Comparing Loudness Normalization (IHAFF) with Speech Intelligibility Maximization (NAL-NL1) when Implemented in a Two-Channel Device

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Abstract
At least two rationales are available for fitting wide dynamic range compression hearing aids. The goal of one rationale is to normalize loudness, and the goal of the second rationale is to maximize speech intelligibility. Neither rationale has been validated against other fitting rationales for the range of input levels common to the hearing aid user in the real world. The goal of the study was to compare the two rationales when implemented in a 2-channel compression hearing aid. Loudness normalization and speech intelligibility maximization were implemented using the Independent Hearing Aid Fitting Forum (IHAFF) and the National Acoustic Laboratories’ Nonlinear (NAL-NL1) prescriptive formulas. Twenty-four subjects (eight for each of three groups of mild flat, moderate/severe flat, and steeply sloping hearing loss) participated in the study. Each subject completed an initial laboratory test, field test, and final laboratory test. The laboratory test consisted of a paired-comparison judgment for each prescriptive formula using four stimuli under both quiet and noisy listening conditions and a sentence recognition test using Bamford-Kowal-Bench sentences. In the field test, subjects evaluated the two rationales in individually selected everyday listening conditions for 4 wk. A digital simulation of the fitting rationales implemented in two channels was used for laboratory testing and a digital 2-memory, 2-channel device was used for field testing. Subjects adjusted the overall gain of each response to their preferred listening level in both the laboratory and in the field. Data collected in the laboratory before and after the field test showed no indication of significant learning or acclimatization effects. For each stimulus presented in the paired-comparison test more subjects preferred NAL-NL1 than preferred IHAFF. For the sentence recognition test, subjects performed significantly better with NAL-NL1 than IHAFF in a low-frequency weighted background noise. Sixteen out of 22 subjects who completed the field test reported a preference for the NAL-NL1 response. The remaining six subjects preferred IHAFF. The paired-comparison test and field test revealed that while the achieved root-mean-square (rms) difference between fittings for an input level of 65 dB SPL was small, the preference for either rationale was small. As the rms difference between fittings increased, the score in favor of NAL-NL1 increased. The correlation between the differences in satisfaction score obtained in the field test and the rms differences between the responses fitted was statistically significant. When the two fitting rationales prescribed substantially different responses for a 65 dB SPL input and these differences were achieved in the fitting, then the subjects preferred NAL-NL1. Even when the difference between fittings was small, the subjects preferred and performed better with NAL-NL1 when listening in a low-frequency weighted background noise.