: a surgeon who sees the air-bone gap and assumes a middle ear problem could perform an unnecessary middle ear surgery that won’t help.
Low-frequency air-bone gaps also show up in Ménière’s disease, where abnormal fluid pressure buildup in the inner ear (endolymphatic hydrops) can alter cochlear mechanics. Research using MRI to visualize the fluid buildup has found that the presence of low-frequency air-bone gaps correlates with more severe endolymphatic hydrops in the vestibular portion of the inner ear.13PubMed Central. Low Frequency Air-Bone Gap in Meniere’s Disease: Relationship With Magnetic Resonance Imaging Features of Endolymphatic Hydrops The gaps in Ménière’s tend to be modest, averaging around 15 dB at 500 Hz in affected ears, but their appearance may signal worsening of the hydrops and can be useful for monitoring the disease.14PubMed. The relationship between endolymphatic hydrops in the vestibule and low-frequency air-bone gaps
Measurement Pitfalls
Bone conduction audiometry is not as straightforward as it seems. The skull doesn’t contain separate compartments for each ear, so vibrating one side sends energy to both cochleae. Audiologists use masking noise in the non-test ear to prevent it from “helping,” but getting the masking level exactly right is a constant balancing act. Under-mask and you get falsely good bone conduction thresholds from the other ear; over-mask and you can actually make the test ear’s thresholds worse.
There’s also a systematic measurement artifact that affects bone conduction results around 2,000 Hz. Research from two independent audiology centers found that bone conduction audiometry introduces a pattern of artificial notching at that frequency, and this error disproportionately affects ears with conductive hearing loss, distorting the audiogram in roughly 17% of such cases. This means some apparent air-bone gaps at 2 kHz may be partly or fully artifactual rather than reflecting a real pathological change. The practical consequence is that clinicians need to interpret bone conduction results at 2 kHz with extra caution, particularly when those results would change a surgical decision.
How Surgeons Use the Air-Bone Gap
For ear surgeons, the air-bone gap is both a diagnostic tool and the primary yardstick for measuring surgical success. The standard benchmark is “gap closure,” meaning how much the postoperative air-bone gap shrinks compared to the preoperative one. A successful outcome is commonly defined as reducing the gap to 10 dB or less.
In stapes surgery for otosclerosis, reported success rates are sensitive to exactly how the gap is calculated. A study drawing on Sweden’s national quality register found that when the calculation included 3 kHz instead of 4 kHz, the average postoperative air-bone gap improved from 10.0 dB to 7.6 dB, and the proportion of cases meeting the 10 dB success threshold jumped from about 63% to 79%.15PubMed Central. Choice of outcome measure methods affects reported results in stapes surgery. Data from the Swedish quality register for otosclerosis surgery That’s a substantial swing created by choosing one frequency over another, which means patients comparing success rates between surgeons or hospitals need to know that the numbers may not be directly comparable if different frequency averages were used.
When comparing surgical techniques, the air-bone gap remains the common currency. Stapedotomy (drilling a small hole in the stapes footplate) and partial stapedectomy (removing part of the footplate) produce similar gap closure overall, though stapedotomy shows an advantage specifically at 4 kHz.16PubMed. Air and bone conduction change after stapedotomy and partial stapedectomy for otosclerosis After surgery, the Carhart effect reverses: bone conduction thresholds improve as the stapes regains mobility, sometimes giving patients a bigger hearing boost than the gap closure alone would predict.
When Surgery Isn’t an Option
Not every air-bone gap can be closed surgically. Some patients have congenital ear canal atresia (the ear canal never fully formed), chronic draining ears that can’t tolerate a traditional hearing aid, or conditions where repeated surgeries have failed. For these situations, bone-anchored hearing devices offer an alternative. A small titanium implant is placed in the skull bone behind the ear, and an external sound processor transmits amplified vibration directly through bone to the cochlea, effectively bypassing the blocked outer and middle ear entirely. These devices have become increasingly popular because they avoid the problems of moisture and feedback that plague conventional aids in abnormal ear canals.17PubMed Central. BAHA: Bone-Anchored Hearing Aid
Bone-anchored devices work best when the air-bone gap is large and the cochlea is healthy, because the device is essentially routing sound via the bone conduction pathway. If bone conduction thresholds are also poor (meaning significant sensorineural loss), the device has less to work with. Candidacy evaluations rely heavily on the audiogram’s bone conduction line, which again underscores why accurately measuring the air-bone gap matters so much.
From Tuning Forks to Modern Audiometry
The concept of comparing air and bone conduction to localize hearing problems dates back centuries. By the 1800s, clinicians were using tuning forks pressed against the skull and held near the ear canal to make this comparison at the bedside. The Weber test (a vibrating fork on the forehead) and the Rinne test (fork on the mastoid bone versus next to the ear) became the classic screening methods for conductive versus sensorineural loss. Modern audiometry has replaced tuning forks with calibrated transducers and insert earphones, but the underlying logic is identical: compare the two pathways, and the gap between them tells you where the problem lives.
Hearing Loss and Broader Cognitive Effects
Understanding what the air-bone gap reveals matters beyond just the ear. Hearing loss of any type, including conductive losses with large air-bone gaps, has measurable effects on cognition. Research has found that people with hearing impairment perform worse on spatial orientation, navigation, and mental rotation tasks compared to those with normal hearing.18PubMed. Does Hearing Impairment Impact Spatial Orientation, Navigation, and Rotation Abilities? The connection likely runs through the brain’s reliance on auditory spatial cues for building an internal map of the environment. For children especially, a persistent conductive hearing loss from chronic ear effusion can affect not only speech development but these broader spatial cognitive skills, adding another reason to take an air-bone gap seriously even when the overall hearing loss seems moderate.
The air-bone gap, then, is far more than a number on a chart. Its size, its pattern across frequencies, and the clinical context in which it appears guide decisions about whether to medicate, operate, fit a hearing device, or investigate further with imaging. When the gap is large and the cause is treatable, closing it can restore hearing to near-normal levels and remove a hidden drag on cognitive function that the patient may not have realized was there.