Reasons, Procedure and Evaluating Results For Impedance Audiometry

Submitted on March 27, 2012

Impedance audiometry is performed by physicians to determine the functioning of the middle ear. Bear in mind, this test does not gauge a child's hearing, but instead helps detect pressure changes in the middle ear. This test can be difficult to perform in toddlers, as it requires that the child remains very still without crying, talking or fidgeting.

Reasons Why It is Condcuted

Impedance audiometry will also verify the state of the tympanic membrane. The other purpose of the test is to assess acoustic reflex pathways including the auditory brainstem and the cranial nerves. This test, however, does not evaluate auditory sensitivity directly.

Acoustic impedance is considered to be a response to acoustic immittance. Acoustic immittance refers to the measurement of air pressure or energy flow in the eardrum, tensor tympani, ossicular chain, ear canal, cochlea, and brainstem. The mass, resistance, and mobility of the middle as well as the outer ear systems influence the tympanometry test.

There are several acoustic impedance tests including tympanometry, perilymphatic fistula, thresholds of acoustic reflex, and Eustachian tube function. These tests help in determining the functioning of the central auditory systems, and can determine conditions like Eustachian tube dysfunction, tympanic membrane hypermobility, tympanosclerosis, middle ear effusion, Otosclerosis, glue ear, brainstem disorders, or even hearing loss.

Tympanometry tests measure sounds reflected from the tympanic membrane even as the physician adjusts the pressure of air in the ear canal. In particular, Tympanometry evaluates the middle and outer ear systems, as well as the Eustachian tube.


A probe with a flexible and soft tip is inserted into the ear canal to obtain an airtight seal. The probe tone in the ear canal is usually 226 Hz and the external ear canal air pressure is varied from +200 to -400 decapascal (daPa). From the middle ear, the acoustic transmission is most when the air pressure in the ear canal and middle ear is equal. This is also referred to as a compliance peak.  Mobility is indicated by the height of the compliance peak; low compliance indicates stiffness of the middle ear and tympanic membrane.

Evaluating Results for Impedance Audiometry

The Jerger system classifies tympanograms. In this system the response recorded can be classified into three types: type A, type B and type C.

A type A response implies normal functioning of the middle ear, although this result could also occur in the early stages of Otosclerosis ears. Compliance peak in type A recordings is -150 to +100 daPa while immittance is 0.2-2.5 milliohms. A type B response indicates a flat trace, i.e. no immittance or compliance peak was observed. A type B response where the ear canal volume is normal implies otitis media. Type B where the ear canal volume is small could be the result of wax or debris occluding the ear canal, or the probe getting squished on the ear canal side, and a Type B response where the ear canal volume is large can be the result of a hole in the tympani membrane.

A type C response is indicative of pressure in the middle ear being negative; this could be caused by otitis media, either in the resolving or developing stage. A type C response could also be the result of Eustachian tube dysfunction.

For infants younger than 7 months, a low frequency tympanometry is best avoided as their ear canal cartilage is too soft and can lead to incorrect tympanometric results. In fact, research is still in progress to develop an accurate immittance audiometry for infants.