Download Ability To Achieve Gain/Frequency Response and SSPL

J Am Acad Audiol 4 : 33-41(1993)
Ability To Achieve Gain/Frequency Response
and SSPL-90 Under Three Prescription
Formulas with In-The-Ear Hearing Aids
Carol A. Sammeth*'t
Barbara F. Peek§
Gene W. Bratt§
Fred H. Bess*t
Susan M. Amberg'
Abstract
Custom in-the-ear (ITE) hearing aids (standard linear amplifiers with single-pole-filter lowfrequency tone control and a class A amplifier output stage) were fit to 90 ears using the revised
National Acoustics Laboratories' formula (NAL-R), and to 20 ears each using Prescription of
Gain/Output II (POGO II) and Memphis State University (MSU) formulas . Both real-ear
insertion gain and 2-cc coupler gain were evaluated. Examination of differences between
prescribed gain and that actually achieved in the fittings revealed that too much gain was often
given in the low- and mid-frequency range and insufficient gain in the high frequencies . There
was little difference among the formulas in the degree of deviation from target . For some
fittings, the deviation resulted in poorer predicted speech recognition scores (modified Speech
Transmission Index) . For the POGO II and MSU mefhods, underfitting of prescribed SSPL90 values was far more common than overfitting.
Key Words: Amplification, frequency response, hearing aids, maximum power output,
prescription gain formulas, probe microphone measurement, real-ear insertion gain, saturation sound pressure level
ecently, there has been a proliferation
of prescriptive hearing aid selection
R methods that provide gain/frequency
response, and sometimes maximum power output characteristics, based on the audiometric
test results of a given hearing-impaired listener. Some of the prescriptive formulas are
based on audiometric threshold data alone (e .g .,
Berger et al, 1984 ; Byrne and Dillon, 1986 ;
Libby,1986 ; Schwartz et al, 1988), while others
*Division of Hearing and Speech Sciences, Vanderbilt
University School of Medicine, Nashville, Tennessee ; t The
Bill Wilkerson Center, Nashville, Tennessee ; t Currentaffiliation : Department of Speech and Hearing Science, Ar izona
State University, Tempe, Arizona ; and §Audiology and
Speech Pathology Service, Department of Veterans Affairs
Medical Center, Nashville, Tennessee
Reprint requests : Carol A . Sammeth, Department of
Speech and Hearing Science, Arizona State University,
Tempe, AZ 85287-0102
require suprathreshold loudness judgments (uncomfortable loudness or most comfortable loudness) on the assumption that these values cannot adequately be predicted from pure-tone
thresholds (e .g ., Shapiro, 1976 ; Skinner et al,
1982 ; Cox, 1985, 1988 ; Tyler, 1986). Each prescriptive procedure has a stated rationale; the
most common goal being that of maximizing
audibility of the speech spectrum or of amplifying the speech spectrum to the patient's most
comfortable listening level (MCL).
With the current wide availability and use
of probe microphone measurement systems,
prescriptive fitting of real-ear insertion gain
(REIG) is rapidly becoming the most accepted
approach to hearing-aid fitting. Despite the
popularity of prescriptive formulas, however,
there is still limited information available to
guide a clinician in determining which of the
many formulas will provide the best amplification characteristics for a specific patient, other
Journal of the American Academy of Audiology/Volume 4, Number 1, January 1993
than knowledge of the theoretical bases of the
approaches . To examine differences between
formulas, Humes (1986) calculated prescribed
REIG values using 10 different prescriptive
approaches with three different hypothetical
audiometric configurations . He found that, although differences in prescribed gain/frequency
response among the 10 formulas were relatively
small, they sometimes caused the selection of
different behind-the-ear (BTE) hearing aid models . Sullivan et al (1988) evaluated modifications of four prescriptive approaches fit using a
digital master hearing aid. Paired comparison
judgments of relative speech intelligibility and
speech quality were made for each of the prescriptions at three different presentation levels .
