Conductive hearing loss shows up on an audiogram as a gap between two lines: the air conduction thresholds drop below normal, while the bone conduction thresholds stay near the top of the chart. This gap, called the air-bone gap, is the defining signature. It tells the audiologist that sound is being blocked or dampened somewhere in the outer or middle ear before it ever reaches the cochlea. The size and shape of that gap, though, vary quite a bit depending on what is causing the problem.
The Air-Bone Gap Explained
An audiogram plots hearing sensitivity at different pitches, from low rumbles on the left side to high tones on the right. Two sets of thresholds are recorded: air conduction, tested through headphones or earphones, measures the entire hearing pathway from the ear canal through the eardrum and middle ear bones into the inner ear. Bone conduction, tested with a small vibrator placed on the skull behind the ear, bypasses the outer and middle ear entirely and stimulates the cochlea directly.
In a person with normal hearing, both sets of thresholds sit close together near the top of the audiogram. In sensorineural hearing loss, both lines drop together because the problem is in the cochlea or auditory nerve. Conductive hearing loss is different: bone conduction stays normal or near-normal, confirming the inner ear works fine, while air conduction falls below that level because something is interfering with sound transmission before it reaches the cochlea.1Operative Techniques in Otolaryngology. Audiometric evaluation and diagnosis of conductive hearing loss The vertical distance between these two lines at each frequency is the air-bone gap, and it is measured in decibels. A gap of 10 decibels or less is usually considered within normal limits. Anything larger points toward a conductive component.
Most conditions that cause conductive hearing loss produce air-bone gaps somewhere between 15 and 60 decibels, though the exact size and the frequencies affected depend heavily on the underlying cause. That variability is what makes audiogram shape diagnostically useful: different pathologies tend to leave different fingerprints.
How Eardrum Perforations Shape the Pattern
A hole in the eardrum is one of the most common causes of conductive hearing loss, and it produces a characteristic audiometric pattern. The air-bone gap tends to be largest at lower frequencies and gets smaller as pitch rises.2PubMed Central. Determinants of Hearing Loss in Perforations of the Tympanic Membrane You might see a gap of 30 or 40 decibels at 250 and 500 Hz that tapers down to 10 or 15 decibels by 4000 Hz. The audiogram often looks like a rising slope on the air conduction line, while bone conduction remains flat and normal across the board.
The size of the perforation is the biggest factor determining how large the gap becomes. Larger holes allow more sound energy to leak through rather than vibrating the eardrum properly, and research confirms that perforation size is the single strongest independent predictor of a large air-bone gap.3PubMed. Effects of tympanic membrane perforation, middle ear cavity volume, and mastoid aeration on hearing impairment A small pinhole perforation might produce only a mild gap, while a subtotal perforation covering most of the eardrum can push the gap to 40 decibels or more at the low end. Perforations that span both the front and back halves of the eardrum or that touch the handle of the malleus (the first middle ear bone) tend to produce worse hearing loss than smaller, more peripheral holes.4PubMed Central. The Effect of Tympanic Membrane Perforation Site, Size and Middle Ear Volume on Hearing Loss
Where the perforation sits on the eardrum matters less than its size, but it is not irrelevant. Perforations in the front versus the back of the eardrum show only small differences in air-bone gap, on the order of a few decibels at lower frequencies.2PubMed Central. Determinants of Hearing Loss in Perforations of the Tympanic Membrane A clinician looking at the audiogram alone would have a hard time guessing perforation location, though they could make a reasonable estimate of perforation size from the gap magnitude.
Middle Ear Fluid and the Flat Conductive Pattern
Fluid trapped behind the eardrum, as in otitis media with effusion, tends to produce a somewhat different audiometric picture from a perforation. Instead of the sloping, low-frequency-heavy gap, you often see a flatter air-bone gap that affects a broader range of frequencies. The fluid adds mass and stiffness to the middle ear system, dampening sound transmission more uniformly. The gap size varies depending on the amount and thickness of the fluid, but it commonly falls in the range of 15 to 40 decibels.
This is one situation where the audiogram alone can look ambiguous, because several different middle ear conditions can produce similar flat gaps. That is why tympanometry is frequently done alongside the audiogram. Tympanometry measures how the eardrum moves in response to pressure changes in the ear canal. When fluid fills the middle ear, the result is typically a flat trace, called a Type B tympanogram, with no identifiable peak of eardrum movement at any pressure setting. In a study of children undergoing tube placement surgery, this flat pattern with normal ear canal volume showed up in about 70% of ears confirmed to have fluid at the time of surgery.5PubMed Central. Otitis media with effusion: Accuracy of tympanometry in detecting fluid in the middle ears of children at myringotomies
The characteristics of the tympanogram trace can even help estimate how large the air-bone gap will be. Research on children with effusion found that specific features of the flat tympanogram, including how much the eardrum was able to move and the area under the tympanometric curve, could reliably distinguish between smaller air-bone gaps (under 20 decibels) and larger ones (above 20 decibels).6PubMed. Characteristics of the type B tympanogram can predict the magnitude of the air-bone gap in otitis media with effusion This combination of audiometry and tympanometry is particularly valuable for young children, who may not be able to cooperate reliably with behavioral hearing tests.
