The stapedial reflex is the involuntary and bilateral contraction of the stapedius muscle, due to excessive stimulation of the inner ear. This muscle’s function consists in protecting the auditory system from overly intense acoustic stimuli; it also helps discriminate sounds.
Impedenzometry consists in inserting a probe inside the acoustic meatus (a canal in the ear). The instrument generates a pure tone (usually, approximately 226 Hz) and it measures the acoustic reflex from the tympanic membrane sound, as the operator changes the air pressure in the external ear canal. The instrument emits an acoustic pressure of varying degree, positive and negative, from a max of +200 decapascal (daPa) to a min of -400 daPa, capable of setting in motion the tympanic membrane and the auditory ossicles connected to it; at the same time, a microphone captures and transduces the sound in continuous current. The instrument registers the resistance to the air flow in the middle ear, at different pressure points.