Results indicated that no procedure was clearly
superior to the others and that speech performance varied across the formulas as a function of
stimulus level. Differences among the approaches were smallest when the level of presentation corresponded to the listener's most
comfortable loudness level.
Neither Sullivan et al (1988) nor Humes
(1986) used commercially available hearing aids
for testing. Most recently, Humes and Hackett
(1990) evaluated differences in prescriptive
methods under more clinically representative
conditions . For each of 12 sensorineural hearing-impaired subjects, three BTE hearing aids
were selected for best match to the gain/frequency response prescriptions under three formulas : the National Acoustic Laboratories revised formula (NAL-R ; Byrne and Dillon,1986),
the Prescription ofGain/Output formula (POGO ;
McCandless and Lyregaard, 1983), and the
Memphis State University formula (MSU ; Cox,
1988). Evaluation of REIG for each aid, and
aided CUNY Nonsense Syllable Test scores,
indicated very little difference in performance
under the three fitting methods.
Humes and Hackett used BTE hearing aids
because they are more flexible electroacoustically
than in-the-ear (ITE) fittings, and are therefore
more able to achieve the prescribed gain/frequency response characteristics. However, ITEs
make up the largest percentage of hearing aids
sold today (Mahon, 1989 ; Cranmer, 1990), a
statistic that is reflected in our dispensing practices. To examine the ability to achieve prescribed electroacoustic characteristics in the
more popular ITE hearing aid, Bratt et al (1987)
evaluated 122 custom ITEs as they arrived from
the manufacturer to determine how closely they
matched the requested (prescribed) 2-cc coupler
gain values using the NAL-R formula. Few of
34
the aids provided a close match to prescribed
coupler values, even though the manufacturer
had been diligent in trying to provide the requested gain function .
This study extends the work of Bratt et al
(1987) and explores the degree of fitting error
(deviation from target) that occurs in custom
ITE hearing aids with REIG as well as with 2cc coupler measurements under three prescription gain formulas : the NAL-R approach, Prescription of Gain/Output II (POGO II ; Schwartz
et al, 1988), and the MSU approach. Specifically, this study addressed the following questions: (1) How closely is prescribed gain/frequency response achieved with ITE hearing
aids fit using the NAL-R formula with 2-cc
coupler and with REIG measurements across a
large group of ears?; (2) To avoid rejecting too
many hearing aids on first shipment, what
degree ofdeviation from prescribed values would
have to be accepted?; (3) Are there differences in
the degree of fitting error among the three
selected prescriptive formulas (NAL-R, POGO
II, or MSU)?; (4) Is speech recognition performance as predicted by a modified Speech Transmission Index poorer due to deviation from
target?; and (5) How closely are prescribed
SSPL-90 values achieved using the POGO II
and MSU formulas?
METHOD
Subjects
Fifty adults with sensorineural hearing
impairment were fit with ITE hearing aids
using the NAL-R prescriptive formula. All but
10 of the subjects were fit binaurally, resulting
in a total of 90 aided ears tested. Subjects were
patients receiving hearing aids at either the
Department of Veterans Affairs Medical Center
or the Bill Wilkerson Center, both in Nashville,
Tennessee.
An additional 14 adults were fit with hearing aids using the POGO II formula, and 11
adults were fit using the MSU formula. Six of
those subjects fit using the POGO II formula
and nine of those fit using the MSU formula
received binaural hearing aids, for a total of 20
additional aided ears tested under each of these
two prescriptive approaches .
Means and standard deviations of audiometric thresholds for the ears fit under each
prescription formula are shown in Figure 1.
These data illustrate that, on average, subjects
had mild/moderate to moderate/severe sloping
Achieving Prescribed Gain/Sammeth et al
FREQUENCY (Hz)
-10
0000
rn
125
250
I
I
500
1000
2000
-TT -
.
