NAL-NL2 empirical adjustments

Abstract

NAL-NL1, the first procedure from the National Acoustic Laboratories (NAL) for prescribing nonlinear gain, was a purely theoretically derived formula aimed at maximizing speech intelligibility for any input level of speech while keeping the overall loudness of speech at or below normal loudness. The formula was obtained through an optimization process in which speech intelligibility and loudness were predicted from selected models. Using updated models and applying some revisions to the derivation process, a theoretically derived NAL-NL2 formula was obtained in a similar way. Further adjustments, directed by empirical data collected in studies using NAL-NL1 as the baseline response, have been made to the theoretically derived formula. Specifically, empirical data have demonstrated that (a) female hearing aid users prefer lower overall gain than male users; (b) new hearing aid users with more than a mild hearing loss prefer increasingly less gain with increasing degree of hearing loss than experienced hearing aid users, and require up to 2 years to adapt to gain levels selected by experienced hearing aid users; (c) unilaterally and bilaterally fitted hearing aid users prefer overall gain levels that vary less than estimated by the bilateral correction factor; (d) adults prefer lower overall gain than children; and (e) people with severe/profound hearing loss prefer lower compression ratios than predicted when fitted with fast-acting compression. The literature and data leading to these conclusions are summarized and discussed in this article, and the procedure for implementing the adjustments to the theoretically derived NAL-NL2 formula is described.

Figure 1.

The preferred overall gain, measured across 0.5, 1, 2, and 4 kHz, relative to NAL-NL1 prescribed overall gain as a function of the degree of hearing loss for 189 adult hearing aid users participating in one of five different research studies

Figure 2.

The average preferred overall gain, measured across 0.5, 1, 2, and 4 kHz, relative to NAL-NL1 prescribed overall gain, for 189 hearing aid users participating in one of five different research studies, shown by gender, degree of hearing loss, and hearing aid experience Note: Bars show plus and minus one standard error.

Figure 3.

The difference in average overall gain relative to NAL-NL1 prescribed overall gain, measured across 0.5, 1, 2, and 4 kHz, preferred by new hearing aid users 1, 4, and 13 months after fitting and users with more than 3 years of hearing aid experience Note: The full and hatched bars show data for participants with a mild and a moderate or greater degree of hearing loss, respectively.

Figure 4.

The implemented gain adjustment in NAL-NL2 as a function of degree of hearing loss for level of experience

Figure 5.

The mean preferred gain deviation from the NAL-NL1 prescription over time Note: The bars show the 95% confidence bands. The broken line shows the average gain to which the participants were expected to adapt (taking a general shift in threshold into account) and the dotted line shows two times the standard error (SE) of that value.

Figure 6.

The difference in gain applied by NAL-NL2 to a unilateral and bilateral hearing loss when the asymmetry between ears is 0 dB

Figure 7.

The maximum compression ratios accepted with fast-acting compression in NAL-NL2 as a function of degree of hearing loss and frequency