How does tympanometry detect fluid behind the eardrum?

Tympanometry is a diagnostic procedure used to assess the condition of the tympanic membrane and the middle ear, particularly when detecting fluid accumulation associated with conditions like otitis media.21

The procedure functions based on the principle that sound entering the ear canal is reflected back by the eardrum. During the test, a handheld instrument produces a pure tone while simultaneously altering the air pressure within the ear canal.2

When the middle ear is healthy and air-filled, the eardrum vibrates normally. However, when fluid accumulates behind the eardrum, it impedes the membrane's movement, causing it to become stiff. This stiffness results in an increased reflection of the sound waves back into the ear canal, which is then measured by a sensitive microphone. By analyzing these changes in sound reflection as air pressure varies, clinicians can obtain evidence of fluid presence in the middle ear space.231