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.
The stapedius reflex is also invoked when a person vocalizes. In humans, the vocalization-induced stapedius reflex reduces sound pressure levels reaching the inner ear hair cells by approximately 20 decibels. The stapedius reflex causes 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 in the ear is connected "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 the hair cells' hairs. Once the hair cells bend, electrical 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 motorcycle helmet. More importantly though, it is an active effect. A muscle is altered in anticipation of the onset of the 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.
Humming when you don't want to hear someone else 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 — thus causing 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) to protect one's hearing from the sound pressure that the impending sound would create. The reflex is not a perceptual reduction in sound; the reflex is a real 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.
The stapedius reflex can also be inconvenient in conversations where inadequate time is allowed between one person vocalizing and the next person beginning his/her vocalization. For example, in a telephone conversation in which one's credit card number is read out in sets of four numbers at a time, if the other person starts echoing them back to you in sets of four and you begin vocalizing the next set of four numbers before the other person's stapedius reflex has subsided, it is likely that the other person will mis-hear the first one or two syllables you spoke.
Finally, the stapedius reflex is not very effective for very low frequency sounds because they are mainly transmitted by bone-conduction to the ear. For such sounds, intra-aural hearing protection devices (earplugs), or even full-face helmets, can provide attenuation.
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