The hearing aid is an instrument that amplifies sounds, particularly speech, for people with hearing impairments. It may be worn comfortably behind the ear, in the outer ear, within the ear canal, in the frames of eyeglasses, or against the body or in the clothing. The main elements of the aid are a microphone, an electronic amplifier to make the sound louder, an earphone or receiver, and an ear mold or plastic shell that serves to couple acoustic energy (sound) from the earphone to the eardrum either directly or through plastic tubes. The sound is converted to an electrical signal, amplified, then reconverted to acoustic energy in the inner ear. A battery, the typical power source, can also be contained in the shell.
The microphone and earphone together form a transducer and determine the performance of the aid over a range of frequencies. The adjustment of tone (low and high frequencies) and gain (volume) can be either manual or automatic so that the user can hear enhanced sounds within a comfortable tolerance level.
The earliest hearing aids were ear trumpets invented sometime in the 17th century. They were long horns with one large opening at one end and a smaller opening at the other end, which was placed in the ear. The principle behind this instrument being that sound pressure waves entering the large end are condensed into smaller volume, thereby increasing the audible sound pressure.
The first electronic aid was a telephone invented by Alexander Graham Bell in 1876 that converted sound pressure waves to a current and then back to waves. By the 1920s, a more sophisticated telephone-type aid was developed resembling the modern hearing aid with a microphone, electrical circuit, diaphragm, and battery. With the invention of the transistor in 1948, the size and weight of the aid was further greatly reduced. Today, tiny aids placed in the ear canal are barely visible to others, offering great cosmetic appeal to the user. The miniaturization of the hearing aid continues to be an area of research and experimentation. Unfortunately, the smaller the hearing aids become, the greater the manual dexterity required of a user to work the controls.
Fabricating a hearing aid takes about two hours. Making hearing aids is a customized process requiring skilled technicians to operate magnification glasses and microscopes in a microminiature manufacturing environment. The tools are generally hand-held and the tasks demand precision movements. The assembler must pay close attention to the wiring diagram and assembly prints so that he or she wires it to produce exact results.
Before fabrication begins, the user is screened by a trained professional. The screening includes a hearing test, and the results are used to create an audiogram covering a variety of parameters. At the screening stage, an impression or mold of the user's outer ear is also taken. The audiogram and the impression are integral to the manufacturing process.
Quality control measures are checked throughout production, some of which have been discussed in the process description above. In addition, the shell is given a serial number after it is constructed for tracking purposes. Appearance is important, and a cosmetic check is made as well as a final function check.
Hearing aids are tested using a computerized ANSI (American National Standards Institute) program that analyzes the production parameters and produces a performance chart. A technician reviews the chart on-screen, checking tolerance levels and other specifications. He or she will print a copy of the results and include it with the finished hearing aid.
The future of hearing aids seems to lie in miniaturization. Today's technology can produce aids the size of a fingertip. Also a recent development, customized digitally programmable aids using microchips found in computers allow users to rapidly switch settings to accommodate different situations. Outdoor events, crowded restaurants, and intimate meetings, each with different sound patterns, can be programmed in the chips. This minimizes the quick adjustments some users must make when they move into a new environment. These custom aids can cost $2,000 each.
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— Peter Toeg