This article was originally written by Mr Neela Mouli Doddi MBBS, MS(ENT), MRCS, and has been updated and expanded for the new site. The original content has been preserved in full. This page is maintained as part of Professor Vik Veer's free clinical education resource for junior NHS doctors.
An audiogram is a graphical record of a patient's hearing. It plots the results of pure-tone audiometry (PTA) — the gold-standard objective hearing test — and tells us the type and degree of hearing loss. It is one of the most important investigations in ENT and is essential to review before seeing any patient with a hearing complaint.
The Axes of an Audiogram
The audiogram has two axes:
- X axis (horizontal) — Frequency: Sound frequency (pitch), measured in Hertz (Hz). The scale runs from low frequencies (125–250 Hz — low-pitched sounds like a rumbling lorry) on the left, to high frequencies (4,000–8,000 Hz — high-pitched sounds like birdsong or the consonants in speech) on the right. The frequencies routinely tested are: 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz. Some audiologists also test 125 Hz, 3 kHz, and 6 kHz for greater detail.
- Y axis (vertical) — Intensity: Sound intensity (loudness), measured in decibels Hearing Level (dB HL). The scale runs from the top (quietest — typically −10 dB or 0 dB) to the bottom (loudest — typically 110–120 dB HL). This is counterintuitive at first — a lower threshold (plotted higher on the graph) means better hearing.
Hearing Threshold and What dB HL Means
The hearing threshold at each frequency is the softest sound a person can hear at least 50% of the times it is presented. These thresholds are plotted as individual data points on the audiogram, and connected by lines to form the audiogram shape (sometimes called the "audiometric configuration" or "audiogram pattern").
It is important to understand that 0 dB HL does not mean the complete absence of sound — rather, 0 dB HL is defined as the average threshold of a young adult with normal hearing at each frequency. The range of normal hearing thresholds is 0–20 dB HL. A threshold of 0 dB HL means the patient's hearing is equivalent to the average of a normal-hearing young adult. A threshold of −5 dB HL means the patient can hear sounds slightly softer than the average normal-hearing person.
Degrees of Hearing Loss
| Threshold Range (dB HL) | Degree of Hearing Loss | Practical Impact |
|---|---|---|
| 0 – 20 dB | Normal | No significant difficulty |
| 21 – 40 dB | Mild | Difficulty with soft speech, especially in noise |
| 41 – 55 dB | Moderate | Difficulty with conversational speech; hearing aid often beneficial |
| 56 – 70 dB | Moderately severe | Significant difficulty; hearing aid required |
| 71 – 90 dB | Severe | Relies on lip-reading, powerful hearing aids, or cochlear implant assessment |
| > 90 dB | Profound | Cochlear implant may be the best option; little or no functional hearing |
Audiogram Symbols — Air and Bone Conduction
At each frequency, two types of threshold are measured:
- Air conduction (AC) — the patient listens to tones delivered through headphones (or insert earphones). This tests the whole hearing system: outer ear, middle ear, cochlea, and auditory nerve. Air conduction thresholds are plotted using circles (O) for the right ear and crosses (X) for the left ear.
- Bone conduction (BC) — a vibrating oscillator is placed on the mastoid process (or on the forehead). This bypasses the outer and middle ear and stimulates the cochlea directly through skull vibration. Bone conduction thresholds reflect cochlear function alone. BC thresholds are plotted using small angled brackets (or triangles) for each ear.
The critical concept is the air-bone gap (ABG): the difference in dB between the air conduction threshold and the bone conduction threshold at the same frequency. In a healthy ear, AC and BC thresholds are within 10 dB of each other (effectively the same). A significant air-bone gap (>10 dB difference) indicates a conductive component to the hearing loss.
