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Comparing loudness normalization (IHAFF) with speech intelligibility maximization (NAL-NL1) when implemented in a
Ear and Hearing
|January 5, 2002
Summary
The National Acoustic Laboratories Nonlinear (NAL-NL1) hearing aid fitting rationale was preferred over the Independent Hearing Aid Fitting Forum (IHAFF) for better speech intelligibility, especially in noise.
Area of Science:
- Audiology
- Hearing Science
- Biomedical Engineering
Background:
- Wide dynamic range compression hearing aids offer multiple fitting rationales.
- Key rationales aim to normalize loudness or maximize speech intelligibility.
- Real-world validation of these rationales is limited.
Purpose of the Study:
- To compare loudness normalization (IHAFF) and speech intelligibility maximization (NAL-NL1) fitting rationales.
- To evaluate these rationales in a 2-channel compression hearing aid.
- To assess user preference and performance across different hearing loss profiles.
Main Methods:
- Implemented IHAFF and NAL-NL1 prescriptive formulas in a 2-channel hearing aid simulation.
- Recruited 24 subjects with varying hearing loss (mild flat, moderate/severe flat, steeply sloping).
- Conducted laboratory tests (paired-comparison, sentence recognition) and 4-week field tests in real-world conditions.
Main Results:
- No significant learning or acclimatization effects were observed.
- More subjects preferred NAL-NL1 in paired-comparison tests.
- Significantly better sentence recognition with NAL-NL1 in low-frequency weighted noise.
- 16 out of 22 subjects preferred NAL-NL1 in the field test.
- Preference for NAL-NL1 increased with larger differences between fitting rationales.
Conclusions:
- Subjects preferred NAL-NL1 when fittings substantially differed.
- NAL-NL1 provided better performance and preference, particularly in noisy environments.
- The study supports NAL-NL1 for enhanced speech intelligibility in hearing aid fittings.
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