A hearing impairment is a decrease in one's ability to hear (i.e. perceive auditory information). While some cases of hearing loss are reversible with medical treatment, many are permanent. Whether temporary or permanent, how severely hearing is compromised not a uniform. In some cases the hearing loss is only mild and or moderate and individuals may not even be aware they are affected. In other cases the degree of deafness is profound and, although vibrations will be sensed.

Hearing loss can also differ in what pitches (frequencies) are affected. Human beings generally are sensitive to even quiet sounds, as long as the sounds are at certain pitches: ranging from about 125 Hz to 8000 Hz. The " human ear " is best tuned to pick up sounds at the same pitches (frequencies) as speech, from about 500 to 4000 Hz. Some people do not hear well throughout this range, but are " hard of hearing " depending on the pitch of the sound (low frequency hearing loss, high frequency hearing loss, mid-frequency or U-shaped hearing loss). Hearing impairment comes from different causes. Most commonly, the ear is affected. Conductive hearing losses involve clogging or abnormalities of the outer or middle ear, and only produce mild or moderate impairment, at worst. Hearing loss due to insensitivity of the inner ear, the cochlea, can also be only mild or moderate but can also be much more severe, even causing complete insensitivity to even the loudest sounds (total deafness). Very unusual hearing impairments involve the auditory portions of the brain.

(by sicologic.com)