NAL-NL3 Fitting System Technical Series (Part 1)

This article marks the first in a three-part technical series introducing the National Acoustics Laboratories NAL-NL3 fitting system: a next-generation, modular hearing aid fitting solution developed by the National Acoustic Laboratories (NAL). NAL-NL3 builds on the globally adopted NAL-NL2, retaining its evidence-based foundations while addressing long-standing clinical challenges.1Figure 1: An overview of the NAL-NL3 fitting system.The NAL-NL3 fitting system comprises a new core prescription, the NAL-NL3 Prescription, alongside specialized modules designed to support specific unmet needs. This three-part technical series will explore each of these components in depth. Part 1 (this article) focuses on the NAL-NL3 Prescription, describing its development, rationale, and the clinical insights that shaped it. Part 2 (forthcoming in 2026) will cover the NAL-NL3 Comfort in Noise Module, a new formula designed to improve listening comfort in challenging acoustic environments. Part 3 will introduce the NAL-NL3 Minimal Hearing Loss Module, tailored for clients with normal or near normal audiograms who nonetheless experience real-world difficulty understanding speech in noise. Together, this series offers a comprehensive look into how NAL-NL3 redefines hearing aid fitting, moving from a single prescriptive formula to a modular, person-centred system designed for real-life listening (See Figure 1). NAL-NL3 Prescription: Interview Insights Behind the Design and Development This first paper in the NAL-NL3 Technical Series focuses on the fundamental NAL-NL3 Prescription, describing its development, design rationale, and supporting evidence. We go behind the scenes of the NAL-NL3 Prescription through conversations with three of its architects: Dr Padraig Kitterick (Head of Audiological Science), Dr Jessica Monaghan (Senior Research Scientist), and Dr Debabrata Mishra (Research Scientist). Their insights reveal how codesign, clinical experience, and AI were harnessed in the early stages to create a new generation of hearing aid fitting prescription. From Hearing Aid Prescription to Fitting System Hearing aid prescriptions are used to determine an appropriate level-dependent gain-frequency response of hearing aids based on an individual’s audiometric thresholds. They serve as the clinical starting point for fitting hearing devices. Widely used, evidence-based prescriptions, such as NAL-NL2, have been adopted globally as the gold standard and implemented in manufacturer fitting software and clinical verification systems.2 Fifteen years after the release of NAL-NL2, NAL has introduced the next generation in evidence-based, hearing aid fitting: the NAL-NL3 fitting system. At the core of this system is the NAL-NL3 Prescription, developed to address key limitations of its predecessor and to better reflect contemporary hearing health care and practical clinical realities. Rethinking the Foundation: Why the NAL-NL3 Prescription? “NAL-NL2 served us well,” reflects Dr Padraig Kitterick, “but it was fundamentally a one-size-fits-all approach. And that’s no longer enough.” To better understand this issue, the NAL research team began by surveying and interviewing clinicians around the world. “We asked clinicians, ‘When the NAL-NL2 fit didn’t work, what did you do? What adjustment did you make and why?’” explains Dr Kitterick who led this early work. “Their answers gave us clear patterns. It wasn’t random. People were making the same types of adjustments for the same reasons.” These survey insights guided NAL researchers when analysing an extensive clinical real ear measures (REM) dataset. Detailed cluster analysis of this data pinpointed fitting patterns that directly informed the refinements now embedded in NAL-NL3. At its heart, the NAL-NL3 Prescription retains the original goals of maximizing speech intelligibility in quiet, while ensuring that loudness is softer than or equivalent to normal hearing, but never louder than normal. “Clinicians were consistently making the same kinds of adjustments in these edge cases,” explains Kitterick. “That told us something fundamental needed rethinking.” Clinicians were reporting consistent challenges, particularly for reverse-slope audiograms, mixed losses, and too much high-frequency gain across most losses. Practitioner-informed insights, workarounds, and learnings formed the starting point for redesigning the NAL-NL3 Prescription, ensuring it addressed clinical needs from the outset. Data-Driven Design: Machine Learning with a Clinical Lens Dr Debabrata Mishra, research engineer on NAL-NL3’s AI framework, offers a technology-focused