The acoustic reflex (also known as the stapedius reflex, stapedial reflex, auditory reflex, middle-ear-muscle reflex (MEM reflex, MEMR), attenuation reflex, cochleostapedial reflex or intra-aural reflex) is an involuntary muscle contraction that occurs in the middle ear in response to loud sound stimuli or when the person starts to vocalize. When presented with an intense sound stimulus, the stapedius and tensor tympani muscles of the ossicles contract. The stapedius stiffens the ossicular chain by pulling the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the tympanic membrane when it pulls the malleus (hammer) in toward the middle ear. The reflex decreases the transmission of vibrational energy to the cochlea, where it is converted into electrical impulses to be processed by the brain. Acoustic reflex threshold The acoustic reflex threshold (ART) is the sound pressure level (SPL) from which a sound stimulus with a given frequency will trigger the acoustic reflex. The ART is a function of sound pressure level and frequency. People with normal hearing have an acoustic reflex threshold (ART) around 70–100 dB SPL. People with conductive hearing loss (-i.e. bad transmission in the middle ear) may have a greater or absent acoustic reflex threshold. The acoustic reflex threshold is usually 10–20 dB below the discomfort threshold. However the discomfort threshold is not a relevant indicator of the harmfulness of a sound: industry workers tend to have a higher discomfort threshold, but the sound is just as harmful to their ears. The acoustic reflex threshold can be decreased by the simultaneous presentation of a second tone (facilitator). The facilitator tone can be presented to either ear. This facilitation effect tends to be greater when the facilitator tone has a frequency lower than the frequency of the elicitor (i.e. the sound used to trigger the acoustic reflex). Characteristics and effects For most animals, the acoustic reflex is the contraction of both middle ear muscles: the stapedius and tensor tympani muscles. However in humans, the acoustic reflex only involves the contraction of the stapedius muscle, not the tensor tympani. The contraction of the stapedius muscle occurs bilaterally in normal ears, no matter which ear was exposed to the loud sound stimulation. The prevalence of bilateral acoustic reflexes in persons 18–30 years old is 85.3% (82.9%, 87.4%) 95th percentile confidence interval N = 3280 and in