The NAL-NL2 prescription procedure

Abstract

NAL-NL2 is the second generation of prescription procedures from The National Acoustic Laboratories (NAL) for fitting wide dynamic range compression (WDRC) instruments. Like its predecessor NALNL1 (Dillon, 1999), NAL-NL2 aims at making speech intelligible and overall loudness comfortable. This aim is mainly driven by a belief that these factors are most important for hearing aid users, but is also driven by the fact that less information is available about how to adjust gain to optimise other parameters that affect prescription such as localisation, tonal quality, detection of environmental sounds, and naturalness. In both formulas, the objective is achieved by combining a speech intelligibility model and a loudness model in an adaptive computer- controlled optimisation process. Adjustments have further been made to the theoretical component of NAL-NL2 that are directed by empirical data collected during the past decade with NAL-NL1. In this paper, the data underlying NAL-NL2 and the derivation procedure are presented, and the main differences from NAL-NL1 are outlined.

Keywords

• hearing aids
• prescription procedures.