Otosclerosis and the Carhart Notch
Otosclerosis is a condition where abnormal bone growth gradually fixes the stapes, the tiny stirrup-shaped bone that transmits vibrations into the inner ear. On the audiogram, it produces a conductive hearing loss that often starts at the lower frequencies and progresses over time. What makes otosclerosis interesting from an audiometric standpoint is a feature called the Carhart notch: a dip in bone conduction thresholds centered around 2000 Hz. Textbooks have long taught this notch as a hallmark of stapes fixation, an artifact of the changed mechanical resonance of the middle ear rather than true inner ear damage.
The reality is less tidy. A study examining ears with surgically confirmed conductive hearing loss found that the Carhart notch appeared in only about 31% of ears with stapes fixation. More telling, it showed up at nearly the same rate in ears with other middle ear problems, like detachment of the joint between the incus and stapes (about 26%) and fixation of other ossicles (30%). The depth of the notch was also similar across groups, averaging around 17 decibels.7JAMA 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 Carhart notch can be a helpful clue, it is not specific enough to diagnose otosclerosis by itself. Plenty of people with otosclerosis do not show it, and some people with other conditions do.
The practical takeaway: if your audiogram shows a conductive pattern with a noticeable dip in bone conduction at 2000 Hz, otosclerosis is one possibility, but the audiologist will need additional information like tympanometry results, acoustic reflexes, and imaging before drawing a conclusion.
When the Ossicular Chain Is Damaged
The three middle ear bones, the malleus, incus, and stapes, form a mechanical chain that amplifies and transmits sound from the eardrum to the inner ear. If any link in that chain is eroded, dislocated, or destroyed, the audiogram shows a conductive loss, but the gap can be quite large. Ossicular discontinuity, where the chain is physically broken, tends to produce the largest air-bone gaps seen in conductive hearing loss, sometimes reaching 50 to 60 decibels.
The condition of each individual bone matters. In cholesteatoma, a type of abnormal skin growth in the middle ear that often erodes the ossicles, the status of each bone independently predicted the size of the air-bone gap. The incus was the most vulnerable and had the strongest relationship to hearing loss. Interestingly, a cholesteatoma merely touching an intact bone did not significantly worsen the gap; actual erosion or disruption was required.8Otology & Neurotology. The relationship between individual ossicular status and conductive hearing loss in cholesteatoma On the audiogram, ossicular disruption tends to produce a relatively flat air-bone gap across frequencies rather than the low-frequency-dominant pattern of perforations. This flat, wide gap with an intact eardrum and normal tympanometric volume is a strong clue that something has gone wrong with the bone chain itself.
When the Audiogram Fakes a Conductive Loss
Not every air-bone gap on an audiogram means the outer or middle ear is the problem. A handful of inner ear conditions can produce gaps that look conductive but have an entirely different origin. The most studied of these is superior semicircular canal dehiscence, a condition where a thin spot or opening in the bone covering one of the inner ear’s balance canals creates an abnormal “third window” for sound energy. The audiogram in these cases shows air-bone gaps concentrated in the low frequencies below 2000 Hz, ranging from 10 to 60 decibels, and the bone conduction thresholds sometimes dip below zero, meaning the person actually responds to bone-conducted sound at levels quieter than the equipment’s baseline.9PubMed Central. Superior semicircular canal dehiscence mimicking otosclerotic hearing loss
This pattern can be nearly indistinguishable from otosclerosis on the audiogram alone, and more than a few patients have been misdiagnosed. The key differentiators tend to come from outside the audiogram: acoustic reflexes (the involuntary muscle contraction in the middle ear in response to loud sound) are typically present in canal dehiscence but absent in otosclerosis. Specialized testing can also show abnormally sensitive balance responses, and a high-resolution CT scan can confirm the bony opening.9PubMed Central. Superior semicircular canal dehiscence mimicking otosclerotic hearing loss If surgery were performed under the assumption of stapes fixation, the surgeon would find a perfectly normal middle ear, as happened in exploratory surgeries reported in the literature before this condition was well recognized.
