Evolving the philosophy: from the NAL rule to NAL-NL3

Abstract

Objective: To describe and discuss the evolution of hearing aid prescriptions for gain and compression from the National Acoustic Laboratories (NAL). Design: Narrative review of the philosophies and scientific approaches underlying the derivation of the NAL prescriptions, from the first linear NAL ‘rule’ to the latest advancements in the NAL-NL3 Fitting System. Study sample: Published studies on the development of the various NAL hearing aid prescription formulas. Results: From the first linear formulas (NAL, NAL-R, NAL-RP) to the subsequent non-linear formulas (NAL-NL1 and NAL-NL2), the NAL philosophy has been grounded in the importance of maximising speech understanding while ensuring loudness remains within normal levels. The NAL-NL3 Fitting System represents both a continuation of this philosophy in the design of the NAL-NL3 Prescription, but also the introduction of alternative philosophies in the NAL-NL3 Modules that address a broader set of needs and characteristics in today’s hearing aid recipients. Conclusions: The NAL prescriptions have evolved to address the needs of hearing aid users and advances in hearing aid technology. Independent evidence-based manufacturer-agnostic hearing aid prescription formulas still represent a key aspect of audiological practice, enabling hearing professionals to provide evidence-based individualized amplification strategies to optimize auditory outcomes.