Download Date of submission: September 30, 2010 Title: Transitioning hearing

Convery and Keidser / Gain transitions for severe and profound losses
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Date of submission: September 30, 2010
Title: Transitioning hearing aid users with severe and profound loss to a new gain/frequency
response: benefit, perception, and acceptance
Authors: Elizabeth Convery, Gitte Keidser
Institutional Affiliation: National Acoustic Laboratories and The HEARing Cooperative
Research Centre
Corresponding Author: Elizabeth Convery, National Acoustic Laboratories, 126 Greville
Street, Chatswood NSW 2067, Australia; Phone: +61 2 9412 6889; Fax: +61 2 9411 8273; Email: [email protected]
Portions of this paper were presented at the 30th International Congress of Audiology in São
Paulo, Brazil, April 2010 and at Audiology Australia’s 19th National Conference and
Workshops in Sydney, Australia, May 2010.
The hearing aids and fitting software used in this study were provided by Siemens
Audiologische Technik, Erlangen, Germany.
Convery and Keidser / Gain transitions for severe and profound losses
ABSTRACT
Background: Adults with severe and profound hearing loss tend to be long-term, full-time
users of amplification who are highly reliant on their hearing aids. As a result of these
characteristics, they are often reluctant to update their hearing aids when new features or
signal-processing algorithms become available. Due to the electroacoustic constraints of
older devices, many severely and profoundly hearing-impaired adults continue to wear
hearing aids that provide more low- and mid-frequency gain and less high-frequency gain
than would be prescribed by the National Acoustic Laboratories’ revised formula with
profound correction factor (NAL-RP).
Purpose: The purpose of the study was to investigate the effect of a gradual change in
gain/frequency response on experienced hearing-aid wearers with moderately-severe to
profound hearing loss.
Research Design: The study was a double-blind, randomized controlled trial.
Study Sample: Twenty-three experienced adult hearing-aid users with severe and profound
hearing loss participated in the study. Participants were selected for inclusion if the
gain/frequency response of their own hearing aids differed significantly from their NAL-RP
prescription. Participants were assigned either to a control or to an experimental group
balanced for aided ear three-frequency pure-tone average (PTA) and age.
Intervention: Participants were fitted with Siemens Artis 2 SP behind-the-ear hearing aids
that were matched to the gain/frequency response of their own hearing aids for a 65 dB SPL
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Convery and Keidser / Gain transitions for severe and profound losses
input level. The experimental group progressed incrementally to their NAL-RP targets over
the course of 15 weeks, while the control group maintained their initial settings throughout
the study.
Data Collection and Analysis: Aided speech discrimination testing, loudness scaling, and
structured questionnaires were completed at three, six, nine, 12, and 15 weeks post-fitting. A
paired comparison between the old and new gain/frequency responses was completed at one
and 15 weeks post-fitting. Statistical analysis was conducted to examine differences between
the experimental and control groups, and changes in objective performance and subjective
perception over time.
Results: The results of the study showed that participants in the experimental group were
subjectively accepting of the changes to their amplification characteristics, as evidenced by
non-significant changes in the ratings of device performance over time. Perception of
loudness, sound quality, speech intelligibility, and own voice volume did not change
significantly throughout the study. Objectively, participants in the experimental group
demonstrated poorer speech discrimination performance as the study progressed, but no
change in objective loudness perception. According to the paired comparison, there was an
overall subjective preference for the original gain/frequency response among all participants,
although participants in the experimental group did show an increase in preference for the
NAL-RP response by the end of the study.
Conclusions: Based on our findings, we suggest that undertaking a gradual change to a new
gain/frequency response with severely and profoundly hearing-impaired adults is a feasible
procedure. However, we recommend that clinicians select transition candidates carefully and
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Convery and Keidser / Gain transitions for severe and profound losses
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initiate the procedure only if there is a clinical reason for doing so. A validated prescriptive
formula should be used as a transition target and speech discrimination performance should
be monitored throughout the transition.
KEY WORDS
Hearing aids and assistive listening devices, Auditory rehabilitation, Speech perception
ABBREVIATIONS
ANOVA = analysis of variance; BEST = Beautifully Efficient Speech Test; BKB = BamfordKowal-Bench sentence lists; BTE = behind-the-ear hearing aid; CR = compression ratio; CT
= compression threshold; ILTASS = international long-term average speech spectrum;
MCL/R = most comfortable loudness level and range; MPO = maximum power output; NAL
= National Acoustic Laboratories; NAL-NL1 = National Acoustic Laboratories’ nonlinear
formula version 1; NAL-RP = National Acoustic Laboratories’ revised formula with
profound correction factor; NR = noise reduction; P1 = program 1; P2 = program 2; PTA =
three-frequency pure-tone average (0.5, 1, and 2 kHz); RECD = real ear to coupler
difference; SNR = signal-to-noise ratio; SRT = speech reception threshold; TTS = temporary
threshold shift; VC = volume control; WDRC = wide dynamic range compression
Convery and Keidser / Gain transitions for severe and profound losses
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INTRODUCTION
Severely and profoundly hearing-impaired adults are often long-term, full-time users of
amplification who, because of their degree of loss, are highly reliant on their devices. Their
amplification needs are unique: individuals in this population require that a wide range of
input levels be made audible, comfortable, and safe within a narrow range of residual hearing
(Kuk and Ludvigsen, 2000). This is potentially achieved by the use of such features as
multichannel wide dynamic range compression (WDRC), microphone directionality, and
noise reduction (NR) algorithms.
The theoretical benefit of WDRC is clear, as the technology enables devices to meet the
needs of hearing aid users who have high gain and output requirements and a narrow
dynamic range. Evidence supporting the use of WDRC for individuals with severe and
profound hearing loss in both real-world and laboratory environments has been reported in
the literature. Subjective benefit has been demonstrated both for adults (Ringdahl et al, 2000;
Barker et al, 2001) and children (Marriage et al, 2005; Flynn et al, 2004) with severe and
profound hearing loss, with participants in these studies judging multichannel WDRC to be
superior to linear amplification for listening to speech in quiet, understanding speech in noise,
watching television, and listening at a distance. The Ringdahl et al (2000), Marriage et al
(2005), and Flynn et al (2004) studies additionally examined objective benefit through the use
of speech discrimination tasks. Participants in the Ringdahl et al (2000) study performed
significantly better with WDRC on tests of speech recognition in quiet, while Marriage et al
(2005) reported that pediatric participants with profound hearing loss showed a significant
benefit from WDRC on one test in their battery. Higher speech recognition scores for the
multichannel WDRC condition were reported by Flynn et al (2004), and this improvement
Convery and Keidser / Gain transitions for severe and profound losses
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was maintained at all test intervals throughout the 12-month study period. Across the three
studies, however, there was variation in the number of channels across which WDRC was
implemented, and the devices used in the studies had a range of different time constants,
compression ratios (CR), and compression thresholds (CT), so it is unknown to what extent
these variables affected the study outcomes.
