Download Using Frequency Lowering with Children Who Wear Hearing Aids

RESEARCH TO REALITY
Using Frequency Lowering with
Children Who Wear Hearing Aids
By Ryan McCreer y and Marc Brennan
R
esearch on the effects of
frequency-lowering signal
processing has proliferated
rapidly over the past few years, leaving audiologists who fit hearing aids
for children with many questions
about whether or not this processing should be activated. As former
pediatric audiologists who became
researchers, we have been interested
in research questions that would
help audiologists know which children who are hard of hearing should
have frequency lowering activated
in their hearing aids. Although there
are still many questions about
frequency lowering left to be
answered, research from our
laboratory and data from
other researchers have
helped to enhance our
understanding of how
this processing might
benefit children who
are hard of hearing.
Audible
Bandwidth
Frequency lowering
is designed to enhance
access to high-frequency
speech sounds by moving
them to lower frequencies
where the hearing aid can
provide more gain, or where
hearing loss is less severe.
Lowering speech sounds has
the potential to increase
audibility of high-frequency speech sounds,
but also carries the risk
of distorting speech
cues. This trade-off
between audibility
and distortion is a key
factor in determining
whether or not frequency
lowering should be
activated. In terms of
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AUDIOLOGY TODAY May/Jun 2015
candidacy, children who have limited
access to high-frequency sounds
with conventional amplification
should be considered candidates
for frequency lowering. The audible
bandwidth represents the range of
frequencies audible to the listener
with conventional processing. The
upper limit of the audible bandwidth
is the frequency range between
where the average and peak of the
long-term average speech spectrum
intersect the audiometric thresholds
in the high frequencies (FIGURE 1). The
upper limit of the audible bandwidth
can be estimated from the SPL-OGRAM during hearing aid verification.
To optimize the audible bandwidth with frequency lowering, a
setting should be selected that lowers the maximum input frequency
with frequency lowering to within
the upper limit of the audible bandwidth. For example, if a patient has
an upper audible frequency limit of
3500 to 4000 Hz, and if the maximum
input frequency for the hearing aid
is 11,500 Hz, then you would want
to lower 11,500 Hz to somewhere
between 3,500 and 4,000 Hz. Doing
this will maximize the bandwidth
that is audible to the patient. To minimize distortion, you want to avoid
selecting settings that lower the
maximum input frequency to a frequency lower than the upper limit of
the audible bandwidth. Our research
has shown that using this procedure
improves speech understanding
when compared to conventional
amplification or to other fitting
methods for frequency lowering
(McCreery et al, 2013; McCreery et
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RESEARCH TO REALITY
al, 2014). If the upper limit of the
audible bandwidth with conventional amplification is greater than
6000 Hz, the potential for increased
audibility with frequency lowering
is marginal and, therefore, might not
have a benefit.
Acclimatization
Several previous studies have
suggested that some of the speech
recognition benefits that occur with
frequency lowering may require listening experience with the processor
before they can be observed (Glista
et al, 2012; Wolfe et al, 2011; Ellis and
Munro, 2015). In other cases, some
improvements in speech recognition
are immediately apparent, even without previous experience listening to
the frequency lowering (McCreery et
al, 2014). These results suggest that
the benefits of frequency lowering
may not be immediately apparent
when assessed as part of clinical
validation, such as an aided speech
recognition assessment. Audiologists
should document aided speech
recognition in children at the initial
fitting when frequency lowering is
activated and then again after at
least 6–8 weeks of consistent hearing
aid use. Clinicians should consider
using speech recognition materials
that contain a sufficient amount of
high-frequency exemplars to ensure
that differences in high-frequency
audibility related to the processing
can be detected.
limited high-frequency bandwidth
to conventional processing with
extended high-frequency bandwidth
and frequency lowering (Brennan et
al, 2014). Preference was assessed
for children with mild to severe
hearing loss for speech and different
types of music. The preferred type
of processing varied widely across
the children, and most children did
not have a preference. While few
children expressed a preference for
restricted bandwidth, most of the
children in the study preferred either
extended bandwidth conventional
processing or frequency lowering, or
did not show a consistent preference.