4000
-1
8000
0
10
L
.- - LIIi
Figure 1 Means and standard deviations of hearing
levels for the ears fit with ITE hearing aids under each
prescription formula (for NAL-R, n = 90; for MSU, n = 20 ;
for POGO 11, n = 20).
audiograms, although for POGO II the mean
audiogram is slightly flatter and for MSU the
mean audiogram is slightly steeper than the
NAL-R.
Procedures
Each subject was fit with custom ITE hearing aids (standard linear amplifiers with singlepole-filter low-frequency tone control and a class
A amplifier output stage) from one of several
major manufacturers who had agreed to work
closely with us to attempt to achieve the desired
gain/frequency responses. The target gain/frequency responses were computer-calculated for
each ear using IBM-compatible software called
"Shaping Hearing Aids for Patients Effectively"
(SHAPE ; Venture 4th, 1987). The NAL-R formula is calculated only on the basis of audiometric thresholds . The MSU approach includes
measurement offrequency-specific upper limits
of comfortable loudness and the POGO 11 approach includes frequency-specific uncomfortable loudness level (UCL) measurements in
order to determine prescriptions for output limitation. For purposes of output limiting with the
NAL-R formula, a broadband speech UCL was
determined under headphones using Hawkins'
closed-set response list (Hawkins, 1980) and a
procedure similar to that used by Sammeth et al
(1989) .
With each ITE order, manufacturers were
given only the earmold impression and the prescribed 2-cc coupler values for gain/frequency
response ; i.e ., audiograms were not supplied .
For POGO II and MSU fittings, the manufacturer was also supplied with the prescribed
SSPL-90 curve values, and for NAL-R fittings,
with the speech UCL value. For all SSPL-90
curve values, 5 dB was added for reserve, since
output potentiometers are typically designed
only to lower the output from maximum. To
provide maximum flexibility in fitting the frequency response, each hearing aid was ordered
with both a tone control and output potentiometer, and with Select-A-Vent variable venting
inserts. All ITEs used in this study were hard
peak-clippers; i.e ., no compression or ASP circuitry was used .
When a hearing aid arrived from the manufacturer, electroacoustic measurements were
accomplished using a Fonix 6500 system . First,
a standard ANSI test (ANSI S3 .22-1987) was
run to ensure that each hearing aid was functioning properly, as indicated by manufacturer's specifications . Then, with a swept puretone input of 50 dB SPL and the volume wheel
turned to maximum, the tone control potentiometer was adjusted to achieve the closest
match to prescribed gain/frequency response .
The SSPL-90 curve was also adjusted with the
output potentiometer so as to most closely match,
but not exceed, if possible, the prescribed values
(POGO II and MSU) . Since tone and output
potentiometers sometimes interact, both measurements were repeated until the best compromise was achieved . These "best-fit" 2-cc coupler
values for the gain/frequency response were
recorded for comparison with the prescribed 2cc coupler full-on gain values to determine the
degree of error that occurred .
Subsequently, the frequency/gain response
of the hearing aid was fit to the patient's ear
using probe-microphone measurement of REIG
with the Fonix 6500 . With a broadband composite signal presented at 70 dB SPL from a loudspeaker positioned 1 meter from the subject at
a 90-degree azimuth, trimpots, venting, and
volume wheel were adjusted, as necessary, to
achieve the closest possible match to target
REIG values . These "best-fit" REIG values were
recorded for comparison with prescribed REIG
values to determine the degree of fitting error
that occurred with the real-ear measurement
procedure. Following the real-ear procedure,
the SSPL-90 curve was again measured on the
2-cc coupler. For POGO II and MSU, these
latter SSPL-90 values were compared to prescribed values to examine degree of SSPL-90
fitting error.