With respect to the efficacy of such noise management strategies as directional microphones
and NR algorithms for individuals with severe and profound hearing loss, directional
microphones have been shown to improve speech recognition in noise for hearing aid users
with severe and profound hearing loss (Kühnel et al, 2001; Ricketts and Hornsby, 2006), and
to improve their subjective ratings of sound quality and speech intelligibility relative to an
omnidirectional microphone (Kühnel et al, 2001). However, both studies were conducted
under controlled laboratory conditions, and a directional advantage measured in the
laboratory is not necessarily indicative of success with directional microphones in the real
world (Cord et al, 2002; Cord et al, 2004; Wu and Bentler, 2010).
Although there is no direct evidence to support or refute the use of NR algorithms in devices
for people with severe or profound hearing loss, it is possible that the feature may reduce
overall audibility and therefore have a negative impact on speech understanding for this
group. The real-life benefit of a combination of directional microphones and NR algorithms
has been evaluated by Keidser et al (2008). The results suggest that a combination of
directional microphones and NR algorithms can be beneficial to hearing-aid users with severe
or profound loss in terms of increasing comfort in noisy environments, but that they should
be enabled in a secondary listening program that targets such environments rather than
activated on a full-time basis for general listening.
Convery and Keidser / Gain transitions for severe and profound losses
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Until relatively recently, the evidence to support the use of advanced signal processing
technology for this population has been sparse, which may contribute to the reluctance of
audiologists to fit their severely and profoundly hearing-impaired clients with the features
described above. Audiologists also may face opposition from the clients themselves, as
hearing-aid users with severe and profound hearing loss may, due to their dependence on
amplification, perceive that the risks of altering their amplification characteristics outweigh
the potential benefits (Keidser et al, 2007; Keidser et al, 2008). When participants in the
Keidser et al (2007) study were initially introduced to nonlinear amplification, using a gainfrequency response shape that often provided less gain across the low and mid frequencies
than they were used to, many commented that the devices seemed insufficiently loud relative
to their own linear or near-linear hearing aids. Consequently, these participants were
concerned that the new lower gain levels for medium- and high-input levels would prevent
them from adequately hearing, and therefore functioning, in their daily life. The combined
effect of clinician and client reluctance to take advantage of the benefits of advanced
technology means that severely and profoundly hearing-impaired clients often continue to
wear older devices with basic technology longer than do hearing aid-users with a milder
degree of loss. For example, older devices had a narrower bandwidth and less sophisticated
feedback management algorithms than do modern hearing aids, meaning that high-frequency
gain targets were often not met for individuals with significant hearing loss in this frequency
range. As a consequence, hearing-aid users with severe and profound hearing loss often were
provided with, and have therefore adapted to, compensatory gain in the low and mid
frequencies. This was evidenced by a comparison of the gain/frequency responses, as
measured for a 65 dB SPL input level, of the participants’ own devices in the Keidser et al
(2007) study and the National Acoustic Laboratories’ revised formula with profound
Convery and Keidser / Gain transitions for severe and profound losses
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correction factor (NAL-RP [Byrne et al, 1991]). A frequency response that provides gain in
the low and mid frequencies that is significantly higher relative to gain in the high
frequencies is likely to have detrimental effects on speech understanding due to upward
spread of masking, particularly in environments with high levels of low frequency-weighted
background noise (Dillon, 2001).
During the recent studies conducted at our laboratory using experienced hearing-aid wearers
with severe and profound hearing loss, we have observed that transitioning this population to
new amplification characteristics requires specific management strategies designed to
decrease the degree of risk perception and to ease the transition process (Convery et al,
2008). Techniques such as the optimization of the gain/frequency response prior to the
implementation of a new feature and the use of multiple memories to allow a pairwise
comparison of settings have been shown to facilitate the transition between old and new
settings, at least for the introduction and optimization of compression (Keidser et al, 2007).
The concept of a commercially-available device that allows the audiologist to alter gain,
compression, bandwidth, and attack time incrementally over time has been described in the
literature (Schum, 2001), and such devices are available today. At least two hearing aid
manufacturers (Beltone, Oticon) have implemented acclimatization managers that operate
automatically, although they are limited to adjustments of overall gain. However, the issue of
a transition to a new and more appropriately prescribed gain/frequency response has not yet
been systematically examined in the context of long-term users of amplification with severe
and profound hearing loss.
The primary objective of this double-blind, randomized controlled trial was to investigate the
impact of a gradual transition to new amplification characteristics among experienced
Convery and Keidser / Gain transitions for severe and profound losses
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hearing-aid users with moderately severe to profound hearing loss, with a particular focus on
the potential subjective and objective benefit of the transition. As the aim of the study was to
transition hearing-aid users from their current gain/frequency response to the response shape
prescribed by NAL-RP, enrollment in the study was limited to those participants whose
gain/frequency response as measured for a medium input level (65 dB SPL) differed
significantly from the NAL-RP prescription. Both the control and experimental groups were
exposed to the same test protocol to eliminate the effects of clinician attention and the
novelty of receiving new devices. Additionally, the study examined whether the experimental
participants noticed the gradual change in gain by exploring the extent of perceptual
disturbance experienced during the period in which gradual gain changes occurred.
