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Figure 1
A fairly typical audiometric pattern for older people.
The audiogram of a fairly typical audiogram can be seen in
Figure 1. (My thanks to Brad Ingrao for creating these figures for me).
Let’s first go through the fundamentals. Frequency (or pitch) is
depicted on the horizontal axis, from low frequencies on the left (250
Hz) to high frequencies on the right (8,000 Hz). For some specific
purposes, it may be useful to test 125 Hz as well as some frequencies
higher than 8,000 Hz (for example, a progressive hearing loss above
8,000 Hz can occur with certain type of ototoxic medications).
The amount of hearing loss is shown on the vertical axis
with the higher numbers indicating a greater degree of hearing loss. A
symbol on this axis (red circle for the right ear and blue cross for the
left) is a measure of the person’s hearing threshold at this frequency;
i.e., the loudness (intensity) point where sound is just audible.
Thresholds from zero to 15 dB are considered to be within
the normal hearing range. After that point, people will usually begin to
display some communication difficulties because of the elevated hearing
thresholds. The higher the number, the greater the impact of the hearing
loss (referring only to the unaided condition).
The 100 dB point
should not be confused with a 100 percent hearing loss, that is a total
lack of hearing.
Hearing sensations do continue past this point, with some audiometers
extending this vertical range to 120 dB. In short, the audiogram
is a chart of a person’s hearing loss, frequency by frequency.
Now note the shaded area extending across the audiogram (the
so—called “speech banana”). This is a general, but static,
representation of the acoustical speech energy across frequency. Some
speech sounds, such as vowels, are predominantly composed of low
frequency energy with less of their power in the higher frequencies.
Other speech sounds, in particular the voiceless consonants, are the
reverse. They have most of their energy in the high frequencies and less
in the lower frequencies.
Only the shaded area of the speech banana above the
threshold curve is audible; portions below will not be perceived. In
spite of portrayals to the contrary on some charts, no speech sound can
he pinpointed to a specific point on the audiogram; all of them spread
their energy somewhat along some segment of the frequency spectrum.
In addition to their energy locations on the frequency
axis, speech sounds also vary in intensity. Some are naturally weaker
(like the voiceless /th/ sound, the weakest sound in the English
language) and some are naturally more intense (like the vowel /aw/, the
strongest). When these are spoken in a normal fashion, as in the word
“thaw” there is a 30 dB intensity range between the two phonemes. The
intensity of all other speech sounds in the English language fall within
these bounds (note, however, that this does not take into consideration
inflected speech, which will move the entire speech banana up and down).
As already noted, the speech banana is a static
representation of speech sounds. In reality, however, conversational
speech is a dynamic and time-varying event, with one speech sound
(phoneme) following another in a rapid succession.
The specific acoustical composition of speech sounds is
partially shaped by the preceding and following sounds in an utterance.
These specific effects, such as the unique modifications in vowels made
by different preceding and following consonants, are actually secondary
cues that can be helpful for understanding speech.
Indeed, the ability to take advantage of these cues may help
explain why some people seem to understand speech much better than one
would predict on the basis of their audiogram. Still, there are certain
anchor locations for all speech sounds, as displayed on the speech
banana, and these can offer valuable insights in understanding the
effects of differing hearing losses.
Looking at the audiogram again, and relating it to the
speech banana, it is apparent that a person with this hearing loss will
hear more of the lower frequency speech sounds than the higher ones,
Indeed, some of the higher frequency sounds, such as the /s/ sound (the
most frequently used phoneme in the English language) will barely be
heard at all. Will this person be able to understand normal speech
without a hearing aid? Yes, but only with some difficulty and then only
if the talker is close by and raises his or her voice slightly.
There are, unfortunately, many people with audiograms
similar to this who do not, for one reason or another, wear hearing
aids. One often sees this type of audiogram in older people whose
hearing loss just “crept up” on them and who are still not fully aware
of the difficulty they are causing themselves, their family and friends.
Conversing with such individuals is a strain on everyone’s
part. Often, they’ll complain that they can “hear” the talker but cannot
“understand” what is being said. This is, indeed, a very frequent
complaint. Looking at the audiogram, we can understand somewhat why this
should occur: they are not hearing the full range of high frequencies
where many of the consonants have their predominant energy. And we know
from many years of research, that consonants contribute more to the
understanding of speech than vowels. Without a hearing aid, this person
would he hearing primarily low frequency vowels energy and some part of
the voiced consonants, but little or no portions of the crucial
voiceless consonant sounds (such as /s/, /t/, and /k/).
