How to Read a Tympanogram

Tympanometry measures the compliance of the middle ear system as a function of ear canal pressure. This guide explains the axes, the Jerger types (A, As, Ad, B, C), acoustic reflexes, and how to correlate the findings with common diagnoses.

Tympanometry is the most useful objective middle ear test available at the bedside and in the audiology clinic. It takes less than a minute per ear, requires no behavioural response from the patient, works in infants and unconscious patients, and provides information that otoscopy alone cannot — it quantifies middle ear compliance and pressure, reveals whether the ossicular chain is mobile, and tests whether the stapedius reflex arc is intact. Yet it is routinely misread or under-interpreted in clinical practice. This article explains what is actually being measured, what each tracing means, and how to combine tympanometry with the audiogram to reach a diagnosis.

What Tympanometry Measures

A tympanometer introduces a probe into the sealed ear canal. The probe emits a low-frequency tone (typically 226 Hz in adults and older children; 1000 Hz in neonates, because the immature ear canal wall compliance confounds the low-frequency probe result) and a microphone in the probe measures how much of that sound is absorbed into the middle ear versus how much is reflected back.

Compliance (also called admittance or mobility) describes how readily the tympanic membrane and ossicular chain move in response to the probe tone. A compliant (mobile) middle ear absorbs more sound energy; a stiff middle ear reflects more. The tympanometer measures compliance continuously as it varies the pressure in the sealed ear canal from positive to negative (the sweep usually goes from +200 daPa to −400 daPa, though the exact range varies by equipment).

At any given pressure, the compliance of the tympanic membrane is maximised when the air pressure on both sides of it is equal. By sweeping pressure and finding the peak of the compliance curve, the tympanometer identifies the pressure at which the middle ear and ear canal are in equilibrium — this is the peak pressure of the tympanogram. In a normal ear, middle ear pressure is close to atmospheric (0 daPa) and the peak falls near zero.

The resulting graph — compliance on the Y-axis, ear canal pressure on the X-axis — is the tympanogram.

The Axes

Y-axis — compliance (admittance): Measured in millimhos (mmho) or cubic centimetres (cc/mL). Normal peak compliance in adults is approximately 0.3 to 1.6 mmho. Values below this indicate a stiff system (reduced compliance); values above indicate an overly compliant (flaccid or disrupted) system.

X-axis — ear canal pressure: Measured in daPa (decaPascals). Normal peak pressure is 0 ± 50 daPa. A peak shifted to the negative pressure side indicates negative middle ear pressure (Eustachian tube dysfunction); a peak at approximately normal pressure with reduced height indicates stiffness.

The Jerger Classification

James Jerger described the standard classification of tympanogram types in 1970. Every tympanogram should be classified, combined with the peak compliance value and ear canal volume measurement.

Type A — Normal

A distinct, symmetric peak near 0 daPa with normal peak compliance (0.3–1.6 mmho). The middle ear is normally pressurised and normally compliant.

Ear canal volume is also measured and displayed — it reflects the volume from the probe tip to the tympanic membrane. Normal volume is approximately 0.6–1.5 mL in adults (smaller in children). This value matters primarily for interpreting Type B traces (see below).

Type As — Shallow (Reduced Compliance)

A peak present at approximately normal pressure, but with reduced height — compliance below approximately 0.3 mmho. The peak shape is normal (symmetric, well-defined) but the amplitude is small. The subscript “s” stands for shallow (or stiffness).

Interpretation: Increased stiffness of the ossicular system. This is the characteristic tympanogram of otosclerosis (fixed stapes footplate), tympanosclerosis (calcification of the middle ear and drum), and also appears in ears with middle ear fibrosis after repeated infections.

Type As is the single most important finding for distinguishing otosclerosis from a normal ear at the tympanometer — it directly reflects the reduced mobility of the stiffened system.

Type Ad — Deep (Increased Compliance)

A peak present at approximately normal pressure, but with markedly elevated height — compliance above approximately 1.6 mmho. The curve rises sharply and may exceed the limits of the tympanometer scale. The subscript “d” stands for deep (or discontinuity/disarticulation).

Interpretation: An excessively compliant middle ear. This occurs in two situations:

Ossicular discontinuity — break in the ossicular chain (typically lenticular process of the incus necrosis, or traumatic dislocation of the incudomalleolar joint). The tympanic membrane no longer has the damping load of the ossicular chain behind it and becomes hypercompliant.
Flaccid or monomeric tympanic membrane — a healed perforation that has produced a thin, poorly supported scar patch (monomeric membrane). This produces compliance well above normal.

A patient with a Type Ad trace and a conductive hearing loss almost certainly has ossicular discontinuity as the cause.

Type B — Flat (No Peak)

A flat, featureless trace with no discernible peak — compliance does not change significantly as pressure is varied. This is the tympanogram of a system that is not compliant at any pressure, or one where the ear canal is not properly sealed.

The ear canal volume is the key to interpreting Type B:

Type B with…	Volume	Interpretation
Normal volume (0.6–1.5 mL)	Normal	Middle ear effusion (fluid-filled), severe tympanosclerosis, or wax occluding the drum but not the canal
Large volume (>2 mL)	High	Tympanic membrane perforation — the probe is measuring the combined volume of the canal + middle ear + mastoid air cells (a large space)
Near-zero volume	Low	Probe blocked by wax, occluded against the canal wall, or probe not sealed

A Type B trace with normal volume and no visible drum abnormality = otitis media with effusion (OME/glue ear) until proven otherwise in a child. In an adult, the same finding should prompt investigation for a cause of Eustachian tube dysfunction (including nasopharyngeal pathology).