METHODOLOGY
Participants
Twenty-three experienced adult hearing aid users, 22 of whom were bilaterally fitted,
completed the study. Participants were selected for inclusion if the gain/frequency response
of their own device(s) for a 65 dB SPL input and their NAL-RP targets (Byrne et al, 1991)
differed by ≥ 4 dB overall, ≥ 6 dB in two or more of four frequency bands, or ≥ 8 dB in a
single frequency band. Twenty-two participants met this criterion in both ears, while one
participant met the criterion in one ear. There were 12 male and 11 female participants who
ranged in age from 22 to 86 years, with a median age of 67 years. The three-frequency puretone average (PTA) of the participants’ fitted ears ranged from 60 to 103 dB HL, with a mean
PTA of 78 dB HL and a standard deviation of 12 dB HL. Figure 1 shows the mean and range
of the participants’ audiometric thresholds. Twelve participants had congenital hearing losses,
Convery and Keidser / Gain transitions for severe and profound losses
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while 11 participants had acquired losses. Twenty-two participants, including the monaurally
fitted participant, had bilateral sensorineural losses, while one bilaterally fitted participant
had a mixed loss with a stable conductive component in the right ear and a sensorineural loss
in the left ear.
Participants were allocated either to an experimental group or to a control group by an
audiologist who was uninvolved with participant testing. Group assignment was doubleblinded and was conducted such that the two groups were matched as closely as possible for
aided ear PTA and age. For both variables, both the mean and range were considered, with
the PTA given greater priority over age. The PTA of the experimental group ranged from 60
to 99 dB HL, while the PTA of the control group ranged from 63 to 98 dB HL. The mean
PTA was 79 dB HL for both groups. The ages of the participants in the experimental group
ranged from 22 to 86 years, with a median age of 64 years. The median age of the
participants in the control group was 68 years and ranged from 24 to 77 years. A t-test for
independent samples revealed no significant differences between the control and
experimental groups in terms of aided ear PTA (p=0.99) or age (p=0.67). The two groups
were also reasonably balanced for gender and origin of hearing loss, with six males and five
females in the control group and six males and six females in the experimental group. Seven
participants in the control group had congenital hearing losses and four had acquired losses,
while the experimental group was composed of five participants with congenital losses and
seven with acquired losses.
The treatment of participants in this study was approved by the Australian Hearing Ethics
Committee and conforms in all respects to the Australian government’s National Statement
on Ethical Conduct in Human Research.
Convery and Keidser / Gain transitions for severe and profound losses
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Test Device and Fitting
The test device used in the study was the Siemens Artis 2 SP, a digital, four-program, 12channel behind-the-ear (BTE) hearing aid. The adaptive features of the Artis 2 SP include
multichannel operation of a directional microphone (omnidirectional, hypercardioid
directional, or automatic switching between the two modes), automatic and adaptive feedback
cancellation (slow or fast base setting), adaptive noise reduction and speech enhancement
(four levels from minimum to maximum), and a wind noise reduction algorithm (on or off).
The device employs automatic listening situation detection and DataLearning™ technology,
the latter of which records such information as the number of hours the devices have been
worn and the percentage of time spent in each of five acoustic environments (speech, speech
in noise, noise, music, and quiet) and in each of the active listening programs. None of the
participants had prior experience with the test device.
The gain/frequency responses of the test devices were adjusted in a 2 cc coupler to match the
amplification characteristics of the participants’ own devices for an input level of 65 dB SPL.
This program was implemented in program 1 (P1) for all participants at the initial fitting and
is referred to hereafter as “mimic fit”. A CT of 50 dB was set in channels 1 and 2, and a CT
of 45 dB was set in channels 3 and 4. Channel 1 had a CR of 1.45:1, channels 2 and 3 had
CRs of 2:1, and channel 4 had a CR of 2.29:1 as per Keidser et al (2007). All adaptive
features were disabled with the exception of the feedback canceller, which was set to the
default fast base setting. An omnidirectional microphone mode was selected for all
participants. The volume control (VC) was enabled for all participants with an 8 dB (±4 dB)
Convery and Keidser / Gain transitions for severe and profound losses
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range. All participants were instructed to use the VC on an emergency basis only to ensure
that they were acclimatising to the gain/frequency response as programmed.
Real ear to coupler difference (RECD) measurements were made preparatory to verification
of the hearing-aid settings in the coupler. Using the 6 kHz notch method (Dillon, 2001) to
position the probe tube, the participant’s earmold was then placed into the ear, with a piece of
tubing connecting the earmold tubing with the output transducer of the probe assembly. The
RECD was measured in response to a 70 dB SPL warble tone. The mimic fit setting was
verified in the coupler in response to international long-term average speech spectrum
(ILTASS [Byrne et al, 1994]) noise presented at 65 dB SPL.
The test devices were then programmed to match NAL-RP targets and verified in the 2 cc
coupler, with individually measured RECD values taken into account. The NAL-RP program
was stored in program 2 (P2) in the fitting software. This program was not available to the
participants during field testing, but was retrieved during laboratory appointments for
comparison tests. Although the test devices were nonlinear, NAL-RP was selected as the
target gain/frequency response as more extensive validation of this formula has been
undertaken for this hearing loss group (Byrne et al, 1990; Byrne et al, 1991) compared to the
National Acoustic Laboratories’ nonlinear formula version 1 (NAL-NL1 [Byrne et al, 2001),
which, due to its derivation method, often does not prescribe targets for low- and highfrequency thresholds in the severe and profound range.
The default maximum power output (MPO) settings for both the mimic fit and NAL-RP
programs were the same. They were evaluated by presenting participants with 5-10 seconds
of a low-frequency-weighted stimulus (traffic noise) at 80 dB SPL, a high-frequency-
Convery and Keidser / Gain transitions for severe and profound losses
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weighted stimulus (percussion sounds) at 75 dB SPL, and a broadband stimulus (applause) at
75 dB SPL in the sound field. All stimuli were recorded sounds presented from a Yamaha
Natural Sound CDX-530 CD player via a Yamaha Natural Sound AX-350 stereo amplifier
and an Aaron loudspeaker. The loudspeaker was positioned 1.5 m away from and at 0°
azimuth relative to the participant. Participants were asked to rate the loudness of each
stimulus using the seven-point categorical loudness scale from the Contour Test (Cox et al,
1997). Two participants required a decrease in the default MPO setting for both programs,
while another participant preferred peak clipping to the default AGC-O in both programs.