Symbols for the Right Ear
Symbols for the Left Ear
Summary of Standard Audiogram Symbols
| Symbol | Meaning |
|---|---|
| O (red) | Unmasked air conduction — right ear |
| X (blue) | Unmasked air conduction — left ear |
| < (red, right-facing) | Unmasked bone conduction — right ear (mastoid placement) |
| > (blue, left-facing) | Unmasked bone conduction — left ear (mastoid placement) |
| [ (red) | Masked bone conduction — right ear |
| ] (blue) | Masked bone conduction — left ear |
| △ (red) | Unmasked bone conduction — right ear (alternative symbol) |
| ▽ (blue) | Unmasked bone conduction — left ear (alternative symbol) |
Masking
Masking is the process of presenting a continuous noise to the non-test ear (the ear not being assessed) in order to prevent it from detecting the test tone by crossover — i.e. by sound or vibration crossing through the skull. Without masking, the non-test ear may "shadow" the test ear and give falsely good thresholds for the test ear.
Masking is required when:
- The difference between the air conduction thresholds of the right and left ears is more than 40 dB (because at this level, the louder sound can be transmitted across the skull to the better ear).
- The difference between the air conduction threshold and the bone conduction threshold of the same ear is more than 10 dB (because BC stimulates both cochleas simultaneously and, without masking, the better cochlea may respond).
When masked thresholds are used, different symbols are plotted on the audiogram to indicate this (e.g. "[" for masked BC right ear, "]" for masked BC left ear). The clinical audiologist will decide when masking is necessary during the test.
Types of Hearing Loss on the Audiogram
Conductive Hearing Loss
What is it? Conductive hearing loss results from a problem in the outer or middle ear that impairs the transmission of sound to the cochlea. The cochlea and auditory nerve remain intact — bone conduction thresholds are normal, but air conduction thresholds are elevated because the middle ear mechanism is not functioning properly.
Audiogram pattern:
- Bone conduction thresholds: within normal limits (0–20 dB HL)
- Air conduction thresholds: elevated (greater than 20 dB HL)
- Air-bone gap: more than 10 dB at the affected frequencies
Common causes: Wax (cerumen) impaction, acute or chronic otitis media with effusion (glue ear), tympanic membrane perforation, otosclerosis (progressive fixation of the stapes footplate), disruption of the ossicular chain (following trauma, infection, or cholesteatoma).
Sensorineural Hearing Loss
What is it? Sensorineural hearing loss (SNHL) results from damage to the hair cells of the cochlea (sensory component), the cochlear nerve (neural component), or the central auditory pathways. The outer and middle ear are intact — sound reaches the cochlea normally, but the cochlea or nerve cannot process it correctly.
Audiogram pattern:
- Bone conduction thresholds: elevated (greater than 20 dB HL)
- Air conduction thresholds: elevated to a similar degree (within 10 dB of bone conduction thresholds)
- Air-bone gap: less than 10 dB (effectively no gap — both pathways are equally affected because the problem lies at or beyond the cochlea)
Common causes: Presbyacusis (age-related), noise-induced hearing loss, Ménière's disease, acoustic neuroma (vestibular schwannoma), sudden sensorineural hearing loss (SSNHL), ototoxic medications (aminoglycosides, platinum-based chemotherapy, loop diuretics in high doses), viral labyrinthitis, congenital hearing loss.
Mixed Hearing Loss
What is it? Mixed hearing loss has a combination of both conductive and sensorineural pathology in the same ear. The cochlea is impaired (raising the bone conduction threshold above normal), and there is an additional middle ear or outer ear problem raising the air conduction threshold further still.
Audiogram pattern:
- Bone conduction thresholds: elevated above 20 dB HL
- Air conduction thresholds: elevated to an even greater degree than bone conduction
- Air-bone gap: more than 10 dB (there is both a sensorineural component and a conductive component)
Common causes: Chronic suppurative otitis media (CSOM) with secondary cochlear damage, otosclerosis in a patient who also has age-related SNHL, head trauma causing both ossicular chain disruption and cochlear concussion.
Disease-Specific Audiogram Patterns
Certain conditions produce recognisable audiogram patterns. Try scrolling down slowly to test your recognition skills before reading the caption.
Otosclerosis — Carhart's Notch
Otosclerosis is a progressive condition in which abnormal bone remodelling leads to fixation of the stapes footplate in the oval window, reducing its ability to transmit sound vibrations into the cochlear fluids. It primarily causes a conductive hearing loss, as expected from a middle ear problem. However, it produces a characteristic feature: a dip in the bone conduction threshold at 2 kHz — this is called Carhart's notch.
