A NAL NL2 and Audiogram+ Showdown!

By Erica Koehler, Au.D.

Each hearing instrument manufacturer implements a proprietary fitting rationale as the “first fit”; the philosophy and logic behind each can vary greatly leaving the end user with very different gain settings depending on which brand they are fit with. Should the fitter decide to avoid these proprietary formulas and choose a generic prescription, such as NAL, DSL or CAMFIT, they are still faced with a wide range of fitting targets. Keidser, Brew and Peck (2003) found when comparing two widely used generic targets to four proprietary, that target gain setting could differ up to 10dB for the same audiogram. So how does ReSound’s Audiogram+ compare to a standard target like NAL or DSL?

Let’s have a look at the literature first. Studies have found that listeners tend to prefer the NAL (NL1 or RP) or NAL-like propriety prescriptions when compared to others that generally provided more gain (Abrams et al, 2012, Mueller, 2005, Stelmachowitz et al, 1998). As suggested by Killion’s 2004 article, proprietary formulas often prescribe less gain than standard methods. Audiogram+ has shown to be very close to the widely used NAL NL1 standard method (Keidser, 2003) and is based on similar rationale. NAL NL1 and 2 use the same rationale of flattening, or normalizing, loudness and maximizing predicted speech intelligibility. Changes made to NAL NL1 were based on the more recent Moore and Glasberg (2004) loudness model and behavioral data of compression ratios and gain preferences (Johnson & Dillon, 2011). This Johnson and Dillon study found that NAL NL2 typically prescribes less insertion gain than its predecessor, which in theory would more closely match the targets of Audiogram+, according to the Keidser study. Audiogram+ is also based on the concept of loudness normalization and is designed around the ReSound compression system. The Audiogram+ white paper states that according to Smeds (2006) and Keidser & Grant (2003), hearing instrument users tend to prefer less gain than a loudness normalization rationale would prescribe. The analysis of fittings based on individual loudness scaling procedures that led to the development of Audiogram+ revealed similar findings. For this reason, Audiogram+ prescribes less gain than a strict loudness normalization rationale would indicate.

Based on the literature we have learned two things. First, listeners tend to prefer rationales that prescribe less gain at a first fitting, such as NAL NL2 and Audiogram+. Second, NAL NL2 and Audiogram+ are based on a similar philosophy but are still fundamentally different. The two have been compared in an objective manner by looking at the targets generated by each. But no subjective evaluation has been carried out. A study was conducted at an off-site research facility to do just that.

The main focus of the trial was to determine if participants preferred to have their hearing instruments fit with targets generated by NAL NL2 or Audiogram+. The amount of fine-tuning required was also investigated based on the fact that when a hearing instrument’s initial fit is close to satisfactory gains, a secondary benefit occurs. Less fine-tuning! Twenty new users were fit with Audiogram+ and NAL NL2, spending two weeks in each. They were then asked to complete a series of questionnaires recording their preference of condition one or two in different domains. Results found equal number of participants preferred Audiogram+ as NAL NL2 when fit to target, however there was increased preference for Audiogram+ after fine-tuning, particularly in noise.

For those that requested fine-tuning, the adjustments were very similar for the two rationales. In six cases the participant did not request fine-tuning, and four of these instances were with an Audiogram+ fitting. This suggests, that when compared with NAL NL2, Audiogram+ may be a better starting point as a “first fit.” Similar fine-tuning from target resulted in a greater number of subjects selecting Audiogram+ as the preferred condition.

Now back to the original question – how does ReSound’s Audiogram+ compare to a standard target like NAL or DSL? The answer seems to be that it compares quite favorably, providing a combination of audibility and comfort, at least for new users. Of course it’s impossible to fit every patient without spending time fine-tuning but when compared to NAL NL2, Audiogram+ resulted in more satisfied listeners with less fine-tuning.

References:
Pastoors-Gebhart A, Koehler E, Jespersen CT, Kießling J (2014) Vergleich einer herstellereigenen Anpassregel mit NAL-NL2 im Feldtest, 17. Jahrestagung der Deutschen Gesellschaft für Audiologie, Proceedings CD