A Type B trace with high ear canal volume = perforation — the “Type B” is not a middle ear finding at all, it is the measurement of an open cavity.

Type C — Negative Peak (Eustachian Tube Dysfunction)

A peak present and within normal compliance range, but shifted to the negative pressure side — typically peak pressure more negative than −100 daPa (some texts use −150 daPa as the cutoff for referral). The subscript “C” refers to the negative pressure characteristic.

Interpretation: The middle ear contains air at negative pressure relative to the atmosphere, because the Eustachian tube is not equalising pressure effectively. The tympanic membrane is retracted (bulging inward). This is the tympanometric finding of Eustachian tube dysfunction before an effusion has developed.

A Type C2 variant (peak beyond −200 daPa) indicates severe Eustachian tube dysfunction and a high likelihood of progressing to effusion.

Acoustic Reflexes

The stapedius reflex (acoustic reflex) is measured by the same probe. A loud sound (typically 70–100 dB above the patient’s threshold, or at fixed levels of 85, 90, 95, 100 dB HL) is introduced to either the probe ear (ipsilateral) or the opposite ear (contralateral), and the tympanometer detects the change in compliance of the probe ear when the stapedius contracts.

Normal reflex: Present at 70–100 dB HL for pure tones at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz.

Ipsilateral reflex arc: Sound → ipsilateral CN VIII → brainstem → ipsilateral CN VII → ipsilateral stapedius. Contralateral reflex arc: Sound → ipsilateral CN VIII → brainstem → contralateral CN VII → contralateral stapedius.

Acoustic reflexes must be interpreted alongside the tympanogram and audiogram. The pattern of absent/present reflexes localises pathology:

Finding	Interpretation
Absent ipsilateral and contralateral reflexes, Type As	Ossicular fixation (stapes or other) — stapedius cannot change compliance of stiff system
Absent ipsilateral reflexes, present contralateral, Type A	CN VII lesion (ipsilateral facial nerve)
Absent contralateral reflexes, present ipsilateral, Type A	Severe ipsilateral SNHL (afferent pathway unable to reach reflex threshold) OR CN VIII lesion
Reflexes present but at elevated levels	Mild-moderate SNHL (reflexes require louder input)
Reflexes absent bilaterally with normal tympanograms	Retrocochlear pathology (acoustic neuroma), CN VII lesion, brainstem lesion

Acoustic Reflex Decay

If the reflex is present, reflex decay tests whether it can be sustained. A 10 dB activating stimulus is applied for 10 seconds; in a normal ear, the reflex amplitude stays sustained or barely fades. If the reflex amplitude drops by >50% within the 10 seconds, the test is positive for decay — a finding characteristic of retrocochlear pathology (acoustic neuroma or other CN VIII lesion). Reflex decay is tested at 500 Hz and 1000 Hz (higher frequencies are too uncomfortable at the required intensity).

Putting It Together — Diagnostic Patterns

Diagnosis	Tympanogram	Acoustic Reflex
Normal middle ear	Type A	Present, normal levels
Otitis media with effusion	Type B (normal volume)	Absent
Eustachian tube dysfunction	Type C	May be present or absent
Otosclerosis	Type As	Absent ipsilaterally
Ossicular discontinuity	Type Ad	Absent (or present if mild)
TM perforation	Type B (large volume)	Absent ipsilaterally
SNHL (cochlear)	Type A	Present, elevated thresholds
Acoustic neuroma (CN VIII)	Type A	Absent, or reflex decay positive
CN VII palsy	Type A	Absent ipsilaterally (effector lesion)

Key Numbers

Parameter	Normal Range
Normal peak compliance (226 Hz probe)	0.3–1.6 mmho
Normal peak pressure	0 ± 50 daPa
Normal adult ear canal volume	0.6–1.5 mL
Type As compliance	<0.3 mmho
Type Ad compliance	>1.6 mmho (often off-scale)
Type C negative pressure cutoff	More negative than −100 daPa
Acoustic reflex threshold (normal hearing)	70–100 dB HL
Reflex decay: positive result	>50% amplitude drop within 10 seconds
Probe frequency (adults)	226 Hz
Probe frequency (neonates)	1000 Hz

Frequently Asked Questions

Why is a different probe frequency used for neonates? In neonates and infants under approximately 6 months, the ear canal wall is highly compliant — the cartilaginous and bony canal has not yet fully stiffened. At 226 Hz, the compliance of the canal wall itself dominates the measurement, making the result unreliable for assessing the middle ear. At 1000 Hz (a higher-frequency probe), the canal wall compliance is less influential and the middle ear compliance signal is more reliably extracted. This is why all neonatal hearing screening programmes that include tympanometry use the 1000 Hz probe.

Can tympanometry detect sensorineural hearing loss? Tympanometry assesses the middle ear system only — it measures tympanic membrane and ossicular chain mobility. It cannot detect sensorineural hearing loss. A patient with a severe SNHL will have a completely normal Type A tympanogram. However, acoustic reflexes will be absent or elevated, because the sensorineural loss means the activating sound does not reach the reflex threshold; this indirect effect of SNHL on reflex thresholds is important to recognise.

Is tympanometry painful? No. The probe hermetically seals the ear canal and the pressure sweep from +200 to −400 daPa can cause mild pressure sensation, but is not painful. Some patients with sensitive ears find it mildly uncomfortable. The probe frequency tone (226 Hz) is typically inaudible or barely perceptible. The test takes approximately 30 seconds per ear. The acoustic reflex stimuli (at 85–100 dB HL) can be startling — patients should be warned — but the brief duration (0.5–1 second per stimulus) limits discomfort.