Hearing Aid

Hearing Aid

A device used by a person with defective hearing to improve his ability to hear. The sense of hearing depends on the ability to detect the pressure of sound waves in the air surrounding the listener. The hearing aid helps the listener by acting as an amplifier; it increases the pressure of sound waves and delivers the amplified sounds to the ear at a point within the ear canal.

Impairment of hearing is a serious problem because the spoken word is the most important means of communication. By wearing a hearing aid, an individual with a hearing handicap can mantain a full life. An estimated 4% of the population can benefit by using the proper hearing aid.

Normal loudness sensation, at a given pressure of the sound waves, is different at different frequencies. Low tones of about 100 vibrations per second and high tones of about 10,000 vibrations per second produce less loudness sensation then tones of about 1,000 vibrations per second. Hearing is deficient when the loudness sensation produced in a particular ear by a given sound pressure is less than normal at any specified frequency. Usually, the loss of loudness is greater for high-frequency than for low-frequency tones. Maximum enchancement of hearing is obtained when the proper amount of amplification is provided at those frequencies at which an individual's loudness sensation is deficient.

Cupped Hand. The cupped hand over the ear-the earliest type of hearing aid-increases the the loudness sensation by increasing the sound pressure received by the ear. Changing the shape and position of the cupped hand enables a person to receive sound pressure increases in the frequency region where they are most needed. This capability makes the cupped hand superior to the ear trumpet, which increases the sound pressure at all frequencies it receives.

Ear Trumpet. The ear trumpet, which was in vented between 1650 and 1700, is a long horn with a large open end in the air and a small open end placed in the ear. Sound pressure waves entering the large end are condensed into a smaller and smaller volume as they travel into the horn, increasing the sound pressure delivered to the ear. The pressure increase depends only on the relative size of the large and small openings; the horn does not use any external energy source, such as a battery.

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