NAL-NL3 and New Modules Introduced for More Precise and Personalized Hearing Aid Fittings

NAL-NL3, a new hearing aid prescriptive fitting formula that addresses unique patient needs, was unveiled by Australia's National Acoustic Laboratories (NAL) on Thursday at the American Academy of Audiology (AAA) Annual Convention in New Orleans. Its forerunner, NAL-NL2, was introduced about 15 years ago, and is the most widely used prescriptive hearing aid fitting formula, incorporated in software used by audiologists and hearing aid specialists worldwide. Prescriptive hearing aid fitting formulas recommend target amplification levels based on a person’s hearing loss to optimize speech audibility and comfort.

NAL Director Brent Edwards, PhD, and Head of Audiology Science Pádraig Kitterick, PhD, introduced NAL-NL3 to audiologists at the AAA Convention. Many of these professionals have spent their entire careers using NAL’s formulas. NAL also announced a series of modules made for two special patient groups: people seeking better hearing in noise, and people who say they have trouble hearing but are diagnosed with hearing thresholds within “normal limits,” a problem that may affect as many as 26 million Americans.

“Consumers want solutions that are best for them, not just a generic solution that is ‘kind of okay’ for everyone,” Edwards told HearingTracker. “They need personalized solutions, and they want a better understanding of what their needs are by the healthcare professional so they can receive solutions that are unique to their challenges when hearing. NAL-NL3 and the new tools we’re introducing today encourage audiologists and hearing care providers to take a more individualized approach when fitting hearing aids.”

One standout feature of all NAL fitting formulas is their extensive use of normative data—the large number of subjects involved in NAL’s research and data collection, which helps establish a more accurate and reliable baseline for individual and group results. The NAL-NL1 and NL2 fitting formulas used audiological data from studies involving about 300 and 1,000 participants, respectively, while NAL-NL3 uses a database of millions of fittings analyzed with neural networks (AI). It was also drawn from over 1,500 real-ear measurements (REM) and real-world feedback through ecological momentary assessment (EMA) on user’s phones, real-time feedback via smartphones about how the hearing aid was performing in specific listening situations.

All of this should result in NAL-NL3 providing more precise and improved prescriptive targets, says Edwards. “There’s nothing really wrong with NAL-NL2, and when I became director of NAL, it was not high on my list for revisions. It works well,” he explains. “However, we've learned a lot more in the past 15 years, and technology has changed substantially. NAL-NL3 is a more precise and improved prescription over NL2, based on millions of fittings. We’ve also ensured that things like reverse slope and mixed hearing losses [where NL2 was lagging] have been accounted for.”

Along with refinements to the previous NAL-NL2 fitting formula, NAL is also releasing new modules that focus on the specific needs of individuals. The first two modules released by NAL are:

• Minimal Hearing Loss (MHL) Module: A new solution for fitting hearing aids to people with speech understanding difficulties but who have normal audiograms or minimal hearing loss (below 25 dB PTA, commonly thought to be “normal hearing”).
• Noise Module: A new solution for improving comfort when listening in noise without compromising speech understanding. For example, a hearing provider could designate this as a separate program in hearing aids that users can turn to when encountering noisy, challenging listening situations.

Edwards says other future modules might focus on improving music listening, extended high-frequency fittings, and profound hearing losses. NAL is actively seeking feedback from hearing care professionals and consumers on what types of future modules might be useful.

Although its first hearing aid fitting formulas date back to 1976, NAL unveiled NAL-NL1 (Non-Linear version 1)¹ in 1999 with the goal of providing more reliable prescriptive “targets”—or the best appropriate amplification for specific hearing losses—in the nonlinear (compression) hearing aids that would come to dominate hearing healthcare. NAL-NL1 was a major step forward that brought practical benefits for everyday speech understanding, allowing for a better balance between speech intelligibility and comfort than previous methods.

During its introduction in 1999, former NAL director and NL1 coauthor Harvey Dillon, PhD, would often draw contrasts between the NAL-NL1’s “equalization” approach—where the fitting target was informed by models of loudness equalization and speech intelligibility—versus the “normalization” approach taken by previous formulas like the Desired Sensation Level-Input/Output (DSL i/o) prescriptive fitting method. These older versions of DSL i/o sought to restore normal loudness perception across all frequencies in the listener’s residual dynamic range while retaining natural loudness (i.e., mimicking normal hearing loudness growth). The DSL i/o is the most widely used hearing aid fitting method for children, although it’s also used for adults.

NAL-NL2² improved upon NAL-NL1 by incorporating a more advanced speech intelligibility model, adjustments for compression ratios, and user-specific factors such as gender, language, hearing aid experience, and binaural fitting. It introduces a modified audibility factor, based on the understanding that severe hearing loss limits speech information extraction. The formula also considers tonal language speakers, user preferences for gain and compression, and differences in monaural versus binaural fittings. These refinements enhanced speech understanding and user satisfaction while ensuring comfortable loudness levels.

The newly revised NL3 fitting formula is undergoing validation in 27 clinics across six states in Australia. With over 160 patients fitted and 595 REM sessions completed, initial feedback is positive. “I set up the hearing aids with both NAL-NL2 and NAL-NL3 targets and he very quickly noticed a difference and strongly preferred the sound of the NAL-NL3 targets,” reports one clinician.

For hearing care professionals, Edwards says the switch between NAL-NL2 and NL3 should be fairly seamless, with hearing aid and special equipment manufacturers gradually introducing the new fitting formula into their systems.

With the advent of NAL-NL3, Edwards encourages audiologists to think beyond “best-fit” approaches and consider how NL3—especially with its new modules—can support more personalized and needs-based care. “If I’m a hearing care professional who wants to create a noise program for my patients, then I’m going to be pretty excited about using the NAL’s new Noise Module,” he said. “And I’m really hoping that the Minimal Hearing Loss Module will open the door to allow audiologists to give themselves permission to treat patients they may have ignored.”

He pointed to a rapidly changing market where managed care and over-the-counter solutions are pressuring hearing care providers to differentiate themselves. Ultimately, NAL-NL3 allows clinicians to demonstrate their expertise and guide patients through truly personalized hearing care. “This is a set of tools where audiologists can make decisions and choices on how to best treat the patient,” said Edwards. “It’s no longer about hitting ‘best fit’ and being done.”