35
Journal of the American Academy of Audiology/Volume 4, Number 1, January 1993
A
2 cc Coupler Gain
+ 25
B
Real Ear Insertion Gain
20
15
10
0
5
w
a,
c
0
5
10
15
20
25
- 30
250
500
1K
2K
3K
Frequency (Hz)
4K
6K
250
500
1K
2K
3K
Frequency (Hz)
4K
6K
Figure 2 Scatterplots of fitting error for 35 ears fit using the NAL-R formula. Each datum
point represents the
difference between prescribed gain and that which was actually obtained, at each frequency. A, data for
2-cc coupler fullon gain measurement. B, data for real-ear insertion gain measurement .
RESULTS
Gain/Frequency Response
The NAL-R Prescriptive Formula
Thirty-five of the 90 ears fit with the NALR formula were selected for depiction in
scatterplot form in Figure 2. These data are
illustrative of the trends found in the full data
set. Data for 2-cc coupler measurements are
shown in Figure 2A, and data for REIG measurements are shown in Figure 2B . Each datum
point represents the difference between the
gain that was actually obtained and that prescribed, with a positive value indicating that the
A
100
gain obtained was greater than target gain, and
a negative value indicating that the gain obtained was less than target gain. It can be seen
that there was a trend toward providing too
much gain in the low and mid frequencies, and
too little gain in the high frequencies relative to
the prescribed values . Note that, for REIG measurements, sufficient gain at 4000 Hz was
achieved in only 3 of the 35 ears .
Tolerance values calculated for the data
from all 90 ears fit with the NAL-R formula are
depicted in Figure 3. Shown is the percentage of
hearing aids that would be accepted for a given
degree of fitting error in dB . Specifically, the
ordinate shows the percentage of fittings in
which deviations from target across the fre-
2 cc Coupler Gain
B
Real Ear Insertion Gain
500 to 3000 Hz
--- 250 to 4000 Hz
90
s0
U)
70
O
60
0Q)
U
U
50
N
_C)
D.
U
U
a
0
5
10
15
20
Difference from Target (dB)
25
Difference from Target (dB)
Figure 3 The percentage of 90 hearing aids fit using the NAL-R formula that would be accepted for
a given degree of
fitting error in dB . Two functions are shown, representing a wider and narrower range of measurement
frequencies . A,
data for 2-cc coupler full-on gain measurement. B, data for real-ear insertion gain measurement.
36
Achieving Prescribed Gain/Sammeth et al
Real Ear Insertion
20
N
Gain
15
C
0
Figure 4 Means of the differences in prescribed realear insertion gain between the three prescription gain
formulas for 15 ears representing a range of audiometric
configurations and degrees of losses .
quency range were less than the dB value on the
abscissa . Figure 3A illustrates data for 2-cc
coupler measurements, and Figure 3B illustrates data for REIG measurements . Two functions are shown, each representing a different
frequency range over which degree of error was
considered . Note that, for 2-cc coupler measurements, an error of more than 15 dB would have
to be tolerated at one or more frequencies in the
frequency range of 250 to 4000 Hz to accept 90
percent of the hearing aids as received from the
manufacturer . To accept 50 percent of the hearing aids, a fitting error of 10 dB would need to be
tolerated at one or more frequencies in this
range. Narrowing the frequency window by
excluding 250 and 4000 Hz results in little
change . The data for REIG measurements are
similar to those for 2-cc coupler measurements .
Comparison of NAL-R, POGO II,
and MSU
In comparing the POGO II and MSU procedures with NAL-R, we initially questioned how
much actual difference in prescribed gain/fre-
m
20
z
A
"
o
x
NAL
POGO
MSU
2 cc Coupler Gain
quency response there is among the formulas .
To explore this issue, prescribed REIG for each
frequency was calculated under each of the
formulas for 15 ears that represented a range of
audiometric configurations and degrees of loss .
Then, for each ear, absolute differences in prescribed REIG between the formulas were calculated by frequency. The means ofthe differences
are illustrated in Figure 4. Although some
audiograms resulted in greater calculated differences than others, mean differences in prescribed gain among the formulas were small,
except that POGO II prescribed an average of 13
dB greater gain at 4000 Hz than did the other
two formulas .
