Introduction to Impedance Audiometry

What tympanometry measures, where it sits in the test battery, and what it can and cannot tell you.

Impedance audiometry — more commonly called acoustic immittance testing — is an objective, non-invasive way to assess the middle-ear transmission system. It does not require a behavioural response from the patient, which makes it valuable across the age range and in patients who cannot reliably perform pure-tone audiometry.

The test rests on a simple physical idea: when sound strikes the tympanic membrane, some energy is absorbed and the rest is reflected. A stiffer drum reflects more energy; a more compliant system absorbs more. By sealing the ear canal and varying its air pressure, the instrument can chart how the mobility of the tympano-ossicular system changes — and that chart is the tympanogram. [American Speech-Language-Hearing Association 1988]

Where it sits in the battery

Tympanometry is one test among several. A case history, otoscopic examination, air- and bone-conduction thresholds, and acoustic-reflex measurements all contribute information about the middle ear. The tympanogram is most powerful when interpreted together with these, not in isolation. [American Speech-Language-Hearing Association 1988][Fowler CG 2002]

What it measures

• Equivalent ear-canal volume (ECV) — an estimate of the air space between the probe tip and the drum.
• Static (peak) admittance — the maximum mobility of the middle-ear system once the canal contribution is removed.
• Tympanometric peak pressure (TPP) — the ear-canal pressure at which mobility is greatest, reflecting middle-ear pressure.
• Tympanometric width (gradient) — how sharp or broad the peak is.

Each of these is compared against age-appropriate normative data. Tympanometry is not reliable in infants younger than about seven months at low probe frequencies because of the highly compliant infant ear canal — a higher-frequency probe tone is used instead. [Onusko E 2004][Margolis RH 1987]

Anatomy & Physiology of the Middle Ear

The structures that make up the conductive pathway and how their mass and stiffness shape admittance.

The middle ear is a mechanical impedance-matching device. Its job is to transfer airborne sound efficiently into the fluid-filled cochlea, overcoming the impedance mismatch between air and perilymph. It does this through the area ratio of the tympanic membrane to the stapes footplate and the lever action of the ossicular chain.

The conductive pathway

• Tympanic membrane — couples ear-canal sound pressure to the malleus.
• Ossicular chain — malleus, incus and stapes, acting as a lever system.
• Stapes footplate and the oval window — the interface with cochlear fluid.
• Middle-ear air space and the Eustachian tube — keep the system aerated and at ambient pressure.
• Stapedius and tensor tympani muscles — modulate stiffness; the stapedius drives the acoustic reflex.

Mass, stiffness and admittance

The ease with which the system accepts sound energy — its admittance — depends on the balance of stiffness and mass. At the low probe-tone frequencies used in conventional tympanometry (commonly 226 Hz), the normal middle ear is stiffness-controlled. Anything that stiffens the system (fluid, fixation, scarring) reduces admittance; anything that adds mass or loosens the system raises it. [Hunter LL 2013]

This is why otosclerosis, which fixes the stapes, tends to produce a shallow trace, while ossicular discontinuity, which removes a mechanical link, produces a deep one. The tympanogram is, in effect, a picture of the system's mechanical state. [Hunter LL 2013][Jerger J 1970]

The Technology of the Tympanometer

Probe assembly, the pressure pump, the probe tone, and how admittance is actually recorded.

A tympanometer's probe seals the ear canal and contains three elements: a loudspeaker delivering a constant probe tone, a microphone monitoring the sound pressure level in the sealed canal, and a pump that varies the canal air pressure. [Onusko E 2004]

How admittance is inferred

The instrument holds the probe tone at a constant voltage. If the middle ear becomes less mobile, more sound energy stays in the canal and the microphone records a higher sound pressure level; if the ear becomes more mobile, more energy is absorbed and the recorded level falls. The instrument converts this into an admittance value, expressed in millimho or, equivalently, in cm³ of equivalent volume. [American Speech-Language-Hearing Association 1988]

Why the pressure sweep matters

Mobility is greatest when the pressure on both sides of the drum is equal. By sweeping ear-canal pressure from positive through to negative, the instrument finds the pressure at which the drum is most mobile — the peak — and that peak pressure estimates middle-ear pressure. At the extremes of the sweep the drum is stiffened by the pressure differential, and the residual admittance there represents the ear-canal volume alone. [Terkildsen K 1959][Fowler CG 2002]

Probe-tone frequency

Low-frequency probe tones (226 Hz) give single-peaked tympanograms in most ears and are the clinical standard for adults and older children. Higher probe frequencies produce more complex, sometimes notched shapes, and are used for infants and for specialised assessment of the middle ear's mass and resonance. [American Speech-Language-Hearing Association 1988][Hunter LL 2013]

Method & Technique

Obtaining a reliable tympanogram: seal, sweep, artefact, and common pitfalls.

