Also known as the stapedius reflex, middle-ear-muscles (MEM) test, the Acoustic Reflex Test measures information about the possible location of a hearing loss problem. Everyone has an acoustic reflex to sound, it is an involuntary muscle contraction that occurs in the middle ear in response to high-intensity sound stimuli or when the person starts to vocalize. A tiny muscle in the middle ear contracts when a loud sound occurs. The loudness level at which the acoustic reflex occurs, or the absence of the acoustic reflex gives information to the audiologist about the type of hearing loss, it is called the Acoustic reflex threshold (ART), which is the sound pressure level (SPL) from which a sound stimuli with a given frequency will trigger the acoustic reflex.
Normally, the acoustic reflex occurs at relatively high intensities; contraction of middle ear muscles for quieter sounds can indicate ear dysfunction. Physically, the pathway involved in the acoustic reflex is very complex and involve the Ossicular chain, which includes the small bones of the middle ear – malleus, incus and stapes. The reflex also includes the cochlea, auditory nerves, brain stem and facial nerves. This limited protective reflex guards against excessive stimulation, especially that of the lower frequencies and has been demonstrated both in man and animals.
Individuals with normal hearing have an acoustic reflex threshold (ART) around 70-100 dB SPL. People with CONDUCTIVE HEARING LOSS or bad transmission in the middle ear, have a higher acoustic reflex threshold. During the test, there will be a contraction of the stapedius muscle bilaterally in normal ears, no matter which ear was exposed to the loud sound stimulation.