Hearing impairment is one of the most prevalent chronic disabilities in the U.S. Approximately 22 million people in the US (8.2%) have hearing impairments. Of those, 2.4 million have severe to profound impairments.
Hearing impairment means any degree and type of auditory disorder, while deafness means an extreme inability to discriminate conversational speech through the ear. Deaf people, then, are those who cannot use their hearing for communication. People with a lesser degree of hearing impairment are called hard of hearing. Usually, a person is considered deaf when sound must reach at least 90 decibels (5 to 10 times louder than normal speech) to be heard, and even amplified speech cannot be understood.
Hearing impairments can be found in all age groups, but loss of hearing acuity is part of the natural aging process. Nearly 23% of those aged 65 to 74 have hearing impairments, while almost 40% over age 75 have hearing impairments. The number of individuals with hearing impairments will increase with the increasing age of the population and the increase in the severity of noise exposure.
Hearing impairment may be sensorineural or conductive. Sensorineural hearing loss involves damage to the auditory pathways within the central nervous system, beginning with the cochlea and auditory nerve, and including the brain stem and cerebral cortex (this prevents or disrupts interpretation of the auditory signal). Conductive hearing loss is damage to the outer or middle ear, which interferes with sound waves reaching the cochlea. Causes include heredity, infections, tumors, accidents and aging (presbycusis, or "old hearing").
The primary difficulty for individuals with hearing impairment is receiving auditory information. This problem can be compensated for by presenting auditory information redundantly in visual and/or tactile form. If this is not feasible, an alternative solution to this problem would be to provide a mechanism, such as a jack, which would allow the user to connect alternative output devices. Increasing the volume range and lowering the frequency of products with high pitched auditory output would be helpful to some less severely impaired individuals. (Progressive hearing loss usually occurs in higher frequencies first.)
Although not prevalent yet, there is much talk of using voice input on commercial products in the future. This, too, will present a problem for many deaf individuals. While many have some residual speech, which they work to maintain, those who are deaf from birth or a very early age often are also non-speaking or have speech that cannot be recognized using current voice input technology. Thus, alternatives to voice input will be necessary to these individuals to access products with voice input.
Familiar coping strategies for hearing impaired people include the use of hearing aids, sign language, lip-reading and TDD's (telecommunication devices for the deaf). Some hearing aids are equipped with a "T-coil" as well, which provides direct inductive coupling with a second coil (such as in a telephone receiver) in order to reduce ambient noise. Some other commercial products could make use of this capability.
ASL (American Sign Language) is commonly used by people who are deaf. It should be noted, however, that this is a completely different language from English. Thus, deaf people who primarily use ASL may understand English only as a second language, and may therefore not be as proficient with English as native speakers.
Finally, telecommunication devices for the deaf (TDD's) are becoming more common in households and businesses as a means for deaf and hard of hearing people to communicate over the phone. TDD's have always used the Baudot code, but newer ones receive both Baudot and ASCII.