|
|
 |
hearing aid
device that increases the loud-ness of sounds in the ear of
the wearer.
The earliest aid was the ear trumpet, characterized
by a large mouth at one end for collecting the sound energy from
a large area and a gradually tapering tube to a narrow orifice for
insertion in the ear.
Modern hearing aids are electronic.
Principal components are a microphone that converts sound into a
varying electrical current, and amplifier that amplifies this current,
and an earphone that converts the amplifies current into a sound
of greater intensity than the original.
Early models were quite large,
but when transistors replaced amplifier tubes and smaller magnetic
microphones became available in the 1950's, it became possible to
build very small hearing aids, some of which were constructed to
fit within the frames of eyeglasses and, later, behind the earlobe
or within the external ear.
Hearing aids have widely differing
characteristics; requirements for suitable aids have been extensively
investigated.
The two characteristics of a hearing
aid that most influence the understanding of speech are the amplification
of the various components of speech sounds and the loudness with
which the sounds are heard by the wearer.
As regards the first characteristic,
speech sounds contain many components of different frequencies,
which are variously amplified by a hearing aid.
The variation of amplification with
frequency is called the frequency response of the aid.
An aid need amplify sound only within
the range of 400 to 4,000 hertz, although the components of speech
cover a much wider range.
With regard to the second characteristic
- the loudness with which sounds are heard- too loud a sound can
be as difficult to understand as one that is too faint.
The loudness range over which
speech is understood best is wide for some users and narrow for
other.
Hearing aids with automatic volume control vary the amplification
of the aid automatically with variations of the input.
A binaural hearing aid
consists of two separate aids, one for each ear.
Such an arrangement can benefit
certain users.
|
