Lecture 8: Prescribing Hearing Aid Performance Flashcards
What is a prescription?
It is something that a medical practitioner that proves authorization for medicine or treatment
What is a prescription as it relates to hearing aids?
A frequency specific gain is prescribed for an individual based on their audiometric information.
How were hearing aids set in the past?
1) Waltson & Knudsen, 1940
- Proposed method for determining gain based on MCLs
2) Lybarger, 1944
- Proposed original half-gain rule
- Based on what listener’s liked to listen to
3) Harvard Report, 1946
- Large scale study
- All subjects had mixed or conductive hearing loss
- All measures were made in the coupler and not in the ear
- Report said that most patients performed will with a gradual response (6 dB per octave)
4) Carhart, Hearing Aid Evaluation (Comparative method), 1980s
- Original method was 2 weeks long
- Functional gain was not sensitive to differences between the hearing aids
5) Pascoe, 1975
- Refuted findings of Harvard report
- Recognized difference in frequency response between the coupler and the ear
6) Prescriptive Methods, 1983
- Brian Walden refuted the comparative method
What are the goals of prescriptive strategies?
- Optimize speech intelligibility
Based on:
- Audiometric thresholds
- Dynamic range
- Need to consider middle ear status
- Need to consider word recognition ability
- Prescriptions can be generic or proprietary
- Proprietary prescriptions are the prescriptions that the manufacturers have in their software
How do we characterize fitting strategies?
Types of Signal Processing
- Linear gain (same amount of gain no matter the input)
- WDRC (different gain for each input)
Audiometric Information required to generate the prescription
- Supra-threshold information (MCL, LDL)
- Pure tone thresholds
Underlying theoretical rationale
- Loudness normalization: to amplify speech so that the vowels are more powerful than the consonants
- Loudness equalization: to amplify the speech sounds so they’re equally loud
What are the key assumptions in prescriptive formulas?
1) Audibility of speech sounds are critical
2) Sound quality and distortion must be considered
What are threshold-based prescriptions? Why might this be a bad prescription method?
- An early method that attempted to mirror the audiogram
- This is bad because all sounds would be overamplified
- This would be way too loud, especially for severe losses
What are some threshold-based linear prescriptions?
1) Lybarger (1944)
- “Half-gain” rule
2) POGO (1983)/ POGO II (1988)
- Half-gain + low-frequency correction factor
3) Berger (1979;1988)
- More than half gain for middle frequencies
4) Libby (1986)
- Gain about 1/3 of the hearing loss
5) NAL-R (1986)
- LTASS placed @ MCL
- Intelligibility is assumed to be maximized when all bands of speech are perceived to have the same loudness
6) NAL-RP
- Change in slope from 46% to 31%
7) DSL
- Achieve audibility & comfort in each frequency region
What are some corrections to prescription gain?
Binaural summation
- Reduce prescription gain by 3-6 dB
- Some formulas might automatically make this correction
Conductive Component
- No reduction in dynamic range
- Add 1/4 of the air/bone gap to prescription gain
What are some supra-threshold formulas?
- Different methods require different supra-threshold measurements
1) Bisection approach - IG in center of dynamic range
2) UCL method
- Specify frequency/gain characteristics so the spectrum of speech falls below UCLs
3) MCL approaches
- Mirror MCL
What are some disadvantages of using supra-threshold techniques?
- Takes more time to make the measurements
- Supra-threshold measures are harder to obtain from children and impaired adults
- Variability of supra-threshold measurements
What is nonlinear amplification?
- Gain and frequency response will vary depending on input level
What is nonlinear prescription?
Specifies gain-frequency response for several input levels or specifying input/output curve for several frequencies
What is the IHAFF/Contour method?
Independent Hearing Aid Fitting Form (IHAFF)
- IHAFF protocol aims to normalize loudness at each frequency
- IHAFF recommends CT of 40-45 dB
What is FIG6?
- Use of average loudness data
- Developed by Killion & Fikret-Pasa (1993)