Importantly, Carhart's notch is a mechanical artefact of the stapes fixation, not a true sensorineural hearing loss — the cochlea itself is not damaged. After successful surgical treatment (stapedectomy or stapedotomy, in which the fixed stapes is replaced with a prosthesis), the Carhart's notch typically disappears, confirming its mechanical rather than sensorineural origin. This is an important point in ENT vivas.
Otosclerosis is more common in women, presents in the second to fourth decade, has a positive family history in many cases, and may worsen during pregnancy. Patients often report hearing better in noisy environments (paracusis of Willis) — this is because in background noise, people naturally raise their voices, which compensates for the conductive loss.
Noise-Induced Hearing Loss — 4 kHz Notch
Noise-induced hearing loss (NIHL) is damage to the outer hair cells of the cochlea caused by excessive sound exposure. It is the most common occupational disease in the UK. The characteristic feature is a 4 kHz notch (also called a high-frequency dip or "audiometric notch") — a selective dip in the hearing threshold at 4 kHz, with partial recovery at 8 kHz. This notch pattern is pathognomonic of NIHL.
The 4 kHz frequency is selectively affected because the outer hair cells in the region of the cochlea that process this frequency (approximately 10 mm from the base of the cochlea, the region corresponding to 4 kHz) are most vulnerable to acoustic trauma. This is partly because the resonant frequency of the external auditory canal amplifies incoming sounds in the 2–4 kHz range, concentrating energy damage in this cochlear region.
NIHL is irreversible — once hair cells are destroyed, they do not regenerate. Prevention through hearing protection is therefore paramount. Audiograms are used in occupational health settings to screen workers in noisy industries and to document progression.
Presbyacusis — High-Frequency Sensorineural Hearing Loss
Presbyacusis is the progressive, bilateral, symmetrical, high-frequency sensorineural hearing loss associated with ageing. It is the most common cause of hearing loss worldwide. The audiogram shows a characteristic downward sloping pattern — thresholds are near normal at low frequencies (250–500 Hz) and progressively worse at higher frequencies (2 kHz–8 kHz).
This pattern reflects the progressive loss of outer hair cells beginning at the basal turn of the cochlea (which processes high frequencies) and advancing towards the apical turn (which processes low frequencies) over time. The basal hair cells are most metabolically active and most exposed to the cumulative effects of oxidative stress, noise exposure, and genetic predisposition throughout life.
Because the high frequencies (particularly 2–4 kHz) are critical for understanding consonants in speech (e.g. "s," "f," "th," "sh"), patients with presbyacusis typically report that they can hear people speaking but cannot understand what is being said — especially in background noise or when the speaker's face is not visible. This leads to the common complaint: "I can hear you, but I can't make out the words."
Ménière's Disease — Low-Frequency Sensorineural Hearing Loss
Ménière's disease is characterised by the clinical triad of: episodic vertigo (lasting 20 minutes to several hours), fluctuating unilateral sensorineural hearing loss, and tinnitus (often with a sensation of aural fullness). The underlying pathology is endolymphatic hydrops — an abnormal accumulation of endolymph in the membranous labyrinth, which distends the endolymphatic space and periodically ruptures the membranes, causing episodes of vestibular and cochlear dysfunction.
The audiogram in early Ménière's disease shows a characteristic low-frequency sensorineural hearing loss, with thresholds worse at 250 Hz and 500 Hz and relatively preserved at 2–4 kHz — giving an "upward sloping" or "rising" audiogram. This is the opposite of the downward slope seen in presbyacusis and NIHL. In advanced disease, the hearing loss flattens and becomes severe across all frequencies, and the fluctuating nature often becomes less pronounced as the disease "burns out."
The low-frequency hearing loss in early Ménière's is thought to reflect the particular vulnerability of the apical cochlea (which processes low frequencies) to the distension caused by endolymphatic hydrops. The apical region contains the most delicate structures and is most susceptible to the pressure changes associated with hydrops.
