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Why can fluid in the middle ear change tympanometry results?

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In tympanometry, fluid in the middle ear alters results primarily by affecting the mobility of the tympanic membrane.

Normally, the middle-ear cavity is filled with air, allowing the eardrum to vibrate freely. When fluid accumulates—a condition often associated with secretory otitis media—it replaces the air and physically impedes the vibratory movements of both the tympanic membrane and the ossicular chain.

Tympanometry functions on the principle that sound entering the ear canal is reflected back by the eardrum; this reflection is measured by a sensitive microphone. When the presence of fluid causes the eardrum to become stiff, the amount of sound reflected back increases. Because tympanometry measures these changes in sound reflection as air pressure is varied within the ear canal, the increased stiffness caused by fluid results in distinct, abnormal readings compared to a healthy, air-filled middle ear.1