A small earpiece seals the ear canal. A pump changes the pressure, and the instrument checks the sound reflected from the eardrum. The resulting line may rise into a rounded hill or remain stubbornly flat. In a child's hearing assessment, that flat trace often raises the possibility of glue ear: fluid behind the eardrum that interferes with movement. But the line alone cannot finish the explanation.[1][2]

This is tympanometry, a test of middle-ear mechanics. Understanding it begins with the difference between asking whether sound can move a system and asking what a person can hear. The National Deaf Children's Society makes that distinction explicit in its guide for families: tympanometry belongs within a hearing assessment, but does not itself measure hearing.[1]

What the pressure sweep changes

Inside the probe are the openings for a sound source, a microphone, and an air-pressure system. The tip must seal the canal so the pump can control pressure in the small space outside the eardrum. As pressure changes, the instrument measures acoustic admittance: how readily the system accepts acoustic energy. A conventional adult test uses a 226 Hz tone.[4]

The pressure sweep is what makes the measurement revealing. With an intact eardrum, a pressure difference across it loads the membrane and makes the system harder to move. Bringing canal pressure close to middle-ear pressure usually produces the greatest admittance at this low frequency. That gives the familiar curve its peak. The machine varies pressure outside the drum to estimate conditions behind it; it does not put a pressure sensor into the middle ear.[2]

The horizontal position of the peak and its height therefore answer different questions. One concerns the pressure at which movement is easiest; the other concerns the acoustic response at that point. A peak shifted toward negative pressure can accompany Eustachian-tube dysfunction, while a flat trace has no clear peak to interpret. These patterns help direct an assessment, but do not identify a unique cause.[4]

A flat line needs a volume reading

Beside the curve is an easily overlooked estimate: equivalent ear-canal volume. With an intact eardrum, it principally represents the enclosed canal space. If the eardrum has an open perforation, the measurement also includes the cavity beyond it. The apparent volume becomes larger.[5]

That is why similar flat traces can point in different directions. An unusually small volume can mean the probe is blocked against the canal wall. An unusually large one can indicate a perforation. The clinician checks the ear, the probe, and the result together, repeating a doubtful measurement.[5]

A flat trace with an ordinary volume supports a different possibility: middle-ear fluid limiting the system's response.[4] The extra number changes the interpretation because it helps establish which space the instrument has actually measured. Calling every flat line “fluid” would turn a useful clue into a premature diagnosis.

A baby's ear changes the experiment

The probe tone matters because the ear's response depends on frequency. Young infants have more yielding ear-canal walls; a low-frequency measurement can be misleading.[4] ASHA's pediatric assessment guidance warns that the conventional low-frequency tone can produce a falsely reassuring result when an infant has middle-ear fluid. It recommends 1000 Hz for infants younger than nine months.[6]

The British Society of Audiology's procedure, amended in March 2026, specifies 1000 Hz below six months corrected age, with 226 Hz at or above that boundary. It also requires professional judgment about the individual examination.[5] The age cutoffs differ between these documents. Their shared point is that interpreting an infant's trace requires an age-appropriate method; an adult-looking curve is insufficient reassurance.[5][6]

This is a useful example of why an objective test still requires choices. The instrument can produce a measurement without a child reporting that they heard anything. Yet the clinician must select a suitable stimulus, check the ear, and interpret the response within the child's wider assessment.[6]

Put the curve back beside the hearing test

Tympanometry was already routine by the early 1970s. ASHA's 1988 technical tutorial emphasized a limitation that remains central: measuring energy entering the middle-ear system does not establish how much reaches the cochlea. Significant hearing loss can coexist with a relatively normal tympanogram.[2]

ASHA's pediatric guidance consequently places tympanometry among several complementary tests. Behavioral measures and auditory evoked responses answer questions that middle-ear measurements cannot. Even the order and selection of those tests depend on the child's age, history, and development.[6]

The practical achievement of the pressure sweep is to make one part of a hearing problem more legible. Read alongside the volume estimate and the rest of the assessment, its curve can help explain where sound encounters difficulty. A reassuring curve closes only the question it was equipped to ask.

Sources

1. National Deaf Children's Society, Understanding your child's hearing tests, “Tympanometry,” printed page 16; patient guide hosted by Salisbury NHS Foundation Trust.
2. ASHA Working Group on Aural Acoustic-Immittance Measurements, “Tympanometry” (1988); measurement principles, clinical history, and limits of interpretation.
3. Max MBAKOP, “Hopital Laquintini-4993 32.jpg,” 27 April 2021; photograph of a tympanometry examination, Wikimedia Commons, CC BY-SA 4.0.
4. Interacoustics Academy, “Tympanometry: An Introduction,” 20 June 2024; manufacturer's explanation of equipment, probe frequencies, and interpretation.
5. British Society of Audiology, Recommended Procedure: Tympanometry and Acoustic Reflex Thresholds (2024, minor amendments February 2025 and March 2026); infant protocols, probe checks, and ear-canal volume.
6. American Speech-Language-Hearing Association, “Hearing Loss in Children,” Practice Portal; acoustic immittance testing and the pediatric assessment battery.