How to Read a Tympanogram
Tympanometry is an objective assessment of the status of the middle ear. It measures the acoustic compliance (how easily the tympanic membrane and ossicular chain move) of the tympano-ossicular system as air pressure in the sealed external auditory canal is varied from positive to negative. The result is plotted as a tympanogram.
- X axis: Air pressure in the external auditory canal, measured in decapascals (daPa) or mmH₂O. Ranges from approximately −400 daPa (strongly negative) to +200 daPa (positive).
- Y axis: Acoustic compliance (or admittance) of the middle ear, measured in millilitres (ml) or millimhos (mmho). Higher compliance = more movement = a taller peak.
The principle is that the tympanic membrane moves most freely (shows maximum compliance) when the air pressure on both sides of it is equal. If the middle ear pressure is negative (e.g. due to Eustachian tube dysfunction), the compliance peak shifts to the left (towards negative pressures). If the TM is stiff or immobile (e.g. due to middle ear fluid or ossicular fixation), the compliance peak is reduced or absent.
Type A — Normal Tympanogram
The Type A tympanogram is the normal pattern. It shows a tent-shaped (peaked) curve located between −100 daPa and +50 daPa, with a compliance peak that falls within normal limits (approximately 0.3–1.6 ml in adults).
A normal Type A tympanogram indicates:
- Normal middle ear pressure
- Normal tympanic membrane compliance
- Normal Eustachian tube function
Type As — Shallow Peak (Stiff Middle Ear)
The Type As tympanogram (the "s" stands for "shallow" or "stiff") shows a peaked curve in the correct pressure range (−100 to +50 daPa) but with a reduced compliance peak — the peak is there but flatter and lower than expected. This indicates that the middle ear system is abnormally stiff, reducing compliance.
Clinical association: Otosclerosis (fixation of the stapes footplate) is the classic cause. Also seen with tympanosclerosis (calcified plaques in the TM or ossicular joints).
Type Ad — Deep Peak (Hypermobile Middle Ear)
The Type Ad tympanogram (the "d" stands for "deep" or "disarticulated") shows a peaked curve in the correct pressure range but with an abnormally high compliance peak — higher than 1.6 ml. This indicates excessive mobility of the tympanic membrane and middle ear system.
Clinical association: Ossicular discontinuity (disruption of the ossicular chain, e.g. following trauma, cholesteatoma erosion, or after stapedectomy in some cases), or a flaccid or thin tympanic membrane (e.g. a healed perforation or myringotomy site).
Type B — Flat Trace
The Type B tympanogram is a flat, horizontal trace — there is no peak at any pressure. The tympanic membrane and middle ear system show minimal or no compliance, regardless of the pressure applied.
The interpretation of a Type B tympanogram depends critically on the ear canal volume (ECV), which is measured as part of the test:
- Type B with normal ECV (0.6–2.5 ml in adults): Indicates fluid in the middle ear (otitis media with effusion — glue ear). The fluid prevents the TM from moving, abolishing the compliance peak. This is the most common cause of a Type B tympanogram in children.
- Type B with large ECV (>2.5 ml in adults): Indicates a perforated tympanic membrane (or a patent ventilation tube — grommet). The probe is effectively measuring the volume of the middle ear and mastoid in addition to the EAC, giving a large combined volume. There is no compliance peak because the TM is not intact.
Type C — Negative Middle Ear Pressure
The Type C tympanogram shows a normal-shaped (tent-shaped) peaked curve, but the peak is located at a pressure more negative than −100 daPa. This indicates that the middle ear pressure is significantly negative relative to atmospheric pressure.
Clinical association: Eustachian tube dysfunction — the Eustachian tube is failing to adequately ventilate the middle ear and equalise pressure. Over time, sustained negative middle ear pressure causes the TM to retract (pulled inward by the relative vacuum), and may progress to an effusion (Type B). The more negative the pressure peak, the more severe the Eustachian tube dysfunction. Type C is commonly seen in patients with early glue ear, upper respiratory tract infections, or allergic rhinitis.
