Optimising hearing aid fittings of children who also use cochlear implants

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Accurate and practical methods for fitting
non-linear hearing aids to infants and children
RECD, REAG, NAL-NL1
Teresa Ching, Louise Britton, Harvey Dillon,
Mandy Hill
National Acoustic Laboratories, Australia
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Overview

• 1. Non-linear amplification for children
• 2. Principles of the RECD / REAG / CG method
• 3. Methods for measuring RECD
• 4. Methods for prescribing and adjusting aids

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Non-linear amplification for children/ infants?
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Would children benefit from compression?

• Compression limiting
• reduces discomfort and distortion
• Wide dynamic range compression
• reduces need for volume control adjustment
• increases audibility of soft sounds
• reduces risk of noise induced hearing loss
• Different gain-frequency responses for different
  input levels
• maximises speech intelligibility

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Reduces need for volume control

• Linear amplification is optimised for typical
  input level for adults and children (Byrne
  Dillon, 1986 Byrne et al, 1990)
• Children/infants - typical input levels are
  highly variable (Stelmachowicz et al, 1993)
• Why should children not have comfortable loudness
  more often?

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Increases audibility of soft sounds

• Children require better signal levels than adults
  for optimal performance
• 25 dB for infants (Nozza et al, 1991)
• 17 dB for 5-year-olds (Byrne, 1983)
• Children have less contextual, linguistic
  knowledge for understanding speech
• Enhances auditory awareness of sounds (Jensen
  Henningsen, 2000)

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Maximising speech intelligibility

• Use NAL-NL1 - aimed at speech intelligibility,
  with loudness no more than normal
• NAL-NL1 applies similar gain-frequency response
  to NAL-RP for typical input level (lots of
  support)
• NAL-NL1 applies more gain for low input level and
  less gain at high input level as required to
  maximise predicted speech intelligibility
• NAL-NL1 validated for adults to be better than
  loudness normalisation procedures for speech
  perception (Keidser Grant, 2000)

Data ?
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Reduces risk of noise-induced hearing loss

• Greatest risk comes from greatest input levels
• WDRC has lower gain than linear for high levels

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Could compression be bad for children?

• Possibly, because compression reduces intensity
  difference cues in speech
• But, so does compression limiting and peak
  clipping.
• Inaudible speech is even worse!

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Children have smaller ears!
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Children have smaller ears
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Children have smaller ears
FOR MORE INFO...
Kruger (1987) Keefe et al(1994)
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Real ear to coupler difference
adults
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RECD relative to adult data
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Insertion gain and Real-ear aided gain
REUG U - F
REAG A - F
REIG A - U REAG - REUG
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Transforming IG to REAG

• IG REAG - REUG
• REAG IG REUG

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Does the different ear resonance suggest the need
for different frequency response?

• No direct evidence on use of superior high
  frequency sensitivity
• Early speech discrimination abilities of normal
  hearing infants draw on low frequency, intensity,
  duration cues
• Modifications of speech to infants include
  extended fundamental frequency range, increased
  duration and intensity on content words, and
  slower rate of delivery
• horizontal localisation.

FOR MORE INFO...

• Kuhl (1987 1992)

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Transforming IG to REAG

• IG REAG - REUG
• REAG IG REUG

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Transforming REAG to CG

• REAG CG RECD MLE
• CG REAG - RECD - MLE

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Measuring RECD
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Measuring RECD
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Summary

• Measure thresholds (preferably in dB SPL in
  canal)
• Use non-linear procedure
• Ignore the variation in canal resonance
• Prescribe in terms of Real Ear Aided Gain (REAG
  ear canal SPL minus field SPL)
• Measure Real Ear to Coupler Difference (RECD
  ear canal SPL minus coupler SPL)
• Calculate coupler gain prescription for the
  individual child
• Adjust aid in coupler

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Questions?
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Measuring RECD results
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RECD Real ear gain Coupler gain
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RECD Real ear gain Coupler gain
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RECD Real ear gain Coupler gain
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RECD real-ear SPL measured using own earmould,
coupler SPL measured using a 2 cc coupler
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RECD repeatability
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Measuring RECD practical issues
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Left and right ears
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Blocked probe tube
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Leakage around earmould
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Probe positioned too far away from the eardrum
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Measured with active child
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Leakage of probe bore into main sound bore
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Fitting hearing aids using RECD
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Fitting hearing aids using RECD

• REAG RECD CG
• Derive individualised coupler gain targets
• Adjust hearing aid parameters and measure in a
  coupler for verification

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Deriving NAL-NL1 targets
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Fitting a Siemens Prisma 2SP
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Fitting a Siemens Prisma 2SP
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Fitting a Siemens Prisma 2SP
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Fitting a Siemens Prisma 2SP
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Fitting a Siemens Prisma 2SP
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Fitting a Siemens Prisma 2SP
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Fitting a Bernafon LS16
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Deriving NAL-NL1 targets
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Fitting a Bernafon Smile III (LS16)
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Fitting a Bernafon Smile III (LS16)
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Fitting a Bernafon Smile III (LS16)
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Fitting a Bernafon LS16Speech display
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Fitting a Bernafon LS16 worst case
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Fitting to NL1 targets for 43 ears
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Summary
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NAL methods for selecting hearing aids for
children and infants

• Measure hearing thresholds and RECD using insert
  earphone custom earmould
• Derive individualised coupler gain targets using
  NAL-NL1 software
• Adjust hearing aid and verify coupler gain
  targets
• Display amplified speech levels in the real ear
• Evaluate effectiveness
• functional assessment based on parents/teachers
  observations in real life (M Hill)
• Electrophysiological assessment (S Purdy)
• Fine-tune hearing aids and/or modify management
  practice if required.

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www.nal.gov.au Teresa.Ching_at_nal.gov.au Louise.Brit
ton_at_hearing.com.au National Acoustic
Laboratories, Australia.