Prescription procedures of the National Acoustic Laboratories
NAL-NL2 (NAL-Non-linear version 2):
Not yet released. Anticipated date of release: August 2008
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.
Keidser G, Dillon H. (2007). What’s new in prescriptive fittings Down Under? In Seewald R (Ed.), Hearing Care for Adults 2006. (pp.133-142).
Keidser G, Dillon H, Dyrlund O, Carter L, Hartley D. (2007). Preferred compression ratios in the low and high frequencies by the moderately severe to severe-profound population. J Am Acad Audiol 18(1):17-33.
Smeds K, Keidser G, Zakis J, Dillon H, Leijon A, Grant F, Convery E, Brew C. (2006). Preferred overall loudness. II: Listening through hearing aids in field and laboratory tests. Int J Audiol 45(1):12-25.
Smeds K, Keidser G, Zakis J, Dillon H, Leijon A, Grant F, Convery E, Brew C. (2006). Preferred overall loudness. I: Sound field presentation in the laboratory. Int Journal Audiol 45(1):2-11.
NAL-NL1 (NAL-Non-linear version 1):
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).
Dillon H. (1999). NAL-NL1: A new prescriptive fitting procedure for non-linear hearing aids. Hear J 52(4): 10-16.
Byrne D, Dillon H, Ching T, Katsch R, Keidser G. (2001). NAL-NL1 procedure for fitting nonlinear hearing aids: characteristics and comparisons with other procedures. J Am Acad Audiol 12: 37-51.
Ching T, Dillon H, Katsch R, Byrne D. (2001). Maximizing effective audibility in hearing aid fitting. Ear & Hear 22: 212-224.
Keidser G, Grant F. (2001). The preferred number of channels (one, two, or four) in NAL-NL1 prescribed WDRC devices. Ear & Hear 22(6): 516-527.
Keidser G, Grant F. (2001). Comparing Loudness Normalization (IHAFF) with Speech Intelligibility Maximization (NAL-NL1) when implemented in a Two-channel Device. Ear and Hear 22(6): 501-515.
NAL-RP (NAL-Revised, Profound):
As its predecessor, NAL, NAL-RP is a threshold-based procedure for prescribing the gain-frequency response in linear hearing aids with the aim of maximizing speech intelligibility. Evaluation of the first NAL formula showed that it did not achieve equal loudness of speech bands for all types of hearing loss. In particular, empirical data showed that the shape of the gain-frequency response varied 0.31 times, rather than the 0.46 times implemented in NAL, the shape of the audiogram. Further, when the hearing loss reached the severe and profound level, hearing aid users wanted 0.66 times, rather than 0.46 times, more gain per dB of loss.
Byrne D, Dillon H. (1986). The National Acoustic Laboratories’ (NAL) new procedure for selecting the gain and frequency response of a hearing aid. Ear & Hear 7(4): 257-265.
Byrne D, Cotton S. (1988). Evaluation of the National Acoustic Laboratories’ new hearing aid selection procedure. J Speech Hear Res 31(2): 178-186.
Byrne D, Parkinson A, Newall P. (1990). Hearing aid gain and frequency response requirments for the severely/profoundly hearing impaired. Ear & Hear 11(1): 40-49.
NAL (National Acoustic Laboratories):
A threshold-based prescription of the gain-frequency response in linear hearing aids that aims at maximizing speech intelligibility. The procedure is based on three principles 1) preferred insertion gain at 1 kHz equals 0.46 times the loss at 1 kHz, 2) speech bands according to the long-term average speech spectrum should be perceived equally loud, and 3) equal loudness at a most comfortable level is modelled using the 60-phon equal loudness contour curve by normal-hearing listeners.
Byrne D, Tonisson W. (1976). Selecting the gain of hearing aids for persons with sensorineural hearing impairments. Scand Audiol 5:51-59.