Test Materials
Paired Comparison: Participants completed a paired comparison between the mimic fit and
NAL-RP settings in response to live dialogues between two female talkers. The dialogues
were 12 extracts of text adapted from various modern novels. One female talker was fixed,
while the second female talker varied across participants according to availability among
staff. With the exception of four participants, the second female talker remained the same at
the repeated measure. Moving in a balanced order between four environments (café, office,
reverberant stairwell, and outdoors with distinct traffic noise in the background), the female
talkers, positioned about 1 m from the participant and facing each other, read the dialogues
aloud with a normal vocal effort. Switching between the two programs during the test was
effected by the fixed female talker via a remote control, and program change beeps were
disabled in the devices so that participants did not know which program was active.
Participants were instructed to prompt program switching in an A-B-A paradigm to determine
their preferred setting, the strength of their preference (much better, moderately better,
slightly better, no difference), and the criterion they used to select their preferred setting
Convery and Keidser / Gain transitions for severe and profound losses
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(speech clarity, loudness comfort, naturalness, other). The task was repeated three times in
each of the four environments, for a total of 12 paired comparisons.
Speech Discrimination Testing: The Beautifully Efficient Speech Test (BEST [Schmitt,
2004]) was used to measure the aided speech reception threshold (SRT) in quiet, or the level
at which 50% speech intelligibility is achieved. Using the same equipment setup described
for the MPO verification testing above, the level of each sentence in the test was altered
adaptively by ±1 dB in accordance with the participant’s response to the previous sentence.
Sentences were scored morphemically. Prior to each list, four sentences from the BamfordKowal-Bench (BKB [Bench et al, 1979]) lists were presented as practice. The presentation
level of the practice sentences were also varied adaptively in 1-dB steps, but reversals were
not included in the determination of the participant’s SRT. The order of BEST lists across
appointments was randomized according to a balanced Latin square. Speech testing was not
completed for three participants who were unable to correctly identify ≥50% of the
morphemes in sentences presented at the maximum presentation level of 80 dB SPL.
Comfort Level and Range: Using the same equipment setup as described for the MPO
verification testing above, the most comfortable loudness level and range (MCL/R) were
measured by presenting participants with a recording of continuous male speech from the
NAL CD of Speech and Noise for Hearing Aid Evaluation (Keidser et al, 2002). Participants
were asked to use an unmarked rotary dial to adjust the speech to levels that were
comfortable but soft, comfortable, and comfortable but loud. Randomly-selected starting
levels between 55 and 70 dB SPL were chosen for each of the four test runs. The MCL was
determined by calculating the median SPL selected for the comfortable category in the last
three runs, while the MCR was determined by calculating the difference between the median
Convery and Keidser / Gain transitions for severe and profound losses
15
SPLs selected for the comfortable but soft and comfortable but loud categories in the last
three runs.
Questionnaire: The questionnaire, which was devised specifically for this study, elicited
information about the participant’s overall perception of loudness comfort, speech
intelligibility, and sound quality; use of the volume control; use of the participant’s own
hearing aids; and changes, if any, in the audibility and comfort of sounds.
Procedure
Participants attended the laboratory for eight appointments. Otoscopy, tympanometry, and
pure-tone air- and bone-conduction audiometry were completed at the first appointment. The
participants’ own hearing aids were measured in a 2 cc coupler in response to ILTASS noise
and RECD measurements were carried out with the participants’ own earmolds. Ear
impressions were taken for those participants who required new earmolds. Baseline SRT and
MCL/R testing was completed with the participants’ own hearing aids.
Preference for either the mimic fit (P1) or NAL-RP (P2) setting was established at one week
and 15 weeks post-fitting through paired-comparison testing. Following the paired
comparison task at one week post-fitting, incremental programs were created and stored in
the hearing-aid fitting software for future retrieval. Each of the incremental programs
represented a 25% progression from the gain/frequency response of the participants’
individual mimic fit settings to their individual NAL-RP prescription. For example, the
mimic fit settings of one participant provided 58 dB of gain at 2 kHz, whereas NAL-RP
prescribed 50 dB of gain at this frequency. His incremental programs were therefore set to
Convery and Keidser / Gain transitions for severe and profound losses
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provide, at 2 kHz, 56 dB at week 3, 54 dB at week 6, 52 dB at week 9, and 50 dB (i.e. the
NAL-RP target) at week 12. The device settings of the participants in the experimental group
were incrementally changed from mimic fit to NAL-RP at three, six, nine, and 12 weeks postfitting, while participants in the control group retained the mimic fit program for the entire
study period. To ensure that neither the test audiologist nor the participant knew whether or
not the incremental programs had been activated (and hence to which group the participant
belonged), all participants were instructed to leave their hearing aids in the test booth at the
end of each appointment. The incremental programs were then recalled only into the devices
of the experimental participants by an audiologist who was otherwise uninvolved in the
study.
Speech discrimination testing, loudness scaling, and questionnaire administration were
completed for all participants at three, six, nine, 12, and 15 weeks post-fitting. Pure-tone air
conduction thresholds were measured bilaterally at 0.5, 1, 2, and 4 kHz as a check for
temporary threshold shift (TTS) at weeks 3 and 15.
RESULTS
Device Usage and Fitting
Device usage was measured using the Artis 2 SP’s DataLogging™ feature. The mean number
of hours the devices were worn during each three-week test period was 252, or an average of
12 hours per day. A repeated measures analysis of variance (ANOVA), with hours of use as
observations, time as the repeated measure, and group as the grouping variable, revealed no
significant main effect of group (F1,21 = 1.96, p = 0.18) or time (F4,84 = 1.39, p = 0.24). There
Convery and Keidser / Gain transitions for severe and profound losses
17
was a tendency for participants in the experimental group to wear the test devices less than
those in the control group, and for this difference to decrease over the course of the study.
However, the interaction between time and group was not significant (F4,84 = 1.30, p = 0.28).
Figure 2 shows the mean achieved 2 cc coupler gains for the mimic fit and NAL-RP
programs for all participants. Error bars show plus and minus one standard error of the mean.
A repeated measures ANOVA revealed a significant effect of frequency (F8,64 = 132.5, p <
0.05) and a significant interaction between frequency and program (F8,64 = 15.4, p < 0.05).
