Includes the following article: In Search of a True
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'IN SEARCH OF TRUE THRESHOLD'
By Max Stanley Chartrand, M.A., BC-HIS
Tie American National Standards Institute (ANSI)
refers to human auditory threshold as 'the minimum
effective sound pressure of the signal that is
capable of evoking an auditory sensation in a
specified fraction of the trials.' In this case, we
seek two out of three responses at a given sensation
level, or better than 50 percent of the time.
The line between imagining a tone and actually
perceiving it as an acoustic sensation can be
difficult to many individuals, especially when the
presentation is pulsed or consistent. This ability
to discern threshold further deteriorates as we
mature and can prove difficult for the elderly.
Try to remember the last time you were given a truly
objective threshold test by another hearing
professional. For even the most astute and trained
individual, finding a true, repeatable threshold can
be a frustrating experience. Is it my imagination or
do I actually hear the tone? you ask yourself.
Perhaps if you stop breathing for a minute or two
you can improve your thresholds another five
decibels.
This presents a tremendous challenge to the
untrained ear; a fact too often taken for granted by
those sitting at the other end of the audiometer.
Indeed, the latest ANSI sound suite ambient standard
has elicited patient reports of 'hearing my heart
beat' or 'deafening silence' sounds and sensations
that, in themselves, can contaminate true threshold
measurements.
Hence, the question, Is it really that important to
find such absolute thresholds since we are, after
all, measuring hearing impaired cases?
CONTROLLING THE TEST ENVIRONMENT
Let's back up for a minute. First, the fact remains
that most adults with aidable hearing thresholds
exhibit normal sensitivity in the low frequencies.
Second, by disregarding test environment sound
levels, the specialist will experience many more
occlusion and over-amplification-in-lows complaints.
This alone should be incentive enough for the
specialist to carefully monitor the testing
environment. Certainly, it can safely be assumed
that those who exhibit a mild loss in the lows when
tested in an uncontrolled environment will probably
exhibit normal low frequency thresholds in a
controlled environment.
Without accommodating this problem, the specialist
is setting up patients with near-normal low
frequencies sensitivities for failure, as they
constitute the single largest group for failed
trials and credit returns today. It is vital to
evoke the best possible thresholds in the best
possible test environment. Patients may not usually
listen in such an environment, but it is the
environment in which to truly ascertain accurate
threshold measures.
Moreover, those receiving hearing tests often
reflect the attitude and thoroughness of the
specialist. For instance, if the specialist treats
the test scores as routine, or hurriedly obtains
thresholds, the patient will perceive that this is
not an important part of the evaluation. Therefore,
the thresholds may not be 'barely discernible'
indications, but may instead be 'comfortable'
responses. This could be as much as 10 or 15dB above
threshold, essentially invalidating the
threshold scores.
Even more important is the cost in time and
frustration experienced by the hearing aid user when
threshold inaccuracies complicate post-fitting
adjustments. Another phenomena that can occur,
although not common, is echoacousia or a
psychological reconstruction of the tone (or 'echo')
causing the listener to believe the tone is still
being given. For that reason, the author strongly
cautions against using pulsated presentation of
pure-tones, as it can evoke a degree of echoacousia
in the unsuspected patient.
YOUNG PATIENTS
In cases of very young children, it often takes a
great deal of patience to train their listening
ability to produce reliable and repeatable threshold
responses. Most need multiple sittings to produce
accurate results.
It behooves the children's specialist to become
familiar with behavioral methods of auditory
testing. Oftentimes, one must develop visual evoked
responses from the child which, coupled with the
usual modes of voluntary response, may provide
greater indication of actual threshold versus
perceived threshold.
As children mature, their ability to communicate
threshold should improve, sometimes creating the
false impression that their hearing acuity has
improved. The recognition of this phenomenon can be
particularly confusing for parents and other casual
observers to understand when one of the early
etiologies of auditory deficit was an Eustachian
tube dysfunction. However, the resolution of that
particular difficulty would affect merely the low
frequencies, leaving the critical speech range
unaffected.
TRAINING ADULTS TO HEAR THRESHOLD
Training for threshold in adults, on the other hand,
generally requires verbally-based approaches. For
young adults through middle age (i.e., 22-64 years
of age), training for threshold detection may not
require more than the simplest
explanations, such as 'Please, raise vour hand when
you hear the tone.'
Older adults may require more explicit instructions
with intermittent assessment checks (i.e.: 'Did you
barely hear the tone, or was it easily heard?') with
instructions repeated as needed.
PARTNERS IN THE PROCESS
The author has found that the best wav to evoke
accurate thresholds for all age groups is to train
the patient to be a partner in the process. By
explaining what is being measured and why the
patient will more likely be cooperative. This
encourages the test to be one of discovery and
anticipation and helps overcome psychosocial
artifacts such as denial. This approach will yield
far more repeatable and accurate results than
leaving them in the dark.