perspective. “With NL2, they had maybe 1,600 simulated audiograms to work with, which were not fully representative of the real-world. We had millions of real-world real ear measures (REMs) and audiograms.” By drawing on real-world insertion gain data extracted from across Hearing Australia clinics, the NL3 team was able to build a much richer training dataset of real-world audiograms, which covered a wide range of losses and audiogram configurations. Cluster analysis was used to ensure the training dataset was much more representative of audiograms that one would see in a clinic. To process that data, the team used a newer generation of AI model. Researchers harnessed the power of cloud computing, which gave them access to much more computational power, allowing them to run more modern machine learning algorithms to optimise the gain for the audiograms in the training data set in shorter amounts of time. The more modern algorithms were better at optimising the gain for challenging audiograms, while the shorter optimization time gave the researchers more opportunities to fine-tune the optimization methods to achieve gains that were closer to clinical practice. Back when NAL-NL2 was developed, machine learning was in its infancy by today’s standards. “They were limited to very simple neural networks, just a few thousand neurons, and trained them on simulated data using what little computing power they had,” Mishra notes. “It took days to train those models, and the assumptions built into the data limited how well the algorithm could generalize.” In contrast, NAL-NL3 leveraged modern computing infrastructure and deeper AI architectures, enabling faster optimization, greater complexity, and improved adaptability. From Clinic to Code: Clinical Relevance at Every Step The NAL-NL3 research team’s strength was in blending engineering sophistication with real clinical insight. “It’s not just about better AI,” says Mishra. “It’s about AI that works for clinicians. We’ve built NL3 to reflect how hearing aids are actually fitted in the real world, not just how they should be fitted based on research studies.” This pragmatic focus was enabled by a rich feedback loop between NAL engineers, audiologists, clinical partners, and hearing aid users. “We made sure that NL3 not only performs better on paper, but also sounds better in someone’s ears,” says Monahan. Clinical Improvements: What’s New in the NAL-NL3 Prescription This emphasis on real-world fitting led to targeted improvements in three longstanding clinical problem areas. “We looked at where NL2 was consistently being adjusted by clinicians — that’s where NL3 needed to change,” explains Kitterick. Mixed losses: NAL-NL3 introduces a more balanced approach to gain in both low and high frequencies, particularly for clients with a conductive component. “Clinicians told us NL2 often over-prescribed in these cases. NL3 brings it back to a more tolerable, usable level,” says Monaghan. Reverse-slope audiograms: NAL-NL2 often prescribed more low-frequency gain than was comfortable. “We saw in the data, and heard from audiologists that it didn’t always work well in practice,” notes Mishra. NL3 applies a more controlled reduction, aligned with observed clinical fitting pattern. First-time users: “High-frequency gain was an issue, especially for new aid users,” says Mishra. “People were often overwhelmed. With NL3, the high-frequency gain is more controlled, so fittings feel more natural from the start and need less fine-tuning.” These refinements reflect the heart of the NAL-NL3 approach: honouring clinical realities, listening to practitioner experience, and using technology to make life easier for both audiologists and clients. As Kitterick puts it, “With NL3, we’re not just fitting hearing aids. We’re fitting people.” CONCLUSION The NAL-NL3 Prescription is not just a refinement of NAL-NL2, it’s a rethinking of what a hearing aid prescription can be: adaptable, evidence-led, and designed to grow. Rollout to hearing aid and REM equipment manufacturers begins in September 2025, setting the stage for a new era of smarter, more individualized hearing care. Brent Edwards, PhD, NAL Director, highlights the shift towards personalized care, stating, “This is a set of tools where audiologists can make decisions on how to best treat the patient,” reflecting his commitment to more tailored hearing aid fittings. That concludes Part 1 of our NAL-NL3 Fitting System Technical Series, exploring the foundations of the new prescription. In the next editions, we’ll take a closer look at the two new modules: the Comfort in Noise Module and the Minimal Hearing Loss Module.