The following article from the SHHH Journal is posted with
permission. The SHHH Journal is published bimonthly by SHHH
(Self Help for Hard of Hearing People, Inc.), an international
nonprofit volunteer organization devoted to the welfare and
interests of those who cannot hear well. For membership and
other information, contact:
7910 Woodmont Ave, Suite 1200
Bethesda, MD 20814
Email: shhh.nancy at genie.com (Nancy Macklin)
Hearing Aids and the Consumer:
An Overview of Current Hearing Aid Wisdom
by Matthew H. Bakke
SHHH Journal 5/95
There are as many kinds of hearing loss as there are people
with hearing loss. This is why there is not one "best" hearing
aid. Here is some valuable information for both those who wear
hearing aids and for those who may be at the point of considering
You will probably read and learn many things about hearing,
hearing aids, and assistive devices. Nevertheless, you should
evaluate your own special hearing health care needs before coming
to any conclusions about your hearing. Your physician,
audiologist, and hearing aid specialist are the best people to
help you do this. This is one case in which what's good for the
goose is not necessarily good for the gander!
Despite the tremendous individual differences among people
who are hard of hearing, we can nevertheless categorize hearing
loss along two dimensions: cause (conductive, sensorineural,
mixed); and severity (mild, moderate, severe, profound).
Conductive hearing loss results from a blockage of sound as it
passes through the outer and middle ears. Sensorineural hearing
loss (often inaccurately called "nerve deafness" in colloquial
language) most often results from problems in the inner ear, or
cochlea. Mixed hearing loss is, of course, a combination of
conductive and sensorineural hearing loss. Medical treatment is
often the best solution for conductive hearing loss, while
sensorineural hearing loss is largely medically untreatable. The
best technological solution for sensorineural hearing loss is
usually the use of hearing aids and other assistive devices.
The most commonly understood feature of hearing loss is that
sensitivity to sound is reduced: sounds need to be louder for
them to be heard. Sensitivity to sound is called hearing acuity.
Hearing acuity can vary with the frequency or pitch of the sound.
(Frequency refers to the number of cycles per second in a
sound wave and is a physical quantity. Pitch is the perceptual
effect of frequency, and is a psychological quantity. A tone of
100 Hz sounds low in pitch, while a tone of 8,000 Hz sounds high
For example, one may be able to hear very soft bass sounds, yet
be able to hear treble sounds only when they are made quite loud.
Hearing aids can address this problem by making some sound
frequencies louder than others based upon information about
the hearing loss of the user.
Sensorineural hearing loss has other important features, however,
which are not as commonly understood. People with hearing loss
often have reduced dynamic range. (Dynamic range is the
difference between the level of sound that is just detectible and
the level of sound that is uncomfortably loud.) For hard of
hearing people, sounds may need to be quite loud before they can
be detected. Yet, sounds that are too loud for normally hearing
people are also too loud for the hard of hearing person. Thus,
there is a smaller range of useful sound intensity for many hard
of hearing listeners. Hearing aids can address this problem by
limiting their output in some way.
People with hearing loss also may have difficulty discriminating
one frequency from another or have more difficulty processing
timing information in sound. These difficulties can interfere
with the understanding of speech even when it is loud enough to
be heard, especially when noise is present. Hearing aids have a
difficult time dealing with these problems. No acceptable hearing
aid solutions, other than reducing interfering noise with FM
transmission or other remote means, have been found.
Hearing aids are designed to address the problem of reduced
hearing acuity by making sounds louder. The essential hearing aid
is nothing more than an amplifier with a microphone and a
loudspeaker that fits in the ear. The fact that sensitivity of
hearing can change with the frequency of sound is addressed by
selective amplification. That is, sounds of different frequency
are amplified differently by hearing aids. We would prefer to
amplify only those sounds that are too soft for the listener,
thus restoring a complete speech signal while maintaining normal
The problem of reduced dynamic range can be addressed by hearing
aids in two ways. The less complex way is to set a simple limit
on the output of the hearing aid. Any sound that exceeds the
limit set by the circuitry of the aid is peak clipped. This word
is used because the peaks of the sound wave are literally clipped
off. As you might expect, peak clipping creates distortion and
makes loud sounds unpleasant to hear even when they are not too
loud. The second way to limit the output is to use compression
Compression requires more complex circuitry, but is easy to
implement in hearing aids. It works by limiting further
amplification after a threshold level of output has been reached.