According to a Tukey HSD post hoc test of means, significantly more gain was implemented
for the mimic fit program at 0.25, 1, 1.5, and 2 kHz, and for the NAL-RP program at 6 and 8
kHz. The mean difference between mimic fit and NAL-RP reflects what is commonly
observed with this hearing loss group: that the participants’ own devices provided more midfrequency gain and less high-frequency gain than is prescribed by NAL-RP.
Effect of Test Device
The SRT and MCL/R was measured with the participants’ own hearing aids at the assessment
appointment and with the study hearing aids at subsequent appointments. To determine the
effect of changing to new devices, a t-test was performed on the data obtained at week 3,
which was the first time the SRT was conducted with the test devices, and at the assessment.
According to this test, there was no significant effect of changing to new devices (p = 0.69).
Similarly, a Wilcoxon matched pairs test was conducted to determine whether there were
significant differences between the intensity levels that were chosen for the comfortable but
soft, comfortable, and comfortable but loud categories with the participants’ own devices at
the assessment and with the study devices at week 3. No significant difference was found for
Convery and Keidser / Gain transitions for severe and profound losses
18
the comfortable (p = 0.12) or comfortable but loud (p = 0.24) categories. However, the mean
level chosen as comfortable but soft was significantly lower (p = 0.05) at week 3 than at the
assessment.
Objective Benefit
Speech Intelligibility: A repeated measures ANOVA was completed to determine whether
there was any effect of participant group (control or experimental) on change in SRT over
time, using the SRT scores as observations, time as the repeated measure, and group as the
grouping variable. Three participants, two from the control group and one from the
experimental group, were excluded from the analysis as they were unable to complete the
speech test. The results of the repeated measures ANOVA revealed no significant main effect
of group (F1,17 = 2.21, p = 0.16) or time (F4,68 = 0.31, p = 0.87) on SRT score. However, there
was a significant interaction between group and time (F4,68 = 5.77, p = 0.0005), indicating a
tendency for participants in the experimental group to produce poorer SRT scores over time,
while the participants in the control group produced better SRT scores as the study
progressed (Figure 3). A Tukey HSD post hoc test of means demonstrated that although there
was no significant difference between SRT scores produced by the two groups at each test
period, the lowest p-levels are seen at weeks 12 and 15.
Loudness Perception: A t-test for independent samples revealed no significant difference
between the levels selected for the baseline MCL and MCR ratings by the control and
experimental groups at the initial assessment, when participants were tested with their own
hearing aids (p = 0.77 for comfortable but soft, p = 0.74 for comfortable, p = 0.69 for
comfortable but loud). A repeated-measures ANOVA was conducted to determine whether
Convery and Keidser / Gain transitions for severe and profound losses
19
there was a significant change in participants’ MCL over time, and whether there was a
significant difference between the levels chosen by the experimental and control groups. The
selected levels were used as observations, time was used as the repeated measure, and group
was used as the grouping variable. No significant main effect of group (F1,20 = 0.95, p = 0.34)
or time (F4,80 = 1.55, p = 0.20) was observed, nor was there any significant interaction
between time and group (F4,80 = 0.96, p = 0.44). There was, however, a tendency for
participants in the experimental group to select slightly higher MCLs over time, which is
consistent with the gradual reduction in mid-frequency gain they experienced as the NAL-RP
gain/frequency response was approached.
Change in MCR was analyzed with a repeated-measures ANOVA using the difference
between the SPLs chosen as comfortable but soft and comfortable but loud as observations,
time as the repeated measure, and group as the grouping variable. There was no significant
main effect of group (F1,21 = 0.73, p = 0.40) or time (F4,84 = 0.67, p = 0.62), nor was there any
significant interaction between group and time (F4,84 = 0.58, p = 0.68), indicating that the
range of comfortable input levels remained constant over time for both groups.
Subjective Benefit
Each participant’s preference for either the NAL-RP or mimic fit program in the paired
comparison task was quantified by assigning 3 points if the preferred program was judged to
be much better, 2 points for moderately better, and 1 point for slightly better. For each of the
24 paired comparisons completed by the participant (12 each at weeks 1 and 15), the nonpreferred program was allocated the equivalent negative number of points. This method of
Convery and Keidser / Gain transitions for severe and profound losses
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analyzing the data increases the likelihood that the observations fall along an interval scale
and thus allows parametric statistical tests to be used (Keidser et al, 1995).
A repeated-measures ANOVA was completed using time, environment, and listening
program as the repeated measures and participant group as the grouping variable. There was a
significant main effect of listening program (F1,20 = 15.5, p = 0.0008), suggesting that the
mimic fit program was preferred overall to NAL-RP. A significant interaction between
environment and listening program was also observed (F3,60 = 3.23, p = 0.03). A Tukey post
hoc test of means showed a significant preference for mimic fit in two environments (office
and outdoors) and a nonsignificant preference for mimic fit in the other, noisier environments
(café and stairwell). Participants also were asked to specify the criterion they used to select
their preferred program. The most commonly-used selection criterion was speech clarity,
followed by loudness comfort and naturalness, and this was irrespective of whether mimic fit
or NAL-RP was chosen.
The interaction between time, listening program, and group neared but did not reach
significance (F1,20 = 3.20, p = 0.09). However, there was a tendency for participants in the
control group to maintain or slightly increase their preference for mimic fit between weeks 1
and 15, while participants in the experimental group showed a greater preference for NALRP and a lesser preference for mimic fit at week 15 than they did at week 1 (Figure 4). This
outcome suggests that experimental participants did undergo some adaptation to the new
settings during the 15-week study period.
Perceptual Disturbance
Convery and Keidser / Gain transitions for severe and profound losses
21
Subjective perceptual disturbance was measured by asking participants to rate both the degree
to which the listening program in the current test period differed from the last test period, and
the degree to which the difference was perceptually disturbing. On a scale of 0 to 3, with 0
representing no perceptual disturbance and 3 representing very disturbing, the average
perceptual disturbance score across all appointments was 0.3 for the control group (despite
this group having experienced no changes to their gain/frequency response) and 0.5 for the
experimental group. While there was a tendency for the control group to report less
perceptual disturbance over time, the ratings provided by the experimental group increased
(Table I). However, according to Friedman ANOVAs, there was no significant effect of time
within either group (p > 0.15), and Mann-Whitney U tests showed no significant effect of
group at any appointment time (p > 0.07).