The degree of fitting error that occurred
under each of the formulas was then explored .
In Figure 5, the means ofthe fitting error across
the audiometric frequencies for the 20 POGO II
and 20 MSU fittings are compared with those
obtained for the 90 ears fit with the NAL-R
formula. For this figure, the direction of the
difference was ignored; that is, absolute values
of the deviations from target were used to calculate the means. Figure 5A illustrates these data
under the three formulas for 2-cc coupler measurements, and Figure 5B illustrates these data
for REIG measurements (REIG data were unavailable for one ear in the POGO II group) .
Overall, there appears to be relatively little
difference in mean absolute fitting error among
the three groups . For both 2-cc coupler and
REIG data, all three prescription formulas have
a mean deviation from target of less than or
equal to 10 dB in the frequency range from 250
to 4000 Hz . Examination of individual differences revealed that, as previously found with
the NAL-R, MSU and POGO II data generally
indicated overfitting in the mid frequencies and
underfitting in the high frequencies.
8
m 20
Real Ear Insertion Gain
15
W
0)
C
10
W
N
5
0
O
0
Q
0
250
500
750
1000
1500
Frequency (Hz)
2500
4000
6000
0
I
I
i
i
250
500
750
1000
1500
Frequency (Hz)
2500
4000
__L_
6000
Figure 5 The degree of fitting error under each of the three prescription gain formulas . Data shown are the means
of the absolute differences between prescribed gain and that actually obtained, at each frequency . A, data for 2-cc coupler
full-on gain measurement . B, data for real-ear insertion gain measurement.
Journal of the American Academy of AudiologyNolume 4, Number 1, January 1993
Predicted Speech Intelligibility
Alogical question to arise from these data
is whether or not significant deficits in speech
intelligibility can be expected to result from
these fitting errors . To explore this issue, we
calculated modified Speech Transmission Indices (mSTIs ; Humes et al, 1986) based on REIG
data for 20 ears randomly selected from the
group fit under the NAL-R method, 19 of the
ears fit under the POGO 11 method (REIG data
was unavailable for one ear), and the 20 ears fit
under the MSU method . The mSTI is an acoustical calculation scheme that is similar to the
Articulation Index. For our purposes, it provided predictions of aided speech-recognition
performance if the CUNY Nonsense Syllable
Test (NST) had been given with speech fixed at
a level of 70 dB SPL, and with a signal-to-babble
ratio of +7 dB in average reverberant conditions. For each ear, the mSTI was calculated for
each prescribed REIG and for each REIG actually obtained with the hearing aid.
Calculated mSTIs ranged from 34 percent
to 82 percent, with similar ranges across the
three formulas . For 2 of the 20 ears under the
NAL-R method, 2 of the 19 ears under the
POGO II method, and 3 of the 20 ears under the
MSU method, the mSTI calculated for obtained
REIG fell outside of the 95 percent confidence
interval for the mSTI calculated for prescribed
REIG . In each of these cases, the mSTI was
poorer for obtained REIG than for prescribed
A
+ 25I
Maximum Output Curves (SSPL-90)
Shown in Figure 6 are scatterplots of individual differences between prescribed SSPL-90
values and those actually obtained on the 2-cc
coupler for the 20 ears under the MSU formula
and 14 ears under the POGO 11 formula (SSPL90 data were not available for 6 POGO II ears).
Data from MSUfittings are shown in Figure 6A,
and from POGO II fittings in Figure 6B . Frequencies shown are those provided by the respective prescriptive formulas . Positive values
indicate that the SSPL-90 value obtained was
greater than that prescribed, and negative values indicate that the value obtained was less
than that prescribed .
