Impedancemetry can only be done if the eardrum is not perforated. It is based on two tests:

• tympanometry: measures the mobility of the tympanic membrane by applying different sound pressures
• the study of the stapedial reflex (or acoustic reflex), which is an involuntary contraction of the two muscles of the middle ear

Tympanometry

The aim is to study the flexibility of the eardrum and the ossicles when they are subjected to variations in pressure.

This involves placing a small probe with a microphone in the external auditory canal. The probe is connected to a generator device which allows:

• to vary the pressure in the external auditory canal

• make a continuous sound in the ear

• and to record the sound partially reflected by the eardrum. The closer the eardrum is to its normal position, the more sound is reflected.

The examination takes place in a soundproof room. It is painless and can be done in children as well as in adults.

Study of the stapedial reflex

The examination consists of stimulating the ear with an intense sound and seeing how the hearing system will protect itself to prevent this sound from reaching the inner ear too loudly.

For this, a probe is placed at the level of the ear studied while the other ear is stimulated by a sound. The probe measures the triggering threshold of the stapedial reflex, i.e. the minimum intensity at which the stapes muscle contracts under the action of sound. When the muscle contracts, the stirrup tilts backwards and this increases the rigidity of the chain of the ossicles. In other words, the vibration of the chain of the ossicles becomes more difficult, the stirrup does not press dangerously on the cochlea and this prevents irreversible damage. It is a protective reflex of the inner ear against too intense sounds. This reflex loop also involves the auditory nerve and the facial nerve. 

In a person with normal hearing, this threshold is 80-85 decibels.

Note that the stapes reflex appears in both ears, regardless of which ear is stimulated.