I don’t want to give the impression that the perception of
vowels is unimportant - on the contrary it is and can he very important
- only that by not actually hearing some of the consonants, people with
hearing loss have to struggle to fill in the acoustical gaps they
produce in the stream of speech. Those individuals with superior
linguistic skills would perform this task better than those with lesser
skills. Often, people with this type of loss are not aware of the sounds
that they don’t hear. They define what they do hear as “normal” as it
is, for them.
In truth, they may be hanging on to speech comprehension
with their finger tips; any further distortion in the speech signal,
such as someone talking rapidly or in a foreign accent, or in the
presence of noise and reverberation, and they lose it. And this
audiogram depicts only a gradual high frequency hearing loss; were this
person’s audiogram something like the one shown in Figure 2, problems
with speech perception would be even more severe.
Figure Two Audiogram –
The “Ski Slope”
Figure 2
A Severe-to-Profound High Frequency Hearing Loss ("Ski Slope" Curve).
This audiogram in Figure 2 is often described as a
“ski-slope” hearing loss. Somebody with this type of hearing loss
actually hears better at 250 Hz than the one whose audiogram is shown in
Figure 1, but much worse at 1,000 Hz and higher. Looking at the speech
banana, it is apparent that while a great deal of low frequency vowel
energy will he perceived, practically no high frequency consonants would
be.
An individual with this audiogram would have even more
difficulty in understanding speech than the one whose audiogram is shown
in Figure 1. These people depend upon whatever low frequency energy they
do receive for comprehending speech. The presence of noise and
reverberation (not exactly an uncommon occurrence in our society!) would
have a disproportionate effect on them, since it would mask the only
speech energy they are able to receive.
Complaints of “hearing” hut not “understanding” would occur
even more frequently. Without a hearing aid (and even with one) this
person needs to depend upon visual cues (speechreading) to supplement
the information received through hearing. Some people, because of their
superior ability to utilize some of the secondary cues in a speech
signal referred to above, or their superior linguistic ability may do
better than these comments would suggest. But there are limits to even
the keenest brain and the most developed auditory integration ability;
at some point, people do need to detect speech energy in order to
understand a spoken message. And this person is missing just too much of
the speech signal to expect easy communication in any oral exchange.
It is usual to describe the extent of a hearing loss with
verbal labels. Thus, someone may be described as having mild, moderate,
severe or profound hearing loss, or some combination of terms (“mild to
moderate” or “severe to profound”). This is an easy and shorthand way of
labeling the severity of a hearing loss. While there is merit and often
necessity to this practice, there is also the danger of
oversimplification. Or perhaps someone’s hearing loss is described by
employing a single figure (derived usually from averaging the hearing
losses at 500 Hz, 1,000 Hz, and 2000 Hz). The numbers themselves then
serve as the basis for the verbal label that is applied. For example
people with an average 40 dB hearing loss are considered to have just a
moderate hearing loss. But such a shorthand label does not reflect the
pattern of a hearing loss, a pattern, as we have seen, that may lead to
more insights into the behavioral implications of a hearing loss. For
example, consider the audiograms shown in Figure 3.
Figure Three –
Behavioral lmplications of Hearing Loss
Figure 3
Two very different ears with the same pure-tone average.
The average hearing loss is the same for both ears in
the audiogram shown in Figure 3. In the left ear, the hearing thresholds
at 500 Hz, 1,000 Hz, and 2,000 Hz are 0 dB, 40 dB, and 80 dB
respectively Their average is 40 dB. In the right ear, the hearing loss
at each of the three frequencies is the same, also resulting in an
average of 40 dB.
But it should he apparent, just from a visual inspection,
that these two ears would perceive speech quite differently. Quite
clearly this example demonstrates that the shorthand description of a
hearing loss with a number or a verbal label can be quite deceptive.
What I have often found useful in explaining the need to
understand the audiogram is commenting that it is possible for someone
to be completely deaf and have completely normal hearing in the same
ear at the same time. Looking at this audiogram, we can state that
this person has perfectly normal hearing in the left ear, hut this is
only true at 250 Hz and 500 Hz.
Or one could assert that he is completely deaf in the left
ear, as again indeed he is (but only at 4,000 Hz and higher). Both
statements are correct and both describe the left ear of this person.
All it would take to resolve this verbal conundrum would be an
understanding of what the audiogram actually signifies (degree of
hearing loss across frequency).
These three examples do not illustrate the full
variety of possible audiograms. In a room full of people with hearing
loss, it is doubtful that any two would exhibit exactly the same
audiogram for both their ears. There would be differences in degree of
hearing loss at the different frequencies as well as divergences between
the ears. People with bilaterally symmetrical hearing losses
(that is, with similar hearing losses in both ears) may have quite
different auditory experiences than people with bilaterally
asymmetrical hearing losses.