The variability in listener preference for the three different types of
processing suggests that we should
ask children about sound quality
when they are old enough to report
a preference. Assessing the child’s
perceptions about sound quality
can allow audiologists to achieve
frequency-lowering settings that
enhance audibility without sacrificing sound quality.
Sound Quality
Although the primary purpose of
frequency lowering is to increase
the audibility of high-frequency
speech sounds, children may be less
likely to use frequency lowering if
the processing reduces perceived
sound quality. Our research assessing multiple dimensions of sound
quality for children compared
conventional amplification with
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FIGURE 1. The aided long-term average speech spectrum (LTASS)
in dB SPL as a function of frequency (Hz). Connected circles
represent the right ear thresholds. The solid green line represents
the average of the aided LTASS and the blue lines represent the
peak (upper) and minimum (lower) of the LTASS. The shaded blue
area of the LTASS is audible. The black arrows represent the points
where the LTASS intersects the audiogram for the average (rightfacing arrow) and peak (left-facing arrow). The solid black lines
demonstrate the frequencies where those intersections occur. The
range between the two arrows represents the maximum audible
frequency range for this patient (approximately 3000–3800 Hz).
May/Jun 2015 AUDIOLOGY TODAY
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RESEARCH TO REALITY
Future Directions for
Research
References
Although significant progress has
been made to advance our understanding of how frequency lowering
affects speech recognition and
sound quality for children who wear
hearing aids, many questions remain
unanswered. Much of the research
conducted with frequency lowering
has focused on a single approach to
frequency lowering, nonlinear frequency compression. Comparisons
of the benefits and limitations of
different types of frequency lowering may help audiologists to better
identify which types of processing to
use with children or with different
audiometric configurations.
Methods of optimizing frequency
lowering for individual patients
would also benefit from further
research. Specifically, there is limited
evidence to support whether or not
frequency-lowering settings should be
symmetrical between ears or allowed
to vary in cases of asymmetrical
hearing loss. Maximizing the audible
bandwidth is a simple way to assess
the potential benefits of frequency
lowering, but different combinations
of frequency lowering parameters may
result in similar enhancements to
audibility. More research is needed to
help clinicians optimize the parameters of frequency lowering to provide
the best outcome.
Brennan MA, McCreery R, Kopun
J, Hoover B, Alexander J, Lewis D,
Stelmachowicz, PG. (2014) Paired
comparisons of nonlinear frequency
compression, extended bandwidth,
and restricted bandwidth hearing aid
processing for children and adults
with hearing loss. J Am Acad Audiol
25(10):983–98.
Ellis RJ, Munro KJ. (2015) Benefit
from, and acclimatization to, frequency
compression hearing aids in experienced
adult hearing-aid users. Int J Audiol (0)1–11.
McCreery RW, Alexander J, Brennan
MA, Hoover B, Kopun J, Stelmachowicz
PG. (2014) The influence of audibility
on speech recognition with nonlinear
frequency compression for children
and adults with hearing loss. Ear Hear
35(4):440–447.
Wolfe J, John A, Schafer E, Nyffeler M,
Boretzki M, Caraway T, Hudson M. (2011)
Long-term effects of non-linear frequency
compression for children with moderate
hearing loss. Int J Audiol 50(6):396–404.
Glista D, Scollie S, Sulkers J. (2012)
Perceptual acclimatization post
nonlinear frequency compression
hearing aid fitting in older
children. J Speech Lang
Hear R 55(6):1765–1787.
McCreery RW, Brennan
MA, Hoover B, Kopun J,
Stelmachowicz PG. (2013)
Maximizing audibility
and speech recognition
with non-linear frequency
compression by estimating
audible bandwidth. Ear Hear
34(2):e24.
Ryan McCreery, PhD, is the director
of the Center for Audiology at Boys
Town National Research Hospital.Marc
Brennan, PhD, is the director of the
Amplification and Perception Laboratory
in the Center for Audiology at Boys
Town National Research Hospital. He
also works as an audiologist in the clinic
at Boys Town.
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