Acceptance of New Amplification
Several items on the questionnaire were designed to elicit information about the participants’
perception of loudness, sound quality, speech intelligibility, and own voice volume and the
extent to which these perceptions changed over time. While speech intelligibility was rated
on a scale from 1 (very muffled) to 10 (very clear), loudness, sound quality, and own voice
volume were rated on a scale from -5 (much too soft, much too dull, and much too soft,
respectively) to +5 (much too loud, much too sharp, and much too loud, respectively), with
zero indicating perfect perception. The mean and standard error values are shown for each
group and for each appointment time in Table I. On average, both groups found the sound
slightly soft and dull. Speech intelligibility was rated as somewhat clear, and own voice
volume was close to perfect. A Mann-Whitney U test showed no significant difference in
perception of these characteristics between the control and experimental groups (p > 0.09),
Convery and Keidser / Gain transitions for severe and profound losses
22
and therefore data from both participant groups were pooled. Using the rated scores for each
descriptor as observations, Friedman ANOVAs revealed no significant change in
participants’ perception of these characteristics throughout the course of the study (p > 0.33).
For all descriptors, the coefficient of concordance was very low (K < 0.05), which indicated a
low degree of agreement among study participants within each group.
Spearman rank order correlation analyses between subjective ratings of speech intelligibility
and objectively measured SRT, and between subjective ratings of loudness and objectively
measured MCL, were conducted. The analyses were based on the mean change in
performance at weeks 6, 9, 12, and 15 relative to week 3. No significant relationship between
the two pairs of variables was demonstrated (p = 0.73 for SRT, p = 0.85 for MCL), which
confirms an inconsistency between participants’ perception of speech intelligibility and
loudness and their performance on related objective tests (Figure 5). Graph 5a further
demonstrates that the improvement in SRT among control participants is mainly due to large
improvements by two individuals of 21.3 and 10.8 dB, respectively. For both participants, the
improvement was evident from week 6 and remained stable thereafter. Conversely, the
experimental participant who showed the greatest reduction in SRT of 11.2 dB required
increasingly higher presentation levels as the response approached the NAL-RP target. This
person transitioned to, on average, 10 dB less gain between 1 and 4 kHz and 25 dB more gain
at 6 kHz and above.
Participants were also asked to rate the overall performance of the test devices at the end of
each three-week test period. Mean ratings for each group and test period ranged from 6.3 to
7.4 out of a maximum possible rating of 10, with an overall average rating of 6.9 (Table I).
While there was a tendency for participants in the control group to rate the test devices more
Convery and Keidser / Gain transitions for severe and profound losses
23
highly in the final test periods than did participants in the experimental group, a MannWhitney U test revealed no significant difference in ratings between the two groups at any
appointment (p > 0.35). Pooling the data across groups, a Friedman ANOVA further revealed
no significant change in performance rating over time (p = 0.94).
DISCUSSION
Performance Differences
The results of this study indicate that experienced hearing-aid users with severe and profound
hearing loss are generally subjectively accepting of a transition to a new gain/frequency
response, as evidenced by non-significant changes in the mean ratings of loudness, sound
quality, speech intelligibility, own voice volume, and overall device performance in real life
over time, and by the lack of a significant difference in these ratings between the
experimental and control groups. Similarly, there was no significant effect of group or time
on perceptual disturbance ratings. However, these data did show that participants from the
experimental group, on average, increased their rating of perceptual disturbance at weeks 12
and 15 (Table I). The increase in mean rating was due primarily to four individuals, of whom
three reported the response to be softer and one reported the response to be louder than in
previous weeks. The reported change in loudness was, for these participants, also reflected in
their ratings of overall loudness in real life. Despite these variations, an outcome suggestive
of adaptation to the new gain/frequency response was demonstrated by the results of the
paired-comparison task, in which the experimental participants showed a decrease in
subjective preference for the mimic fit program by the end of the study (Figure 4).
Convery and Keidser / Gain transitions for severe and profound losses
24
The results of the speech tests, however, suggest that objective benefit is not apparent after a
15-week transition period. Participants in the control group showed a mean improvement in
SRT of 4 dB between the initial and the final test sessions, while the experimental group
demonstrated an average performance decrement of a similar magnitude over the same time
period (Figure 3). One variable that may have affected the change in speech discrimination
performance was the use of compression parameters in the test device that differed from
those in the participants’ own hearing aids. The mimic fit program matched the
gain/frequency response of the participants’ own devices for a 65 dB SPL input level only,
and did not attempt to match output at any other level. The CRs in the test device were
selected based on previous research into the compression parameters most preferred by
hearing-aid users with severe and profound hearing loss (Keidser et al, 2007), and were in
many cases higher than the linear or near-linear amplification participants had been wearing
prior to entering the study. Further, CTs between 45 and 50 dB in the test device tended to be
lower than those in the participants’ own devices, which reflects the fact that the test devices
provided more gain at lower input levels than did the participants’ own hearing aids. One
effect of the difference in compression parameters was observed when comparing the
baseline MCL/R results, which were completed at the initial appointment with the
participants’ own devices, with the first MCL/R results obtained with the test devices, at
week 3. The mean SPL chosen as comfortable but soft was significantly lower (p = 0.05) at
week 3 (58.5 dB SPL) than at the initial appointment (60.3 dB SPL). Other possible
differences between the test devices and the participants’ own devices include the use of
adaptive features, which were disabled in the test devices in an attempt to control extraneous
variables. These differences may account for the improvement in speech performance
measured for a few participants in the control group, possibly due to increased audibility of
lower level speech components. Specifically, the improvement in speech performance
Convery and Keidser / Gain transitions for severe and profound losses
25
observed among control participants occurred between three and six weeks after they were
fitted with the test device and its new compression parameters. This finding is therefore in
agreement with Kuk et al (2003) and Flynn et al (2004), both of whom demonstrated an
objective improvement in speech performance four and six weeks, respectively, after their
participants had been fitted with test devices with new compression parameters.