Beyond canal dehiscence, air-bone gaps can occasionally appear in other inner ear conditions such as enlarged vestibular aqueduct syndrome, endolymphatic hydrops, and even certain vascular anomalies near the ear.10PubMed Central. Inner-Ear Disorders Presenting with Air-Bone Gaps: A Review These are uncommon, but they remind clinicians that the audiogram is a starting point, not the final diagnosis. An air-bone gap says sound is not getting through normally by the air route compared to the bone route. It does not always explain why.
Testing Pitfalls That Create False Gaps
Sometimes what looks like a conductive loss on the audiogram is actually a testing artifact. One of the most common culprits is ear canal collapse. In some people, particularly older adults with soft ear canal cartilage, the pressure of standard audiometric headphone cushions can push the ear canal walls together, partially or completely blocking the canal. This artificially raises air conduction thresholds while bone conduction remains unaffected, creating a spurious air-bone gap. The effect is most pronounced at higher frequencies above 2000 Hz, though in more severe cases the attenuation extends to lower pitches as well.11The Journal of the Acoustical Society of America. Pure-tone thresholds for listeners with normal hearing and with ear-canal collapse under various earphone conditions Using insert earphones, which sit inside the canal and hold it open, largely eliminates this problem. An audiologist who recognizes the pattern, often a sudden unexplained conductive component in an older patient with otherwise sensorineural loss, will typically retest with inserts before concluding that a true conductive component exists.
Another technical challenge is masking. When one ear hears much better than the other, sound presented to the worse ear through headphones can cross through the skull and be detected by the better ear, producing falsely good thresholds that can distort the air-bone gap picture. Proper masking, which involves playing controlled noise in the better ear to prevent it from “helping,” is essential for accurate measurement. In children, research suggests cross-hearing should be suspected when the difference between ears reaches about 40 decibels with insert earphones, the point at which masking becomes necessary.12PubMed. Estimates of interaural attenuation in children and the implications for masking in clinical audiometry Without masking, an audiogram might show a conductive pattern in the worse ear that is actually an artifact of the better ear picking up the signal.
What the Audiogram Looks Like After Surgery
If the audiogram is the diagnostic snapshot, the postoperative audiogram is the report card. Successful treatment of conductive hearing loss should close or substantially narrow the air-bone gap by restoring the mechanical pathway for sound.
In otosclerosis surgery, where the fixed stapes is either partially removed or bypassed with a prosthesis, the postoperative air-bone gap commonly closes to around 6 decibels on average across the speech frequencies. A study comparing two surgical techniques, partial stapedectomy and stapedotomy, found virtually identical gap closure overall, with the average gap dropping to about 6 decibels in both groups. Stapedotomy did show a modest advantage at higher pitches, with better closure at 2, 3, and 4 kHz.13PubMed. Air and bone conduction change after stapedotomy and partial stapedectomy for otosclerosis For the patient, a postoperative gap under 10 decibels is generally considered an excellent result, meaning the middle ear is conducting sound almost as efficiently as a healthy ear.
For eardrum perforations repaired with cartilage grafts, the audiometric results are similarly encouraging. Tympanoplasty using cartilage plug grafts achieves successful closure of the air-bone gap in roughly 92% of cases overall, though success rates dip somewhat for larger perforations requiring grafts bigger than about 5 millimeters.14PubMed. Effect of Graft Size on Air-bone Gap Closure in Cartilage Button Tympanoplasty On the audiogram, a successful repair looks like the air conduction line climbing back up toward the bone conduction line, ideally overlapping it. If a residual gap remains, the audiologist can quantify exactly how much conductive component persists, which guides decisions about whether further intervention or amplification might be helpful.
Mixed Hearing Loss and Overlapping Patterns
Real-world audiograms are not always textbook examples. Many people, especially older adults, have mixed hearing loss, where a conductive component sits on top of underlying sensorineural loss. On the audiogram, both the air and bone conduction lines are depressed below normal, but air conduction is worse than bone conduction, preserving a measurable gap. The bone conduction line tells you how much inner ear damage exists, the air conduction line adds the conductive component on top, and the gap between them reveals the portion that might be medically or surgically treatable.
Reading mixed loss correctly matters because the conductive piece is often fixable. A person whose bone conduction shows a mild sensorineural loss at 30 decibels but whose air conduction sits at 55 decibels has a 25-decibel conductive component. If the conductive cause, say middle ear fluid or a fixable ossicular problem, is addressed, their hearing should improve to the level of the bone conduction line. They will still have a mild loss, but the functional improvement from eliminating that extra 25-decibel barrier can be substantial. The audiogram makes this math visible in a way few other medical tests can, giving both clinician and patient a clear picture of what is recoverable and what is permanent.