Summary Table — Tympanogram Types
| Type | Peak Location | Compliance | Clinical Meaning |
|---|---|---|---|
| A | −100 to +50 daPa | Normal (0.3–1.6 ml) | Normal middle ear |
| As | −100 to +50 daPa | Reduced (<0.3 ml) | Stiff middle ear — otosclerosis, tympanosclerosis |
| Ad | −100 to +50 daPa | Increased (>1.6 ml) | Hypermobile TM — ossicular discontinuity, flaccid TM |
| B (low ECV) | Flat — no peak | Absent | Middle ear effusion (glue ear) |
| B (high ECV) | Flat — no peak | Absent | Perforated TM or patent grommet |
| C | More negative than −100 daPa | Usually normal | Eustachian tube dysfunction / negative middle ear pressure |
Frequently Asked Questions
How do I read an audiogram step by step?
Follow these steps: (1) Identify the axes — frequency (Hz) on the X axis (left = low pitch, right = high pitch) and intensity (dB HL) on the Y axis (top = quiet, bottom = loud). (2) Identify which ear you are reading — red symbols are the right ear, blue are the left. Circles (O) = right AC, crosses (X) = left AC. Brackets or triangles = bone conduction. (3) Note the pattern of thresholds — are they elevated? At which frequencies? (4) Compare AC and BC — is there an air-bone gap (ABG > 10 dB)? An ABG indicates a conductive component. (5) Classify: CHL (normal BC, elevated AC, ABG present), SNHL (both elevated, no ABG), or Mixed (both elevated, ABG present). (6) Note the shape — sloping high-frequency loss (presbyacusis/NIHL), rising low-frequency loss (Ménière's), notch at 4 kHz (NIHL), notch at 2 kHz on BC (Carhart's notch in otosclerosis).
What is an air-bone gap and what does it mean?
The air-bone gap (ABG) is the difference in dB between the air conduction (AC) threshold and the bone conduction (BC) threshold at the same frequency. In a healthy ear, AC and BC thresholds are within 10 dB of each other — there is essentially no gap. A gap greater than 10 dB indicates that air conduction is significantly worse than bone conduction — meaning the outer or middle ear is not transmitting sound effectively to the (functioning) cochlea. This is the hallmark of a conductive hearing loss. The larger the ABG, the greater the conductive component. In sensorineural hearing loss, both thresholds rise together — the ABG remains small (<10 dB).
What is Carhart's notch and which condition causes it?
Carhart's notch is a characteristic dip in the bone conduction threshold at 2 kHz seen in patients with otosclerosis. It is named after Raymond Carhart, who first described it in 1950. Importantly, it is a mechanical artefact caused by fixation of the stapes footplate — it does not represent true sensorineural hearing loss. The fixed stapes cannot vibrate normally, and this reduces the efficiency of bone conduction transmission at this resonant frequency. After successful stapedectomy (surgical replacement of the fixed stapes with a mobile prosthesis), the Carhart's notch typically improves or resolves, confirming its mechanical rather than cochlear origin. This is a classic exam and viva question.
What does a flat Type B tympanogram mean?
A flat (Type B) tympanogram — with no compliance peak at any pressure — indicates that the tympanic membrane is not moving in response to pressure changes. The interpretation depends on the ear canal volume (ECV): a normal ECV with a Type B trace indicates middle ear fluid (otitis media with effusion — glue ear), preventing TM movement; a large ECV with a Type B trace indicates a perforated TM or patent grommet, where the middle ear cavity is in open communication with the EAC, making the entire connected volume too large for a compliance peak to be demonstrated.
How do you distinguish presbyacusis from noise-induced hearing loss on an audiogram?
Both conditions cause bilateral sensorineural hearing loss with worse thresholds at higher frequencies, but the key distinguishing feature is the audiogram shape. Presbyacusis typically shows a gradual downward slope from low to high frequencies — the loss increases progressively from 1 kHz to 8 kHz without a specific notch. Noise-induced hearing loss (NIHL) shows a characteristic notch at 4 kHz with partial recovery at 8 kHz — the 4 kHz frequency is disproportionately affected compared to adjacent frequencies. The notch pattern is pathognomonic of NIHL. In practice, many older patients have a combination of both conditions (adding noise exposure on top of age-related changes), which can obscure the classic notch.
What does the Ménière's disease audiogram look like?
