The NAL-NL3 Prescription

NAL-NL2 is used worldwide to prescribe hearing aid gain and compression for new and experienced hearing aid users. The NAL-NL3 Prescription was developed to further optimise fitting targets to maximise predicted speech intelligibility without exceeding normal-hearing loudness, and to address clinical feedback about NAL-NL2. Clinician reports identified cases where fittings departed from NAL-NL2 targets; clinical records confirmed systematic deviations. Multi-stage non-linear optimisation with constraints on low- and high-frequency gain identified optimal targets. Clinical review guided target refinement for deep neural-network training. Output-stage limiting enforced fast-compression ratio limits. Modelling assessed predicted intelligibility, loudness, and sharpness; in-clinic trials gathered clinician usability ratings. 76 clinician survey respondents; 1.33 million adult-fitting records; 29 clinicians evaluated usability across 263 adult fittings. Model-based evaluation identified statistically significant increases in predicted intelligibility and significant decreases in sharpness while maintaining loudness within normal-hearing limits. Relative to NAL-NL2, gain decreased below 0.5 kHz and above 6 kHz, with modest increases between 1 and 4 kHz. Compression ratios were within desired limits. Clinicians reported improved ease of fitting and fewer fine-tuning adjustments. The NAL-NL3 Prescription retains NAL’s validated approach to prescribing gain and compression while generating targets with higher predicted intelligibility that are clinically appropriate for a wider range of fittings.