The acoustic reflex refers to the contraction of a middle ear muscle in response to sound. The contraction causes a stiffening of the middle ear system and, consequently, the flow of acoustic energy to the cochlea is impeded. By measuring the change in admittance in the auditory system during sound stimulation it is possible to indirectly monitor the middle ear muscle contractions. Such measurements provide useful information regarding the integrity of the auditory system and the location of the auditory pathology.

A standard clinical immittance test battery includes measurements of the acoustic reflex, or more specifically, the middle ear muscle reflex (MEMR). The MEMR is the contraction of the stapedius muscle in response to high-level acoustic stimulation. Clinical MEMR measurements are made according to the American National Standards Institute standard for immittance instruments (S3.39–2012) by use of a 226-Hz probe tone in conjunction with a reflex-activating stimulus presented to the ipsilateral or contralateral ear. The MEMR is a bilateral response, which means that presenting the activator to one ear will elicit the response in both ears. In traditional clinical measurements, the reflex activator stimulus is a pure tone (500, 1000, 2000, or 4000 Hz) or a broadband noise, and the MEMR is measured at tympanometric peak pressure (TPP) as measured on the tympanogram. If the admittance of the test ear decreases by a criterion amount in the presence of the activator, for example, 0.02 to 0.03 mmho, the reflex is considered to be present. The lowest level at which an activator is presented and reliably elicits at least the criterion change in admittance is considered the MEMR threshold. The MEMR is typically present in ears with pure-tone behavioral thresholds of ≤60 dB HL at the activator frequencies (Gelfand 2009).