From a counseling context the specialist will find
several considerations that may affect the validity
of pure-tone thresholds. Alertness and motivation of
both the client and third party are essential to
continue the 'journey of discovery' began earlier in
the case history.
FINDING AN ACCURATE SRT
The Speech Reception Threshold (SRT) provides a
relative correlation with the pure-tone audiogram,
and is expected to come out at about plus or minus
5-lOdB of the pure-tone average (PTA). A shortcut
method of checking SRT accuracy is to note if the
SRT is within 5dB of the threshold at 1KHz.
The SRT will also act as a calibration standard or
reference point for comfort and discomfort levels.
In totality, the SRT represents the 'floor' of the
dynamic range in hearing complex sounds (i.e.,
speech). Consequently, if the SRT does not correlate
with the pure-tone scores, the other speech scores
cannot be trusted.
In performing the SRT, the patient begins
consciously 'training' their ear to listen for
discernible speech sounds at threshold levels. The
word lists used are spondaic or two syllable CID/W-1
and W-2 spondee words, which provide evenly
presented familiar words.
These words also provide fairly redundant speech
context and inflectional cues for ease of
understanding at the lowest intensity possible. As
stated above, when comparedr.) the pure-tone
air-conduction thresholds, the SRT should occur
within 5-lOdB of the pure-tone average (PTA @ .5KHz,
1KHz and 2KHz) of the audiogram.
On the other hand, an accurate SRT can help expose a
patient that did not clearly understand pure-tone
threshold instructions, or one that is a malingerer.
When the SRT comes out lOdB better than the PTA of
the pure-tone rest, it behooves the specialist to
retest pure-tones before proceeding.
Exceptions to the rule include ski-slope, corner or
reverse slope audiograms. In cases of central
auditory processing disorder (CAPD) such as in
phonemic regression temporary) or auditory agnosia
or aphasia (which, coinciientally, require clinical
therapy) the SRT score may be signlificantly more
elevated than the PTA, or may be entirely impossible
to measure (marked 'N/A' or 'CNT').
In difficult-to-test cases, or those in which
language or vocabulary limitations interfere with
the familiarity of the standard spodaic word lists
used for SRT, the specialist may instead administer
a Speech Detection Threshold SDT), also known as a
Speech Awareness Threshold SAT). When performing the
SDT, ask the patient to raise heir hand when they
begin to hear speech and record their indications as
dBHL thresholds. The SDT will usually corelare
closer to 500Hz or the best pure-tone threshold,
vhile the SRT generally correlates closely with the
PTA ind 1KHz.
To administer the SRT utilizing an appropriate
sponaic word list, the specialist would provide the
followig instructions:
"Now, you will hear a list of words. Please, repeat
the words the best you can. I will reduce the volume
to a very low level, which will eventually fall
below the level you will need to understand. Don't
let this frustrate you. Just do the best you can by
repeating that which you do hear. For example, I
will say, 'Say the word BASEBALL' and you will
repeat BASEBALL back to me. Do you have any
questions?'
When live voice testing is utilized, the evaluator
should se the carrier phrase 'Say the word..."
before each instruction. Generally speaking, the
purpose of the carrier phrase is two-fold: 1) for
consistent modulation and control of the evaluator's
voice, and 2) for the psychological prepaition of
the patient. The carrier phrase should be
approxnately 5dB louder than the presented spondaic
word.
SUMMARY
As we search for the often elusive threshold, we
must reiove all obstacles along the way. The first
obstacle is in assuring adequate insulation from
ambient noise, which could otherwise prevent
accuracy and confidence. The next potential obstacle
involves examiner attitude. The patient mav perceive
a hurried, cursory testing procedure as indication
that the examiner is nor careful in their work, or
that the thresholds are unimportant.
We must consider all the tools and approaches
available that are, appropriate for the age group
and personal status of the patient. A child may be
either trying to please, thereby providing false
responses, or not be mature enough to report tones
at threshold. An older adult with psychosomatic
overlay problems may not understand the directions
given by the examiner.
Finally, we must be able correlate, with few
exceptions the SRT or SAT with the pure-tone
threshold to some degree, to assure proper
calibration and accuracy. In following these and
other guidelines the specialists may better ful-fill
ethical and professional expectations.
ABOUT THE AUTHOR
Dr. Chartrand serves as director of research and
professional training at United Hearing Systems,
Inc., and is a long-time faculty member of the
International Institute for Hearing instruments
Studies. Correspondence by fax to: 702.269.0575.
REFERENCES
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Young Child, Denton, TX, Texas Woman's University
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American National Standards Institute, American
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New York (1977).
American National Standards Institute, Manual
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American Speech-Language Hearing Association,
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Carhart, R., "The determination of hearing loss,"
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