Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 170

Some are born with the severe handicap of deafness. Others suffer sudden hearing loss later in life or (much more commonly) gradually lose their hearing over a period of time. For all these persons, deafness is a crippling loss of one of the most important senses. Deafness can be very slight or complete. There are different legal and medical definitions of deafness. For our purpose we will define the onset of impaired hearing when a person's hearing level is 25 dB, averaged at 500, 1000, and 2000 Hz. We will define total deafness when a person's hearing level is 92 dB averaged at the same important speech frequencies [47] (see Section 5.7). Someone who is hard‐of‐hearing, then, may have a hearing level anywhere between these two extremes of 25 and 92 dB. The average hearing level of a nonimpaired person is 0 dB.

Deafness not only causes a lowering in the level at which sound is heard, but in many cases also causes a loss in hearing quality so that consonants cannot be distinguished and words are confused. In addition, some deaf people also suffer from tinnitus or a ringing in the ears which is disturbing and competes with other sounds.

Because of their affliction, deaf people tend to withdraw from society. This is sometimes reinforced by society itself. In ancient Rome, the deaf were classed with the mentally incompetent. In modern societies, education, advances in medicine, and use of hearing aids are now improving the plight of the deaf. Strict control of noise levels in industry will also reduce the number of people receiving needless hearing damage and consequent deafness. A review on noise‐induced hearing loss is available in the literature [48].

Hearing disorders (deafness) can be divided into two main types: conduction deafness and sensory‐neural deafness. Conduction deafness is related to disorders in the outer and middle ear, while sensory‐neural deafness occurs in the inner ear in the organ of Corti, the auditory nerve, or the auditory cortex in the brain.