Download Greater Fitting Flexibility with Three Input Levels

Greater Fitting Flexibility with Three Input Levels in Aventa
Jennifer Groth
Abstract
ReSound was the first manufacturer to offer Wide Dynamic Range Compression (WDRC) to account for loudness
recruitment. The proprietary, threshold-based Audiogram+ provides a reliable starting point for fitting this WDRC
system by prescribing gains for soft and loud input levels. Beginning with the ReSound Verso product family in
Aventa 3.4, an extra compression kneepoint and control for moderate level inputs is provided. This gives fitters remarkable flexibility for addressing fine-tuning issues, and fitting varying degrees and types of hearing losses, as well
as unprecedented precision in matching the targets of generic prescriptive formulas.
Since the first ReSound hearing instruments were introduced to the market in the early 1990s, the fitting
rationale has remained rooted in the same philosophy
inspired by the workings of the normal ear. Based on
the seminal work of Villchur1, ReSound pioneered
Wide Dynamic Range Compression (WDRC), offering
the first system to account for loudness recruitment by
applying progressively less gain with increasing input
levels. To provide a reliable starting point for use of this
WDRC system, a fitting algorithm based on individual
psychoacoustic measures of loudness growth was
initially used. Years of clinical experience encompassing thousands of fittings with this procedure provided
the foundation for development and refinement of the
threshold-based Audiogram+ fitting algorithm2.
Audiogram+ calculates insertion gain targets for
narrowband inputs of 50 and 80 dB SPL at 11 audiometric octave and interoctave frequencies from
125 Hz through 8 kHz. The underlying compression
system has always used linear compression. This
means that the slope of the input-output function between the lower and upper compression kneepoints
was described by a straight line. In other words, only one
compression ratio has been possible between the lower and upper kneepoints. This is illustrated in Figure 1.
For this system, the 50 and 80 dB input level controls are always between the compression kneepoints.
Changing the gains for both of these input levels
simultaneously would shift the entire input-output
curve up or down, thus preserving the compression
ratio. Adjusting gain for one or the other of the two
levels would change the compression ratio.
Figure 1. The ReSound compression system has been based on two compression kneepoints, allowing only one compression ratio in the WDRC range.
For most fittings, the degree of control over gain
for different sound levels provided by the ReSound
system has proven adequate. However, there are cases when additional control is desirable. Beginning with
the ReSound Verso in Aventa 3.4, an extra compression kneepoint at 65 dB SPL is available for fitting. This
allows independent adjustment of gain for moderate
level inputs, as well as two compression ratios within
the range of compression, as shown in Figure 2. No
changes to the Audiogram+ prescription have been
made. Thus, the default gain setting for G65 when a
fitting based on Audiogram+ is prescribed will be halfway between G50 and G80. By providing this extra
control, numerous fitting issues can be solved. These
include a better ability to address user complaints
when fine-tuning, better ability to accommodate different hearing loss configurations, and better ability to
match targets to generic prescriptions.
Figure 2. The updated ReSound compression system includes a gain control
for moderate level sounds. This allows for two compression ratios within the
WDRC range.
Using G65 for fine-tuning
One way in which the G65 control enhances fitting
flexibility is that it gives more options for addressing
user complaints regarding loudness, feedback and
speech audibility. By having control over soft, moderate and loud level sounds independently, it is easier to find a balance between these often opposing
complaints. For example, Mr. V is fit with a Verso 62
RIE open model. At the initial fitting, the G50 curve is
at the maximum stable gain limit, and there is a slight
tendency for the instruments to feed back. Therefore,
the fitter reduced G50 and G65 in the high frequencies
in order to reduce this tendency. When Mr. V returned
for a follow up, he reported that feedback was not an
issue, and that the sound levels were comfortable in
all kinds of listening environments. However, he often
felt the need to turn up the volume somewhat when
conversing with individuals and at meetings. Based
on Mr. V’s experience, the fitter increased the G65 by
several steps from 1kHz and above to improve the
volume and clarity for moderate level speech. Technically, this reduced the compression below 65 dB, and
increased it slightly above. For Mr. V, this meant that
the comfort for loud sounds was not affected, feedback tendency was not reintroduced, and moderate
level speech was made more audible.