This system has the virtue of limiting the output of the hearing
aid without clipping the sound wave. The result is far less
distortion of the hearing aid output while maintaining control
over the maximum loudness that the aid can produce.
Besides limiting output, compression circuitry can also control
the loudness of the sounds coming from the hearing aid. This
process is called automatic volume control (AVC), or automatic
gain control (AGC). The goal of AGC or AVC is to limit the
necessity for the user to adjust the volume wheel as
environmental conditions change, and to ensure user comfort.
Wide dynamic range compression involves the use of a compression
circuit to ensure that both loud and soft speech sounds can be
heard at comfortable levels. It reduces the differences between
loud and soft sounds, bringing more speech information into the
dynamic range of the hearing aid user.
Perhaps the primary limitation of hearing aids is that they do
not restore hearing. If someone does not have hearing at 2,600
Hz, for example, then he or she will not hear a tone at this
frequency no matter how loud it is made by a hearing aid. For
someone with no hearing at any frequency, other options than
hearing aids must be considered.
Another limitation is that selective amplification never seems to
work as well as the example illustrated above might indicate. One
problem is that speech sounds are slippery little things that
vary in frequency content and loudness level from person to
person and word to word. The other is that speech and noise
typically are composed of the same frequencies. For this reason,
hearing aids typically do not work well in noise.
While we wish the hearing aid could amplify only speech, most
often background noise is amplified along with speech. I remember
the first time I understood this. My brother, who is deaf, had a
hearing aid when we were children. One day, I "borrowed" it with
the intention of giving myself super hearing so I could overhear
my friends talk in the next house. To my disappointment, instead
of juicy bits of secret information, I got a heavy dose of the
hum of life: automobile engines, airplanes, rustling leaves,
wildlife noises and wind.
Because of the noise factor, hearing aids work optimally when
used in conjunction with assistive devices (audio loops, FM,
etc.) when the person with the hearing loss is at a distance from
the speaker or in a large room. The hearing aid may couple with
the assistive device by two means: direct input or by means of a
telecoil (T-switch). Direct audio input is accomplished by
connecting your behind-the-ear hearing aid to the assistive
device by a wire that is supplied by the hearing aid company. The
hearing aid dispenser should request this option when your
hearing aid is ordered. More commonly, the telecoil of your
hearing aid will serve as the means of coupling.
Many of the assistive devices that are available can be used with
neck loops. These are wires that hang around the neck and
stimulate the telecoil in your hearing aid. This is a good reason
to be sure that your hearing aid comes with a telecoil, even if
you do not prefer to use it with the telephone. It is also
important to consider whether you prefer having a behind-the-ear
(BTE) or in-the-ear (ITE) hearing aid versus an in-the-canal
(ITC) hearing aid. Telecoils are available in BTE and ITE cases,
but not yet readily available in ITC cases.
There are other difficulties with hearing aids. For one thing, we
modern humans tend to live in rooms, rather than out on the high
plains like our ancestors. Rooms have walls that reflect sound
resulting in reverberation. This can be considered a form of
noise, since it interferes with our understanding of speech.
Anyone who has tried to understand a sermon in a vast cathedral
or spacious synagogue can appreciate the terrible effect that
reverberation can have on speech. Hearing aid amplification can
actually worsen the effects of reverberation, especially when
only one hearing aid is being worn. (The use of both ears at the
same time, or binaural hearing, is one way that we can compensate
for the effects of reverberation.)