A reliable tympanogram begins with a good hermetic seal. The probe tip must be the right size for the canal and seated so that no air leaks during the pressure sweep. Otoscopy first — to clear the canal of obstructing cerumen and to confirm the drum is visible — is part of good technique. [British Society of Audiology 2013]

The recording sweep

• Seat the probe and confirm a stable seal.
• The pump sweeps ear-canal pressure, conventionally from positive (around +200 daPa) toward negative.
• The instrument records admittance continuously and identifies the peak.
• ECV, static admittance, TPP and tympanometric width are reported.

Common artefacts and pitfalls

• Leak — a poor seal mimics a flat or noisy trace; re-seat the probe.
• Probe against the canal wall — blocks the opening and produces a flat trace with a very small ECV.
• Cerumen occlusion — also yields a flat trace; otoscopy distinguishes it.
• Patient movement, swallowing or vocalisation during the sweep introduces irregularities. [British Society of Audiology 2013]

Reading the ECV is the key to disambiguating flat traces: a flat trace with a very small ECV suggests an occluded probe or impacted wax, a normal ECV suggests middle-ear effusion, and a large ECV suggests a perforation or a patent ventilation tube. [Fowler CG 2002][Onusko E 2004]

Reading the Tympanogram

The Jerger classification, normative ranges, and a systematic reading method.

The typing of tympanometric patterns popularised by Lidén (1969) and Jerger (1970) remains in the most widespread clinical use. It is a quick visual shorthand — but it works best when backed by the actual numbers. [Lidén G 1969][Jerger J 1970][American Speech-Language-Hearing Association 1988]

The Jerger types

• Type A — peak near 0 daPa with normal amplitude: normal middle-ear pressure and mobility.
• Type As — peak at normal pressure but shallow: a stiff system, e.g. otosclerosis or tympanosclerosis.
• Type Ad — peak at normal pressure but abnormally deep: a hypermobile system, e.g. ossicular discontinuity or a flaccid drum.
• Type B — flat, no identifiable peak: e.g. effusion (normal ECV), perforation (large ECV), or occlusion (small ECV).
• Type C — peak shifted markedly negative: significant negative middle-ear pressure, e.g. Eustachian tube dysfunction. [Jerger J 1970]

Adult normative ranges

Commonly cited adult reference values are an equivalent ear-canal volume of roughly 0.6–2.0 cm³, a static admittance of roughly 0.3–1.6 mmho, a tympanometric peak pressure between about -100 and +50 daPa, and a tympanometric width up to roughly 110 daPa. Normative data are population- and age-dependent, and institutions may adopt their own; the values used here follow widely cited screening criteria. [Margolis RH 1987][Koebsell KA 1986][British Society of Audiology 2013]

A systematic reading method

• Check the ECV first — it tells you whether the system is sealed and intact.
• Find the peak pressure — is the middle ear aerated?
• Read the static admittance — is the system too stiff or too mobile?
• Read the width — a broad peak suggests early or partial loading.
• Only then assign a type, and always cross-check against audiometry, otoscopy and reflexes.

A normal-looking tympanogram does not guarantee a normal ear. Otosclerosis can produce a marked conductive loss with a near-normal trace; the absent acoustic reflex is often the more sensitive sign. Conversely, a flaccid or scarred drum can look strikingly abnormal while hearing is barely affected. [American Speech-Language-Hearing Association 1988][Jerger J 1970]

Acoustic Reflexes

The stapedial reflex, threshold testing, reflex decay, and site-of-lesion logic.

A sufficiently loud sound presented to either ear triggers a bilateral, involuntary contraction of the stapedius muscle. This stiffens the ossicular chain and changes middle-ear immittance — a change the probe microphone can detect. The acoustic reflex therefore extends immittance testing from the middle ear outward to the cochlea, auditory nerve and brainstem. [Hunter LL 2013][Borg E 1973]

The arc has an afferent and an efferent limb. The afferent limb runs from the cochlea along the auditory nerve to the ventral cochlear nucleus; from there, projections reach the superior olivary complex on both sides. The efferent limb runs from the facial nerve nucleus along the facial nerve to the stapedius muscle. The crossover at the superior olivary complex is what makes the reflex bilateral — a loud sound in one ear contracts both stapedius muscles. [Mukerji S 2010][Borg E 1973]

Acoustic reflex thresholds

The acoustic reflex threshold is the lowest stimulus level at which a reliable immittance change is seen, typically measured at 500, 1000, 2000 and 4000 Hz. In a normal ear the reflex appears at a high sensation level. Reflexes can be tested ipsilaterally and contralaterally, and the pattern across these conditions helps localise a lesion. [Hunter LL 2013][Katz J (Ed.) 2015]