The decrement in speech performance seen among experimental participants in our study
was, however, more gradual. In addition to changed compression parameters, participants in
the experimental group had their gain/frequency responses altered every three weeks, which
means that experimental participants were exposed to four new and different gain/frequency
responses over the course of the study. One of the rationales for introducing the gain
transition gradually was to avoid a significant change to amplification that may be rejected
before the hearing aid user has had a chance to fully acclimatize to the new response. In
designing this study, it was hypothesized that the incremental steps (ranging from 2-8 dB on
average, depending on frequency) would be less intrusive and therefore easier to adapt to
over a three-week period. It is possible, however, that individuals with severe to profound
hearing loss are more sensitive to small changes in amplification than are those with milder
degrees of loss. If this were the case, acclimatization to each incremental change would have
been an issue for our participant group. Past research suggests that a period of three weeks is
insufficient for acclimatization to a new gain/frequency response to occur. For example, in a
study of monaurally-fitted hearing-aid users with mild to moderate hearing loss, Gatehouse
(1992) demonstrated that objective benefit from a new gain/frequency response, as measured
by aided speech discrimination testing, required between six and 12 weeks to become
apparent. A review of the acclimatization literature by Arlinger et al (1996) supports this
conclusion. These observations would also suggest that changing the shape of the
Convery and Keidser / Gain transitions for severe and profound losses
26
gain/frequency response may have a greater effect on speech discrimination performance
than does altering compression parameters.
Although the SRT scores of the experimental group as a whole worsened as participants
reached the NAL-RP target, an individual improvement in SRT between weeks 3 and 15 was
demonstrated by one participant, whose average improvement over time was 5 dB (Figure
5a), while others produced no change at all. It is possible that the individual variation in
speech test results is partially accounted for by variations in frequency resolution, as
indicated by the presence or absence of dead regions, among the study participants.
Cochlear dead regions are relatively common among individuals with severe and profound
hearing loss, with Moore et al (2003) reporting a prevalence rate of approximately 70%
among a group of severely to profoundly hearing-impaired teenagers. The presence of a
cochlear dead region affects the ability to extract meaningful information from a speech
signal, and the provision of amplification at frequencies at which a dead region is present
may increase audibility but not speech intelligibility (Moore, 2001). Findings reported in the
literature indicate that for individuals with cochlear dead regions, amplification should only
be extended to a frequency that is approximately 1.7 times that of the edge frequency of the
dead region, and that extending the frequency response beyond this range either confers no
further benefit, or can have detrimental effects on speech discrimination for some hearingimpaired people (Vickers et al, 2001; Baer et al, 2002). In contrast, both studies also showed
that people without cochlear dead regions demonstrate benefit from amplification over a
bandwidth of 7.5 kHz, both for speech in quiet (Vickers et al, 2001) and in noise (Baer et al,
2002). However, experiments by Ching et al (2005) indicate that age and audiometric
threshold, rather than frequency resolution, are more consistent predictors of the degree to
Convery and Keidser / Gain transitions for severe and profound losses
27
which an individual will be able to extract usable information from a speech signal. We
therefore believe it is unlikely that knowledge about participants’ dead regions would have
explained the individual variation in SRT observed in this study.
Comments on Methodology
During the study, the VC range was restricted to ±4 dB, and participants were instructed to
reserve VC use for emergencies only. At the end of each three-week test period, participants
were asked to report on their VC use. Ten participants in the experimental group reported
using the VC during at least one test period, while nine participants in the control group did
so. Of the VC users, the majority in both groups (10 experimental, seven control) indicated
that they mostly used the VC to increase the overall volume. However, according to VC use
data downloaded from the device’s DataLogging™ feature, all participants used the VC
during at least one test period. According to a Mann-Whitney U test, there was no significant
difference between the mean VC changes that were made by the experimental and control
groups, nor was there any systematic change in VC adjustments over time, suggesting that the
experimental subjects did not, on average, use the VC to compensate for the incremental
changes in gain in a systematic way. It is therefore unlikely that VC usage during the study
had a significant impact on the results.
For the paired comparison task, participants were asked to rate their preference for either the
mimic fit or the NAL-RP program in response to live dialogues performed by two female
talkers. One talker participated in all dialogues, while the second talker varied across
participants. While efforts were made to use the same pair of talkers at both test intervals, this
was not possible for two experimental and two control participants due to conflicts between
Convery and Keidser / Gain transitions for severe and profound losses
28
the participants’ appointment times and the work schedules of the talkers, who were all
employees of NAL. However, replacement talkers were selected to match, as closely as
possible, the voice characteristics (e.g. accent, fundamental frequency) of the original talkers.
Additionally, as an equal number of participants from each group was affected, it is unlikely
that talker variation in the paired-comparison task had a significant impact on the results.
At the end of the study, participants were given the opportunity to guess whether they had
been members of the experimental or the control group before this information was revealed
to them. Three control participants believed that their hearing-aid settings had been
systematically altered throughout the study and that they had therefore been part of the
experimental group. This perception was reflected in the control participants’ responses to the
questionnaire items that probed changes in sound perception. Although the gain/frequency
response of the test device did not change over time, some participants in the control group
reported the emergence of new sounds and the cessation of familiar sounds at all test intervals
subsequent to week 3. Such a pattern of responses cannot be explained by changes in gain, as
the control group wore the same settings throughout the study. Conversely, we cannot
exclude the possibility that participants in the experimental group reported changes in their
soundscape on the basis of their group membership belief, rather than on their actual
perception.
Previous research has shown that hearing-aid users with severe and profound hearing loss
require a great deal of supportive counseling when transitioning to new devices or settings
(Convery et al, 2008). Due to the double-blind nature of the study design, participants in the
current study did not know whether or not they were undergoing the transition, and hence
those who did undergo the transition did not receive a greater degree of clinical support than
Convery and Keidser / Gain transitions for severe and profound losses
29
did members of the control group. The constraints of a research study precluded a detailed
explanation by the audiologist of the expected perceptual differences between the old and
new settings or the option to adjust the pace of the transition based on client feedback, which
may have eased the transition process for some of the study participants. In a clinical context,
however, such support is not only possible but also strongly recommended. The role of
counselling and the most appropriate length and rate of a transition were not explored in this
study, but both issues could offer avenues for further investigation.
Assuming mean differences of 4 dB in the SRT test and 5 dB in the MCL test, the statistical
power was close to 80% ( = 0.05) for the repeated measures with about 12 participants in
each group, while the power for the effect of group was about 20%. This suggests that the
performance differences measured over time can be reported with more confidence than those
measured between the control and experimental groups.
