Acoustic Reflex - Vocalization-induced Stapedius Reflex

Vocalization-induced Stapedius Reflex

The stapedius reflex is also invoked when a person vocalizes. In humans, the vocalization-induced stapedius reflex reduces sound intensities reaching the inner ear by approximately 20 decibels. The stapedius reflex causes an acousto-mechanical increase in impedance.

Understanding the basic signal flow through the ear is important. Essentially everything in the ear is connected "in series": The outer ear → the eardrum → the ossicles → the fluid-filled cochlea. Inside the cochlea, a traveling wave of displacement occurs in the fluid, and deflects the inner hair cells' hairs, which causes the primary auditory neurons to send impulses to the brain. If any effect decreases transduction from one element to the next (in the items of the above paragraph), there will ultimately be less signal sent to the brain. The stapedius reflex that is invoked upon vocalization works in the ossicles of the middle ear, and is an active effect. A muscle is tightened in anticipation of the onset of 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 works through the stapedius reflex. The effect is active; the reflex is not a purely psychological effect or perceptual masking effect or a question of poorer signal-to-noise ratio when "noise" is boosted. Rather, the effect is that less of the sound wave is coupled into the inner ear, making it less loud.

The vocalization-induced stapedius reflex can indeed be used for hearing protection purposes. Just before an impulse noise (door slam, electromagnet lock slapback, gunshot, 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.