What abnormal reflexes mean

• Absent reflexes with a conductive loss point to a middle-ear cause — effusion, fixation or discontinuity.
• Elevated reflexes can accompany cochlear hearing loss, though they often appear at a reduced sensation level.
• Elevated or absent reflexes with normal middle-ear function raise the question of a retrocochlear lesion. [Katz J (Ed.) 2015]

Reflex decay

Reflex decay testing presents a sustained tone (commonly at 500 and 1000 Hz) and watches whether the reflex contraction is maintained. A normal (negative) result holds the contraction; a positive result is a fall in reflex magnitude of 50% or more within the first five seconds. Positive reflex decay is highly suggestive of retrocochlear pathology and warrants onward referral. [Hunter LL 2013][Katz J (Ed.) 2015]

In advanced otosclerosis the reflex may show a characteristic biphasic on–off pattern before disappearing as fixation progresses — a reminder that reflex morphology, not just presence or absence, carries information. [Hunter LL 2013]

Wideband & Multi-Frequency Tympanometry

Beyond the 226 Hz probe tone: multi-frequency tympanometry, wideband absorbance, and the resonance frequency.

Conventional tympanometry uses a single low-frequency probe tone — usually 226 Hz — and works well for the everyday question of whether the middle ear is sealed, aerated and mobile. But a single low frequency only sees one slice of a system whose behaviour is strongly frequency-dependent. Two ears with identical 226 Hz tympanograms can transmit sound very differently across the rest of the spectrum. [Hunter LL 2013]

From one probe tone to many

Multi-frequency tympanometry extends the measurement by sweeping pressure at several probe frequencies — 226, 678, 800 and 1000 Hz are commonly reported together. Higher probe frequencies are more sensitive to changes in the mass and stiffness of the ossicular system, which is why a 1000 Hz probe tone is the standard for infants, whose highly compliant ear canals make a 226 Hz trace unreliable. [Hunter LL 2013][Hunter LL 2013]

The 1000 Hz trace does not look like the familiar 226 Hz peak. In many ears it is more complex and can show a double peak, so its interpretation is less a matter of reading a single peak height and more a matter of recognising a shape. [Hunter LL 2013]

Wideband acoustic immittance

Wideband acoustic immittance — also called wideband absorbance or wideband tympanometry — goes further again. Instead of pure tones it uses a brief click, which contains energy across a broad band, commonly 226 Hz to 8 kHz. The instrument reports how much of that energy the middle ear absorbs at each frequency. Absorbance is a fraction between 0 (all energy reflected) and 1 (all energy absorbed); the reflected fraction is the mirror-image reflectance. [Keefe DH 1993][Hunter LL 2013]

Because a wideband measurement can be made at ambient pressure and at the tympanometric peak pressure, and across the whole frequency range at once, it builds a far richer picture of middle-ear function in the same time a conventional tympanogram takes. [Hunter LL 2013]

The normal absorbance pattern

In a normal adult ear, absorbance is low at low frequencies — the stiffness-controlled region reflects most of the energy — then rises to a broad peak through the mid frequencies before falling away again at the high end. The frequency at which the stiffness and mass elements of the middle ear are balanced is the resonance frequency, and it sits near the absorbance peak — around 900 Hz in normal adult ears. [Keefe DH 1993][Karuppannan A 2021]

Why it matters: stiffness versus mass

The resonance frequency moves in a clinically meaningful direction. Anything that stiffens the system shifts resonance higher: group studies of otosclerosis report resonance frequencies well above the normal range, around 1400 Hz, with reduced low-frequency absorbance. Anything that adds mass or loosens the system shifts resonance lower: ossicular discontinuity has been reported with resonance frequencies around 670–750 Hz and raised low-frequency absorbance. [Karuppannan A 2021]

This is the real payoff of the wideband approach. Otosclerosis and ossicular discontinuity can both present with a conductive loss, and on a 226 Hz tympanogram the distinction rests largely on peak height. The wideband absorbance pattern and the resonance frequency separate them more directly — one is a stiffness problem, the other a mass problem, and the curve shows which. [Karuppannan A 2021][Hunter LL 2013]

Middle-ear effusion, by contrast, damps the system broadly: wideband absorbance is reduced across a wide span of frequencies, and studies using higher probe frequencies and wideband measures report improved sensitivity for detecting effusion compared with the 226 Hz tympanogram alone. [Hunter LL 2013][Onusko E 2004]

Wideband immittance is still maturing as a routine clinical tool, and interpretation is less standardised than the Jerger types. But as an addition to the battery — quick, objective, and richer than a single probe tone — it is increasingly part of how the middle ear is assessed. Use the explorer below to see how the absorbance curve and the resonance frequency move with stiffness and mass. [Hunter LL 2013]

Disease Signatures — Summary