While most people exhibit a gradually sloping hearing loss
across frequency (such as in Figure 1), some people have audiograms that
are very atypical. This would include people with rising thresholds with
frequency as well as those with perfectly normal hearing at the very low
and high frequencies, but with moderately or severe hearing losses in
the mid frequencies. The varieties are almost (but not quite) endless
and many of these differences have behavioral implications. One has to
look at the curve and not just some average figure or verbal label.
It is also true that some people with very similar
audiograms “hear” quite differently. While an audiogram can help explain
much of the auditory behavior of a person, it does not explain it all.
We know that there are other dimensions to auditory experiences that
cannot be explained by the audiogram. Still, the audiogram is a good
place to begin when trying to understand someone’s speech perception
problems. Hearing losses greater than about 70 dB may produce
qualitatively different effects than losses less than this.
The reason for this phenomenon is that more severe hearing
losses usually involve different structures within the cochlea (i.e.,
more inner hair cell damage) and that these different structures will
impact upon the cochlea’s ability to separate incoming speech sounds
into their various frequency components. A hearing loss may also involve
central auditory pathways and these, too, may affect speech perception
quite severely. Nevertheless, in spite of all these caveats, the
audiogram is still the most fundamental auditory dimension of all.
There is another dimension of hearing loss that is sometimes
also included on an audiogram and that is the discomfort threshold.
(But whether included or not, it is a dimension that every hearing aid
dispenser has to take into consideration when fitting a hearing aid).
This is the loudest sound that an individual is willing to tolerate.
When measured with tones across frequencies and compared to the hearing
thresholds (i.e., the audiogram) at these same frequencies, the
difference between them is the usable “listening area.” (It is also
referred to as the “dynamic range”). It is the auditory area - the
“target” - within which a hearing aid is expected to provide amplified
sound. Sounds delivered by the hearing aid below the threshold of
hearing would not be audible; those exceeding the threshold of
discomfort would not he tolerated (the hearing aid user would either
turn the volume control down or simply remove the hearing aid).
The dilemma of the threshold of discomfort for people with
hearing loss is that it is often the same as that observed with normally
hearing individuals (perhaps 90 dB or so across frequency). But because
their threshold of hearing is elevated, but not their threshold of
discomfort, this means that people with hearing loss generally have a
reduced listening area (or narrow dynamic range).
For example, if someone’s thresholds at 1,000 Hz were 50 dB
and their threshold of discomfort were 90 dB; their dynamic range would
be 40 dB at this frequency. This is an adequate listening space within
which to deliver amplified speech sounds. If, however, the loss were 75
dB and the tolerance level at 95 dB, then the resulting dynamic range
would only be 20 dB. In this instance, it would be more of a challenge
to package sound within this more restricted dynamic range (this is
where advanced technology can he very helpful, as in hearing aids with
fast acting automatic gain control circuits).
Finally, it is important for people to be familiar with the
details of their audiogram so that they can track any changes with time.
Hearing loss, particularly adult onset hearing losses, may get gradually
worse. (Anybody experiencing rapid changes in their auditory thresholds
should check with an otolaryngologist as soon as possible). After a
while, these changes will likely have behavioral implications that may
require reprogramming one’s hearing aid (if only a few frequencies are
involved) or changing to another hearing aid if the deterioration
extends across frequency.
It is a good idea for people to keep copies of all the
audiometric tests administered to them so that comparisons can be made
(and to be sure that they are dated correctly).
In brief, the audiogram is perhaps the most important
indictor of one’s hearing function, the particulars of which everybody
who has a hearing loss should be aware of.
Mark Ross, Ph.D., is an audiologist and associate at the
Rehabilitation Engineering Research Center (RERC) at Gallaudet
University in Washington, D.C. He dates his emergence into the field of
audiology to the time he attended the Army Aural Rehabilitation program
as a patient in 1952, He received his doctoral degree from Stanford
University and taught at the University of Connecticut and worked as a
clinical audiologist at the Newington Children’s hospital. Dr. Ross is
the former director of research and training at the League for the Hard
of Hearing and has served on the boards of SHHH and the International
Federation of Hard of Hearing People. He has written a regular column on
developments in research and technology in the Journal since 1990. He
and his wife, Helen, live in Storrs, Connecticut.
You can find more articles from Dr. Ross regarding
technology for consumers at the following web site:
www.hearingresearch.org/ross.htm
“This article is supported in part by GRANT #H133E030006
from the US Department of Education, NIDRR, to Gallaudet University. The
opinions expressed herein are those of the author and do not necessarily
reflect those of the Department of Education”.
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