An acoustic reflex is a type of muscle contraction that takes place in the ear. More specifically, this uncontrollable muscle movement occurs in the middle ear when the area is stimulated by a high intensity sound. It may also be known as the attenuation reflex, the auditory reflex, or the stapedius reflex.
Several muscles and bones in the middle ear facilitate acoustic reflexes. When a high-pitch sound reaches the middle ear, two muscles — the stapedius and the tensor timpani — located in a bony portion of the ear called the ossicles contract. As these muscles move, they each pull other muscles away from the thin tissue of the eardrum. The stapedius pulls the stapes of the middle ear while the tensor tympani pulls a hammer-shaped part of the middle ear called the malleus.
The acoustic reflex's function is to temper the force of sound vibrations that reach the cochlea through the eardrum. The cochlea is vital because it converts vibrations into electrical impulses that are then sent to the brain, thus creating hearing. This part of the ear is fluid-filled and contains tiny hairs that move in response to vibrations, and these hairs can be easily damaged by forceful pressure. The acoustic reflex protects from this kind of damage.
A type of acoustic reflex may also occur after an individual speaks. The act of talking can raise sound pressure in the cochlea, so the reflex may prove necessary. Like its peers, this reaction reduces sound measurement levels known as decibels. On a spectrum of sound-intensity measurement like the equal-loudness contours, acoustic reflexes are triggered at around 70 to 90 decibels.
Although the acoustic reflex usually operates in response to sounds of high intensity, it occasionally may be triggered by less intense sounds. If this response occurs frequently, however, it may signal an ear problem like hyperacusis, which is extreme sensitivity to certain sounds. Likewise, the failure of any sounds to summon an acoustic reflex can hint at a problem as well. Another condition that has a relationship to the acoustic reflex is facial nerve damage. Since the ear’s stapedius muscle and the facial nerve are connected, a dysfunction in one area will often affect the other area.
Due to its role in detecting certain ailments, many physicians will test the acoustic reflex. Professionals specializing in audiometry, or hearing ability testing, are particularly interested in auditory reflexes. A device called a tympanometer can measure the effects of various levels of sound on the inner ear. Abnormalities may indicate hearing loss or even a nervous system deficit. These tests may produce inconsistent results, however.