In early Ménière's disease, the audiogram shows a unilateral low-frequency sensorineural hearing loss — thresholds are worst at 250 Hz and 500 Hz, with relatively preserved hearing at 2–4 kHz, giving an "upward sloping" or "rising" configuration. This is distinctive and the opposite of the high-frequency loss seen in presbyacusis. The loss is fluctuating — it may improve between attacks, especially in the early stages of the disease. In late-stage or "burnt-out" Ménière's, the audiogram typically shows a severe flat sensorineural loss across all frequencies as the fluctuations become less prominent. Serial audiograms documenting this fluctuation are an important part of the diagnostic criteria.
What is masking in audiometry and when is it needed?
Masking in audiometry is the delivery of a broadband noise (typically narrowband noise centred on the test frequency) to the non-test ear, to prevent it from detecting the test tone by crossover through the skull. The skull attenuates sound by approximately 40–50 dB (the inter-aural attenuation). If the test tone is more than 40 dB above the threshold of the non-test ear (for air conduction) or more than 0–10 dB above it (for bone conduction — since bone conduction effectively stimulates both cochleas simultaneously), the non-test ear may respond, giving falsely good thresholds for the test ear. Masking is particularly important when there is a marked hearing asymmetry, and when bone conduction is being measured.
What does a Type C tympanogram indicate clinically?
A Type C tympanogram (compliance peak at a pressure more negative than −100 daPa) indicates negative middle ear pressure, caused by Eustachian tube dysfunction. The Eustachian tube normally opens briefly during swallowing and yawning to equalise middle ear pressure with atmospheric pressure. When the tube fails to open adequately — as in upper respiratory tract infections, allergic rhinitis, or barotrauma — the middle ear absorbs air faster than it is replenished, creating negative pressure. The TM is drawn inward (retracted). If untreated, the negative pressure progresses to a middle ear effusion (glue ear), and the tympanogram evolves from Type C to Type B. Type C is common in patients presenting with aeroplane-related ear discomfort.
How would you present audiogram findings in an ST3 ENT viva?
Structure your answer systematically: "This is the pure-tone audiogram for the right/left ear. Looking at the air conduction thresholds, I can see [thresholds at each frequency, or a summary e.g. a high-frequency sloping loss]. The bone conduction thresholds are [within normal limits / also elevated]. The air-bone gap is [absent / present — approximately X dB at Y Hz]. Overall, this is most consistent with [conductive / sensorineural / mixed] hearing loss of [mild / moderate / severe / profound] degree, with a configuration consistent with [presbyacusis / NIHL with a 4 kHz notch / Ménière's with low-frequency loss / otosclerosis with Carhart's notch]. I would also want to see the tympanogram to assess middle ear function." This demonstrates systematic interpretation rather than just guessing the diagnosis.
References
- Gleeson M, Clarke R, eds. Scott-Brown's Otorhinolaryngology, Head and Neck Surgery, 8th ed. CRC Press; 2018. Chapters on Audiology and Pure-Tone Audiometry.
- British Society of Audiology. Recommended Procedure: Pure-tone air-conduction and bone-conduction threshold audiometry with and without masking. BSA; 2017.
- British Society of Audiology. Recommended Procedure: Tympanometry. BSA; 2013.
- Katz J, ed. Handbook of Clinical Audiology, 7th ed. Wolters Kluwer; 2015.
- National Institute for Health and Care Excellence. Hearing loss in adults: assessment and management. NICE Guideline NG98; 2018.
- National Institute for Health and Care Excellence. Otitis media with effusion in under 12s (surgical management). NICE Guideline NG149; 2023.
- Carhart R. Clinical application of bone conduction audiometry. Arch Otolaryngol. 1950;51(6):798–808.
- Dhillon RS, East CA. Ear, Nose and Throat and Head and Neck Surgery, 4th ed. Churchill Livingstone; 2013.
- Merchant SN, Rosowski JJ. Conductive hearing loss caused by third-window lesions of the inner ear. Otol Neurotol. 2008;29(3):282–289.
- Lopez-Escamez JA, Carey J, Chung WH, et al. Diagnostic criteria for Menière's disease. J Vestib Res. 2015;25(1):1–7.