Acoustic reflex

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


The acoustic reflex (or stapedius reflex) is an involuntary muscle contraction that occurs in the middle ear of mammals in response to high-intensity sound stimuli.

When presented with a high-intensity sound stimulus, the stapedius and tensor tympani muscles of the ossicles contract. The stapedius pulls the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle pulls the malleus (hammer) away from ear drum. The reflex decreases the transmission of vibrational energy to the cochlea, where it is converted into electrical impulses to be processed by the brain. The acoustic reflex normally occurs only at relatively high intensities; activation for quieter sounds can indicate ear dysfunction and absence of acoustic reflex can indicate neural hearing loss.

Vocalization-Induced Stapedius Reflex

More important than the passive reflex discussed above is an active, though automatic, stapedius reflex that is invoked when a person vocalizes. Just prior to vocalization (when there is no "turning back") the stapedius reflex is invoked. In humans the vocalization-induced stapedius reflex reduces sound pressure levels reaching the inner ear hair cells by approximately 20 decibels. The stapedius reflex may be termed an acousto-mechanical increase in impedance.

Continuing the electronics analogy (impedance is "resistance" for AC signals), understanding the basic block diagram is important. Essentially everything is "in series". The outer ear --» the eardrum --» the three little bones --» the fluid-filled cochlea. Inside the cochlea a pressure wave occurs in the fluid. Where this pressure wave has maxima, it pushes the long basilar membrane against hair cells' hairs. Once the hair cells bend signals are sent to the brain. (This is simplified; there are inner and outer hair cells, for example).

If any effect decreases transduction from one element to the next (in the items of the above paragraph) there will ultimately be less bending of the delicate hair cells of the inner ear. Ear wax decreases airborne sound transduction to eardrum vibrations, for example. Ear wax does not prevent the bone conduction pathway by which waves impinging on the head and torso can vibrate the three little bones directly (without going to eardrum vibrations first). A full-face motorcycle helmet decreases the bone-conduction pathway from sound waves that would otherwise have hit the head bones -- as well as, of course, reducing the air-conduction transduction to the eardrum. The stapedius reflex that is invoked upon vocalization works deeper in the series of transducers than either earwax or the morotorcycle helmet. More importantly though, it is an active effect. A muscle is altered in anticipation of the onset of you vocalizing.

While the vocalization-induced stapedius reflex in humans results in about a 20 dB reduction in transduction to the inner ear, birds have a stronger stapedius reflex that is invoked just before the bird tweets. Reference: Scientific American August 1989 pp 74-80

While the human does not have as large a vocalization-induced stapedius reflex as had by the tweeting bird, there are several practical applications to be aware of. The long-known and rude thing to do of humming when you don't want to hear someone really works -- and the reason is because of the stapedius reflex. While rude, the behavior provides the opportunity to make two points: first, the effect is active, not passive; second, and a more subtle point, is that the reflex is not a psychological effect or perceptual masking effect or a question of poorer signal-to-noise ratio when "noise" is boosted. The effect is not that of a sound seeming less loud; rather, the effect is that less of the sound waves is transducted into the inner ear -- and thus less bending of the hairs of the hair cells.

The vocalization-induced stapedius reflex can indeed be used for hearing protection purposes. Just before an impulse noise (door slam, electromagnet lock slapback, gun shot, pound of hammer on nail) one could vocalize (or cough or hum) and one would literally be protecting one's hearing from the sound pressure that the impending sound would create. Again, the reflex is not a perceptual or seeming reduction in sound; the reflex is an actual reduction in sound level reaching the inner ear -- an actual reduction in how far one's delicate hair cells will be bent by that sound. An identical hammer blow when one engaged in no vocalization is more damaging to one's hearing than that same hammer blow if one began vocalizing just a few tens of milliseconds prior to the blow.

While the stapedius reflex protects you from the sound of your own vocalizing and can be purposefully exploited for hearing protection of impulsive sounds (do wear earplugs and earmuffs for gunfire though; the 20 dB of the stapedius reflex is not adequate protection), there are occasions in which the stapedius reflex can be inconvenient. All these cases revolve around a conversation with another person in which inadequate time is allowed between one person vocalizing and the next person beginning his/her vocalization. For example, in a telephone mail-order situation in which there is the stating of one's credit card number in sets of four numbers at a time, if the order-taker starts echoing them back to you in sets of four and you begin vocalizing the next set of four numbers before the order-taker's stapedius reflex has subsided, it is likely that the order-taker will mis-hear the first one or two syllables you spoke. Similar situations can occur on "special instructions" at a take-out drive-thru. There is the tendency to speak as soon as the other has stopped, but without allowing any time for the other speaker's stapedius reflex to subside.

Finally, the stapedius reflex is not very effective for very low frequency sounds and it will provide little protection for the "bass notes" of a big impending boom. For that type of dangerous sound you need intra-aural (earplug) hearing protection and perhaps even a full-face motorcycle helmut to eliminate at least some of the bone-conduction vibration. 16:59, 11 September 2007 (UTC)

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