Hearing aid

The invention: Miniaturized electronic amplifier worn inside the
ears of hearing-impaired persons.
The organization behind the invention:
Bell Labs, the research and development arm of the American
Telephone and Telegraph Company
Trapped in Silence
Until the middle of the twentieth century, people who experienced
hearing loss had little hope of being able to hear sounds without the
use of large, awkward, heavy appliances. For many years, the only
hearing aids available were devices known as ear trumpets. The ear
trumpet tried to compensate for hearing loss by increasing the number
of sound waves funneled into the ear canal. A wide, bell-like
mouth similar to the bell of a musical trumpet narrowed to a tube that
the user placed in his or her ear. Ear trumpets helped a little, but they
could not truly increase the volume of the sounds heard.
Beginning in the nineteenth century, inventors tried to develop
electrical devices that would serve as hearing aids. The telephone
was actually a by-product of Alexander Graham Bell’s efforts to
make a hearing aid. Following the invention of the telephone, electrical
engineers designed hearing aids that employed telephone
technology, but those hearing aids were only a slight improvement
over the old ear trumpets. They required large, heavy battery packs
and used a carbon microphone similar to the receiver in a telephone.
More sensitive than purely physical devices such as the ear trumpet,
they could transmit a wider range of sounds but could not amplify
them as effectively as electronic hearing aids now do.
Transistors Make Miniaturization Possible
Two types of hearing aids exist: body-worn and head-worn.
Body-worn hearing aids permit the widest range of sounds to be
heard, but because of the devices’ larger size, many hearing impaired persons do not like to wear them. Head-worn hearing
aids, especially those worn completely in the ear, are much less conspicuous.
In addition to in-ear aids, the category of head-worn hearing
aids includes both hearing aids mounted in eyeglass frames and
those worn behind the ear.
All hearing aids, whether head-worn or body-worn, consist of
four parts: a microphone to pick up sounds, an amplifier, a receiver,
and a power source. The microphone gathers sound waves and converts
them to electrical signals; the amplifier boosts, or increases,
those signals; and the receiver then converts the signals back into
sound waves. In effect, the hearing aid is a miniature radio. After
the receiver converts the signals back to sound waves, those waves
are directed into the ear canal through an earpiece or ear mold. The
ear mold generally is made of plastic and is custom fitted from an
impression taken from the prospective user’s ear.
Effective head-worn hearing aids could not be built until the
electronic circuit was developed in the early 1950’s. The same invention—
the transistor—that led to small portable radios and tape
players allowed engineers to create miniaturized, inconspicuous
hearing aids. Depending on the degree of amplification required,
the amplifier in a hearing aid contains three or more transistors.
Transistors first replaced vacuum tubes in devices such as radios
and phonographs, and then engineers realized that they could be
used in devices for the hearing-impaired.
The research at Bell Labs that led to the invention of the transistor
rose out of military research duringWorldWar II. The vacuum tubes
used in, for example, radar installations to amplify the strength of electronic
signals were big, were fragile because they were made of
blown glass, and gave off high levels of heat when they were used.
Transistors, however, made it possible to build solid-state, integrated
circuits. These are made from crystals of metals such as germanium
or arsenic alloys and therefore are much less fragile than glass. They
are also extremely small (in fact, some integrated circuits are barely
visible to the naked eye) and give off no heat during use.
The number of transistors in a hearing aid varies depending upon
the amount of amplification required. The first transistor is the most
important for the listener in terms of the quality of sound heard. If the
frequency response is set too high—that is, if the device is too sensitive—the listener will be bothered by distracting background noise.
Theoretically, there is no limit on the amount of amplification that a
hearing aid can be designed to provide, but there are practical limits.
The higher the amplification, the more power is required to operate
the hearing aid. This is why body-worn hearing aids can convey a
wider range of sounds than head-worn devices can. It is the power
source—not the electronic components—that is the limiting factor. A
body-worn hearing aid includes a larger battery pack than can be
used with a head-worn device. Indeed, despite advances in battery
technology, the power requirements of a head-worn hearing aid are
such that a 1.4-volt battery that could power a wristwatch for several
years will last only a few days in a hearing aid.
Consequences
The invention of the electronic hearing aid made it possible for
many hearing-impaired persons to participate in a hearing world.
Prior to the invention of the hearing aid, hearing-impaired children
often were unable to participate in routine school activities or function
effectively in mainstream society. Instead of being able to live at
home with their families and enjoy the same experiences that were
available to other children their age, often they were forced to attend
special schools operated by the state or by charities.
Hearing-impaired people were singled out as being different and
were limited in their choice of occupations. Although not every
hearing-impaired person can be helped to hear with a hearing aid—
particularly in cases of total hearing loss—the electronic hearing aid
has ended restrictions for many hearing-impaired people. Hearingimpaired
children are now included in public school classes, and
hearing-impaired adults can now pursue occupations from which
they were once excluded.
Today, many deaf and hearing-impaired persons have chosen to
live without the help of a hearing aid. They believe that they are not
disabled but simply different, and they point out that their “disability”
often allows them to appreciate and participate in life in unique
and positive ways. For them, the use of hearing aids is a choice, not a
necessity. For those who choose, hearing aids make it possible to
participate in the hearing world.