Note that the vast majority of fittings provided lower SSPL-90 values than were prescribed. For most of these hearing aids, output
potentiometers were turned to maximum; that
is, they were providing the greatest output of
B
SSPL - 90 (MSU)
SSPL - 90 (POGO)
+ 25
20
20
m 15
m 15
10
0
L
REIG . The mean difference between the mSTI
for prescribed REIG and that for obtained REIG
in these seven fittings was 18 .7 percent (SD =
3.6).
For most of the remainder of the ears (n =
29), the mSTI for obtained REIG was also poorer
than that for prescribed REIG, although not
exceeding the 95 percent confidence interval . In
10 ears, the two values were equal. In only three
ears was the mSTI for obtained REIG slightly
better than that for prescribed REIG.
~10
0 5
L
w 0
~, 5
5
Lw 0
o, 5
10
i~ 15
d
10
1
i~ 15
20
25
20
25
30
30
- 35
-35
250
500
1K
1 .5K
750
Frequency (Hz)
2.5K
250
500
750
1K
2K
Frequency (Hz)
4K
Figure ii Scatterplots of fitting error for SSPL-90 values . Each datum point represents the difference between
prescribed output and that actually obtained, at each frequency . A, data for 20 ears fit with the MSU method . B, data
for 14 ears fit with POGO 11 .
38
101
Achieving Prescribed Gain/Sammeth et al
which they were capable. Only a few of the
SSPL-90 values fit exceeded those prescribed .
For these hearing aids, values exceeding prescribed output at one frequency were accompanied by values below prescribed output at other
frequencies. Sometimes the clinician chose to
have output at one frequency above the prescribed value in order to more closely match
output for the remainder of the frequencies.
DISCUSSION
n this study, the degree of deviation from
target demonstrated for 2-cc coupler measurements of gain/frequency response is similar
to that found previously by Bratt et al (1987) .
The same trend was found with REIG measurements as with 2-cc coupler measurements . Most
commonly, gain was overfit in the mid frequencies and underfit in the high frequencies .
Differences in fitting error among the three
formulas (see Fig. 5) were generally quite similar to differences in prescribed gain between the
formulas (see Fig. 4) . These data do not imply
that one formula has more or less merit than
another; rather, they simply suggest that there
is no strong rationale for using any one of the
three formulas over another until better fitting
accuracy is achieved .
If a hearing aid is supplied with gain/frequency response and/or SSPL-90 values that
appear to be too deviant from the target values
ordered, the fitter must make a decision as to
whether it is reasonable to return it with instructions that the manufacturer try to match
the order more accurately . Amajor problem lies
in determining what is a significant discrepancy, since it is neither time- nor cost-efficient
to return too many hearing aids upon receipt,
especially if the manufacturer is unlikely to do
better on subsequent attempts . To make the
issue more difficult, none of the published formulas, to our knowledge, specifically states how
much fitting error can be tolerated before the
principles driving the method are violated . As
indicated by the data of Figure 3, acceptance
criteria for the ITEs that we used would have to
be lenient.
It is difficult to determine how much difference the deviations from target would make in
terms of patient performance and satisfaction .
Although the mSTI predictions do not account
for all subject and amplification variables or all
listening situations experienced, they suggest
that the degree of fitting error found would
detrimentally affect speech understanding in at
least some of the fittings . In addition, our philosophy, like most of the developers of prescriptive approaches, is to consider the target gain/
frequency response as only a starting point in
the fitting process. The "best" gain/frequency
response for an individual patient may deviate
from the prescribed gain/frequency response for
several reasons. First, most formula approaches
are based on averaged data for a number of
parameters, including real ear unaided response
(REUR) and real-ear-to-coupler correction factors . Some authors have recently recommended
measurement of an individual's REUR for incorporation into the calculation of target 2-cc
coupler gain (Mueller, 1989 ; Punch et a1,1990) .
Had this approach been used here, fitting error
for REIG may have been reduced for some ears .