Figure 3. Separate adjustments of gain for soft and moderate input levels
helped to eliminate feedback, maintain comfort for loud sounds, and increase
audibility for moderate level speech.
Fitters who are accustomed to working with three input level controls will be able to apply their experience
when working with the updated ReSound system. For
those who are less familiar with this degree of control,
the Aventa Guide provides suggestions for addressing
common fitting complaints that incorporate the new
65 dB input level control.
hearing losses. Although many fitting prescriptions,
including Audiogram+, prescribe additional gain to
overcome some of the attenuation resulting from the
conductive impairment, there is no consensus on
how that should be done. In such cases, the added
flexibility of the moderate level control may be invaluable. For example, a user with a conductive loss might
report that loudness and clarity are generally fine, but
that loud sounds are uncomfortable or tend to be
distorted. In this case it might be helpful to decrease
G80. This should help with comfort without degrading
what the user likes about the fitting.
Figure 4. Aventa Guide incorporates the G65 control in providing suggestions
to solve common fitting complaints.
Using G65 for fitting different hearing
loss configurations
The ReSound WDRC fitting system was originally
designed to compensate for mild-to-moderate sensorineural hearing losses. For more severe losses, this
type of compression scheme presents a greater challenge to fit successfully. When fitting WDRC to individuals with severe-to-profound hearing losses, the
compression characteristics will result in more gain
for soft sounds and less gain for loud sounds relative
to a linear scheme with the same gain for moderate
level sounds. This can create problems with feedback
as well as inadequate loudness. Nevertheless, those
with more severe losses can potentially benefit greatly
from compression if an appropriate balance in comfort
and audibility can be achieved. Barker and colleagues3
found that fast-acting WDRC could be fit successfully
to individuals with severe-to-profound hearing losses.
However, many of the participants in their trial preferred higher compression kneepoints than typically
characterize WDRC schemes. With the addition of the
65 dB input level control in Aventa, the fitter can effectively decrease compression below the moderate
input level if preferred by individuals with more severe
losses. This is achieved by decreasing the G50. If the
G50 is decreased to the level of G65, the compression
kneepoint is in fact raised to this level. In doing this, the
fitter can preserve audibility for moderate level speech,
maintain comfort for loud sounds, and decrease feedback risk for those with severe-to-profound losses.
Fitters may also find the 65 dB input level control useful when fitting individuals with conductive or mixed
Using G65 for more accurate fitting to
generic prescriptions
Many professionals follow fitting protocols that require
fitting and verification according to scientifically validated, peer-reviewed prescriptions. Aventa facilitates this
process by offering automatic adjustment to a number
of such prescriptions, including NAL-NL1, NAL-NL2,
DSL i/o and DSL 5. While Audiogram+ prescribes
linear compression between the lower and upper
compression kneepoints, generic prescriptions may
differ. In the example in Figure 5, the same mild-tomoderate sloping audiogram was used to prescribe
fittings based on NAL-NL2, DSL i/o and Audiogram+.
The same signal type is assumed in each case. As
can be seen, these three formulas each prescribe the
compression in a different way. While NAL-NL2 prescribes a more linear response as input level increases, DSL i/o does the opposite. Without the separate
control for G65, the hearing instrument would not be
able to meet each of these prescriptive targets.
References
1. Villchur E. Signal processing to improve speech
intelligibility in perceptive deafness. Journal of the
Acoustical Society of America. 1973; 53: 1646-1657.
2. Audiogram+: The Resound Proprietary Fitting Algorithm. ReSound white paper. 2009.
3. Barker C, Dillon H, Newall P. Fitting low ratio compression to people with severe and profound hearing
losses. Ear and Hearing. 2001; 22(2): 130-141.
Figure 5. Fitting rules often differ in how compression is prescribed for a particular audiogram. With the added flexibility of a separate control for moderate
inputs, targets can be more accurately met for a variety of different formulas.
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DK-2750 Ballerup
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Tel.: +45 45 75 11 11
Fax: +45 45 75 11 19
www.resound.com
M200770-GB-12.05-Rev.A
Summary
Fine-tuning to patient preferences, accounting for
different types of hearing losses, and accurate fitting
to generic fitting formulas are benefits of the flexibility
afforded by control over gain for soft, moderate and
loud sound levels. Beginning with Resound Verso,
fitting professionals have access to this important
fitting feature.