Hearing Rehabilitation
Prescriptive Procedures
Over the years hearing aids have become increasingly complicated with many styles and parameters to choose from. To enable clinicians to select the best solution for the hearing-impaired person, many procedures and methodologies have been introduced over the years. At the National Acoustic Laboratories (NAL) we believe that such clinical procedures should be practical and well-founded and it has been a main objective of NAL research for several decades to deliver scientifically sound and clinically feasible procedures to assist clinicians with the hearing aid fitting process. A brief historical overview of NAL's early contributions to the development of hearing aid prescription procedures can be found here Newer developments include prescriptive procedures for fitting non-linear hearing aids (NAL-NL1 and NAL-NL2) and outcome measures (COSI).
NAL-NL1
NAL-NL1 is a threshold-based procedure that prescribes gain-frequency responses for different input levels, or the compression ratios at different frequencies, in wide dynamic range compression hearing aids. The aim of NAL-NL1 is to maximize speech intelligibility for any input level of speech above the compression threshold, while keeping the overall loudness of speech at or below normal overall loudness. The formula is derived from optimizing the gain-frequency response for speech presented at 11 different input levels to 52 different audiogram configurations on the basis of two theoretical formulas. The two formulas consisted of a modified version of the speech intelligibility index calculation and a loudness model by Moore and Glasberg (1997).
The NAL-NL1 formula is not publicly available. Manufacturers of hearing aids and analysing equipment who wish to include NAL-NL1 in their product can obtain a license to the formula. Click here for more information
A stand-alone software program that enables clinicians to extract the NAL-NL1 prescription from threshold data is available from NAL. Click here for more information about the software and on how to obtain a copy.
Further Reading:
1. Byrne D, Dillon H, Ching T, Katsch R and Keidser G (2001). NAL-NL1 procedure for fitting nonlinear hearing aids: characteristics and comparisons with other procedures. Journal of the American Academy of Audiology, 12:37-51.
2. Ching T, Dillon H, Katsch R and Byrne D (2001). Maximizing effective audibility in hearing aid fitting. Ear & Hearing, 22:212-224.
3. Dillon H (1999). NAL-NL1: A new prescriptive fitting procedure for non-linear hearing aids. Hearing Journal, 52(4):10-16.
4. Keidser G and Grant F (2001). The preferred number of channels (one, two, or four) in NAL-NL1 prescribed WDRC devices. Ear & Hearing, 22(6):516-527.
5. Keidser G and Grant F (2001). Comparing loudness normalization (IHAFF) with speech intelligibility maximization (NAL-NL1) when implemented in a two-channel device. Ear & Hearing, 22(6):501-515.
NAL-NL2
NAL-NL2, like its predecessor, NAL-NL1, is a threshold-based procedure that prescribes gain-frequency responses for different input levels, or the compression ratios at different frequencies, in wide dynamic range compression hearing aids. The aim remains to maximize speech intelligibility for any input level of speech above the compression threshold, while keeping the overall loudness of speech at or below normal overall loudness. Evaluation of NAL-NL1 showed that prescribed overall gain was slightly too high for adults, particularly for higher input levels, and slightly too low for lower input levels for children. Further, the compression ratios prescribed to severe and profound hearing loss were too high. The optimization procedure used to derive NAL-NL1 will be repeated with improved versions of both the speech intelligibility index formula and the loudness model. In particular, new speech perception data collected in quiet and noise on 75 listeners with hearing ranging from normal to profound hearing loss have been collected together with a variety of psychoacoustic parameters, including information about cochlear dead regions, to improve the calculation of the effective audibility in the speech intelligibility index formula.
Further Reading:
1. Dillon H, Keidser G, Ching T, Flax M and Brewer S. Development of the NAL-NL2. XIX Australian Audiological Society National Conference, Sydney, May 2010. Click here to download presentation slides.
2. Keidser G and Dillon H (2007). What's new in prescriptive fittings Down Under? In Seewald R (Ed.), Hearing Care for Adults 2006. (pp.133-142).
3. Keidser G, Dillon H, Dyrlund O, Carter L and Hartley D (2007). Preferred compression ratios in the low and high frequencies by the moderately severe to severe-profound population. Journal of the American Academy of Audiology, 18(1):17-33.
4. Smeds K, Keidser G, Zakis J, Dillon H, Leijon A, Grant F, Convery E and Brew C (2006). Preferred overall loudness. II: Listening through hearing aids in field and laboratory tests. International Journal of Audiology, 45(1):12-25.
5. Smeds K, Keidser G, Zakis J, Dillon H, Leijon A, Grant F, Convery E and Brew C. (2006). Preferred overall loudness. I: Sound field presentation in the laboratory. International Journal of Audiology, 45(1):2-11.