While it would have been desirable to conduct this study with more participants in each
group, we note that recruitment of this specific population is especially difficult due to their
reluctance to trial new technology, an obstacle we have discussed earlier in this paper.
Additionally, we have found that there is a relative scarcity of people with severe and
profound hearing loss who do not have co-existing cognitive and/or physical disabilities that
would impact upon their ability to successfully participate in a research study. During the
recruitment phase of this study, we also noted that many people with our target degree of
hearing loss have obtained cochlear implants. Consequently, controlled trials with this
population are always likely to have low power, and clinicians are advised to look for
consistency across the results of multiple studies to find evidence for or against a particular
clinical technique.
Convery and Keidser / Gain transitions for severe and profound losses
30
Clinical Recommendations
Based on the findings of this study, we suggest that not only is there sufficient evidence to
support undertaking a transition to a new gain/frequency response with hearing-aid users who
have severe and profound hearing loss, but that the need to transition to a more appropriate
gain/frequency response is prevalent among hearing-aid users with severe and profound loss.
As detailed in the methodology section, one criterion for inclusion in the study was that the
response shape of the participants’ own devices must differ significantly from the NAL-RP
prescription. The first 23 hearing-aid users to volunteer for this study met this criterion,
entering the study with devices that, on average, overfitted them across the low and mid
frequencies and underfitted them in the high frequencies relative to NAL-RP. This
occurrence, combined with similar observations made during previous NAL studies that
focused on hearing-aid users with severe and profound loss, suggests that a significant
proportion of such users may benefit from a systematic transition to a new gain/frequency
response. To achieve this, it may be desirable for hearing-aid manufacturers to incorporate a
feature that automatically and gradually changes gain over a specified period of time into
high-power devices.
Due to the individual variation in SRT score that was observed in this study, the motivation
for undertaking a gain transition should stem from empirical evidence that the transition
would be appropriate for a particular individual. For example, temporary and permanent
threshold shift are risks for any hearing-aid user, and these risks increase significantly for
those users with severe and profound hearing loss (Humes and Bess, 1981; Macrae, 1991).
Clients whose current hearing-aid settings exceed the gain and/or MPO values recommended
Convery and Keidser / Gain transitions for severe and profound losses
31
by a validated prescriptive formula should have their settings adjusted to safer levels, and the
use of a gradual transition to achieve such levels is recommended. Client reports of poor
sound quality that may be related to upward spread of masking is also a valid reason for
initiating a gain transition. Similarly, if a client yields poorer aided speech discrimination
scores than would be expected relative to unaided scores and/or PTA, and objective
verification of the hearing-aid settings reveal that the client is not receiving optimum
audibility across the frequency range, a gradual transition to a more appropriate
gain/frequency response could be undertaken. We recommend that the aided speech
performance of clients undergoing a gain transition be monitored closely over time, with the
clinician prepared to slow or stop the transition if significant decrements in performance are
observed.
SUMMARY
The impact of a gradual transition to new amplification characteristics on experienced
hearing aid users with moderately severe to profound hearing loss was investigated in a
double-blind, randomized controlled trial. Twenty-three participants, whose current hearing
aids provided an average of more mid-frequency gain and less high-frequency gain than
would be prescribed by NAL-RP, were fitted with digital hearing aids matched to their own
devices’ gain/frequency response, with half the group progressing to their NAL-RP target
over the 15-week test period and half the group maintaining their initial settings throughout
the study. Speech discrimination and loudness scaling testing was completed at three-week
intervals, and subjective feedback was gathered through structured questionnaires and a
paired-comparison task.
Convery and Keidser / Gain transitions for severe and profound losses
32
The results of the study showed that participants were subjectively accepting of their new
amplification characteristics, as perception of loudness, sound quality, speech intelligibility,
and own voice volume did not vary significantly throughout the study. Participants in the
experimental group reported a similar level of perceptual disturbance to those in the control
group throughout the course of the study. Objectively, participants in the experimental group
demonstrated poorer speech discrimination as the study progressed, although there was no
change in their objective loudness perception. According to paired comparison tests, there
was an overall subjective preference for the original gain/frequency response among all
participants, although participants in the experimental group did show an increase in
preference for the new gain/frequency response by the end of the study. The various
subjective findings suggest that a gradual change is a viable procedure for managing the
transition to a new and more appropriate gain/frequency response among hearing aid users
with severe and profound loss. However, because of the impact on speech performance, on
average, the procedure should be undertaken only if there is a valid clinical reason for doing
so, and speech performance should be closely monitored during the transition period.
ACKNOWLEDGEMENTS
We would like to acknowledge Anna O’Brien, Margot McLelland, Ingrid Yeend, Megan
Gilliver, Vivian Fabricatorian, Pamela Jackson, Emma van Wanrooy, and Elizabeth Beach of
the National Acoustic Laboratories and Simone Siltmann and Dirk Junius of Siemens
Audiologische Technik for their contributions to this study.
FIGURE LEGENDS
Convery and Keidser / Gain transitions for severe and profound losses
33
Figure 1. Mean (black circles, solid line) and range (dotted lines) of the pure-tone airconduction thresholds of the participants’ fitted ears.
Figure 2. Mean 2 cc coupler gain values for the mimic fit program (white squares, dotted
line) and the NAL-RP program (black squares, solid line). Error bars show one standard error
of the mean.
Figure 3. Mean SRT scores at each test interval for the control group (white circles, dotted
line) and the experimental group (black circles, solid line). Error bars show 95% confidence
intervals.
Figure 4. Mean preference ratings for mimic fit relative to NAL-RP at each test interval for
the control group (white circles, dotted line) and the experimental group (black circles, solid
line). A positive score indicates a preference for mimic fit and a negative score indicates a
preference for NAL-RP. Error bars show 95% confidence intervals.
Figure 5. The correlation between (a) relative SRT score and relative reported speech
intelligibility rating, and (b) relative MCL and relative reported loudness rating. Relative
values were calculated by setting the scores and ratings obtained at week 3 to zero, and
calculating the difference between the actual score and rating obtained at this test interval and
the scores and ratings obtained at subsequent test intervals.
Convery and Keidser / Gain transitions for severe and profound losses
34
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