Also, the MSU procedure recommends accomplishing audiometrics and loudness measurements with the use of 2-cc calibrated insert
earphones, which directly compare to 2-cc calibrated hearing aids, rather than with the 6-cc
calibrated supra-aural earphones that were used
in this study.
Second, and perhaps more importantly, most
of the prescriptive approaches are based only on
audiometric threshold and loudness measurements, while it is well known that patients may
differ in suprathreshold, psychoacoustic, and
speech recognition abilities despite similarities
in audiograms . For this reason, and as discussed by Bratt and Sammeth (1991), we believe that evaluation of an individual patient's
aided performance using speech or speech-like
stimuli, and adequate follow-up appointments,
should be crucial components of the clinical
protocol, following a reasonable approximation
to target gain . Unfortunately, adjustments to
improve speech intelligibility would most commonly be desired in the direction of greater
high-frequency gain, which these data indicate
would be difficult, if not impossible, to achieve
without considerable alteration in earmolds and/
or hearing aid circuitry.
The degree of deviation from target found in
this study for SSPL-90 curves under the POGO
II and MSU methods was also of concern. Although it is important to limit maximum power
output so that there is neither danger of
overamplification nor significant loudness discomfort, we found only a few instances where
inadequate limiting occurred . Rather, the majority of hearing aids arrived from the manufacturers with less available output than was requested, even though a 5 dB reserve had been
added. Significant underfitting of maximum
Journal of the American Academy of Audiology/Volume 4, Number 1, January 1993
output may have detrimental effects on a patient'shearing aidsatisfaction, just as overfitting
does . High gain combined with low SSPL-90 in
a hearing aid can lead to a condition known as
"inadequate headroom" (Preves and Newton,
1989). When a hearing aid has inadequate headroom, clipping may occur too frequently, producing distortion products that may affect speech
intelligibility and sound quality. Recently, Fortune and Preves (1992) reported that aided
UCLs of hearing-impaired subjects decrease as
SSPL-90 decreases, presumably because of increases in the distortion produced . As a consequence, measurement of UCLs under audiometric headphones for purposes of setting SSPL90 in limited-headroom hearing aids, as proposed by the majority of formula approaches,
may not be justifiable .
In summary, a relatively large mean degree
of deviation from target for gain/frequency response and SSPL-90 were found with the custom linear ITE hearing aids that we evaluated.
Although the prescribed gain/frequency response
is considered to be only a starting point in the
hearing aid fitting process, it would certainly be
desirable to achieve closer approximations .
Customizing the prescription using the individual's own REUR is one approach that may
lead to a closer match to target . It is also important to note that there have been recent technological advances that allow greater flexibility in
shaping the frequency response . The ITEs used
in this study represent technology available
from 1989 to 1991 . Specifically, these ITEs had
an amplifier with a class A output stage, tone
controls with a limited range of variability, and
receivers with a primary frequency response
peak around 2000 Hz and a relatively narrowband response . Many ITEs currently being produced use either class D or class B amplifier
output stages, receivers with a wider band frequency response and a higher frequency primary resonance peak, and tone controls consisting of two- or even four-pole filters . With these
newer-technology hearing aids and with wider
availability of multichannel technology, better
approximations to target values can be expected
(e .g., see Sammeth et al, in press) . Finally, the
use of compression circuitry or of new class D
amplifiers that provide higher output levels
may help to reduce problems with excess distortion resulting from inadequate headroom .
Acknowledgment . Portions of these data were previously presented in achapter by Bratt G, Sammeth C. In :
Studebaker G, Bess F, Beck L, eds. The VanderbiltIVA
Hearing Aid Report II. Parkton, MD : York Press, 1991 .
Appreciation is extended to Blake Lazenby, Susan
Logan, Valerie Matlock, and Andrea Williams for assistance with data collection, to Donna Buckman for database programming, to Christine Hughes for data entry,
and to Don Riggs for graphics support.
This research was supported by Veterans Administration funded project #C307-RA .
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