Hearing aids can also create their own kinds of noises. For
example, a particularly annoying attribute of hearing aids is
their tendency to produce feedback, or to whistle. This problem
occurs when the seal of the earmold or in-the-ear hearing aid
case is not tight, and the volume wheel of the aid is turned up
high. It can also happen when an obstruction is placed in front
of the hearing aid microphone, such as when trying to use the
telephone without using a telecoil. This can happen at the most
inopportune times. A friend of mine became a bit nervous one
day when his hearing aid whistled as an attractive young woman
walked by arm-in-arm with her very young, very strong boy friend.
Another kind of hearing aid noise is called distortion.
Distortion occurs when you get something out of a system that you
did not put in. For example, when you play your CD of "Swan Lake"
on a $1,000 stereo system, you hear the music much as it was
recorded in the studio. If you play the same CD on a $50 player
with $3 headphones purchased at the drug store, you hear things
never intended by the artist. These things can also be considered
noise, because they interfere with the intended signal. Hearing
aids are not inexpensive, but they are imperfect and they can
create distortion. This is not because of poor quality in design
or construction but because distortion may be the cost of doing
some other important things, such as signal processing and
miniaturization, which can improve the performance of the hearing
Finally, hearing aids also have what is called internal noise.
This is the noise introduced by the amplifiers, switches,
filters, and other internal components of the aid. This noise is
usually very low and cannot be heard by most users. Even when it
is loud enough to be audible in a sound-treated room, the
environmental noises of normal environments mask it so that it is
not usually heard.
There are several hearing aid types and styles to choose from
these days. The terminology can be confusing, especially for
consumers who may not have backgrounds in technology.
Conventional hearing aids use ordinary analog electronic
components to achieve their function. Most hearing aids on the
market today are analog systems. The audiologist or hearing aid
dealer can adjust their functions with a screwdriver.
Programmable systems, on the other hand, use computer circuits
(logic) to perform their functions. This gives them greater
flexibility. Instead of screwdriver adjustments, the fitting of a
programmable aid requires the use of a programming unit
(sometimes facetiously called an electronic screwdriver). The
programming unit is purchased by the audiologist or hearing aid
dealer, not by the user, and may work with more than one kind of
You may also have heard of digital hearing aids. To the extent
that a programmable aid has computer components, it may be called
digital, but a true digital hearing aid operates much like a
compact disk: it changes sound into numbers, operates on the
numbers mathematically, and changes the numbers back into sound.
One advantage of a digital hearing aid is that it can
theoretically be programmed to perform exquisite signal
processing. Another advantage is that it can change its
characteristics quickly and easily, and be used in the process of
hearing aid selection.
There are no true digital hearing aids now on the market,
probably because they require extra size (they do not fit in the
ear) and power consumption. Most of the programmable hearing aids
are hybrid systems, having both analog and digital components.
These have more limitations than true digital systems, but still
provide greater flexibility than conventional hearing aids.
By far the greatest advantage of the programmable hearing aid is
the ease of fitting and adjustment. Very fine changes in the
hearing aid's function can be made quickly and easily by the
audiologist. The advantage to the consumer is a better-fit
hearing aid. Other features of programmable hearing aids include
multiple memories and special signal processing features, such as
multiple channels and automatic signal processing (ASP). The down
side of programmable hearing aids is their added cost to the
Several programmable hearing aids offer multiple memories. This
means that one hearing aid can function in many different ways.
The philosophy behind the use of multiple memories is that one
hearing aid characteristic cannot be optimum for every hearing
situation. Rather, the user should have the option of changing
hearing aid characteristics as listening conditions change. It is
impractical to carry a pocketful of hearing aids and change from
one another when conditions change. Therefore, many people find
that the multiple memories option gives them the ability to push
a button and find the best "hearing aid" for that particular
Hearing aids with multichannel capacity may offer consumers a
better hearing aid fit, especially when their hearing loss
differs greatly between frequencies. Audiologists find these
kinds of hearing loss difficult to fit with conventional hearing
aids. Multichannel hearing aids divide the range of frequencies
into two or more channels (most often a low frequency channel and
a high frequency channel). Each channel can then be independently
controlled. This approach allows greater flexibility of fit.
Another development that has been helpful with difficult-to-
fit hearing losses is the Class D Amplifier. It is helpful for
fitting high frequency hearing losses because it can deliver
greater power in the higher frequencies with less distortion.
Automatic Signal Processing (ASP) is a general term that can
mean almost anything. In the hearing aid world, it is generally
taken to mean that the hearing aid automatically responds to
changes in the level of input sounds. This approach is also
called automatic frequency response (AFR). There are two
philosophies currently on the market: BILL and TILL.
BILL stands for Bass Increase at Low Levels. This strategy is
geared toward reducing the effects of noise. In BILL, bass (low
frequency) information is filtered out of loud sounds, but
allowed to remain for quiet sounds. This is based upon the
assumption that most environmental noise is low in pitch. It is
expected that removing low frequencies will reduce the negative
effects of noise. Quiet sounds, on the other hand, can only be
heard in quiet environments, so no filtering is necessary. BILL
is offered in most hearing aids that advertise ASP.
TILL stands for Treble Increase at Low Levels, and is the
opposite of BILL. The philosophy behind TILL is that loud sounds
do not need to be amplified at all: the hearing aid should do
nothing to loud sounds. Quiet sounds, however, need to be
amplified to be heard by the hard of hearing person. Since most
hard of hearing people hear better at low than high frequencies,
raising the treble, or high frequency, components of speech at
low levels will benefit the user. TILL is used in the K-Amp
circuits that many hearing aid manufacturers offer.
Which works better, BILL or TILL? As I mentioned in the first
paragraph of this article, there is not one best hearing aid.
Many people find BILL wonderful, others find TILL to be the best
for them. This is an issue that must be worked out between every
user of hearing aids and his or her audiologist.
The problem of feedback in hearing aids, mentioned earlier in
this article, can be a significant problem for some users,
especially when hearing loss is severe. A recently introduced
scheme for suppressing feedback uses computer logic. It permits
the hearing aid to detect feedback and cancel it out
acoustically. If you have had problems with feedback control, you
may want to discuss these options with your audiologist.
We have seen that there is a wide range of hearing aid technology
that is now available. This is good news because it is likely to
result in better hearing aid performance and more user
satisfaction. The down side, however, is that we may be subject
to information overload.
I hope that this brief article has helped you sort out some
issues and language about hearing aids that may have confused you
in the past. If you have any questions about what you have read,
or about any other hearing issues, please contact me at the
Rehabilitation Engineering Research Center (RERC), (718) 899-8800
Voice/TTY; or you may write to me at RERC, The Lexington Center,
Inc., 30th Avenue and 75th Street, Jackson Heights, New York
The preparation of this article was supported by Grant
#H133E30015 from the U.S. Department of Education, NIDRR, to the
Lexington Center for a Rehabilitation Engineering Research Center
(RERC) on Hearing Enhancement and Assistive Devices.
Matthew H. Bakke, M.Ph., is the director of research at The
Lexington Center, Inc., Jackson Heights, New York. He is an
audiologist and is a former teacher of deaf students. He is
preparing a dissertation on auditory localization for the City
University of New York, and expects to receive his doctorate
degree in audiology within the year.
Sidebar: You Are Not Alone
One in Ten Americans Has a Hearing Loss
"Thousands of us, people of all ages, wear hearing aids. They
help us to hear and keep us tuned into life. We vividly remember
how difficult it was to make up our minds to get help when we
suspected we were losing our hearing. Some of us waited too long
and became dependent on our spouses or strained our relationships
by constantly asking for sentences to be repeated or by arguing
over how loud we turned up the TV."
It's easy to be confused about hearing loss -- whether or not you
need help. Sometimes, you hear fine. Other times, not so well.
Help is available.
Write today for the SHHH booklet Hearing Loss: How to Get Help.
Send a self-addressed, stamped, business-sized envelope to SHHH
National, 7910 Woodmont Avenue, Suite 1200, Bethesda, MD 20814.