NAL-NL1 procedure for fitting nonlinear hearing aids: characteristics and comparisons with other procedures

Area of Science:

• Audiology
• Hearing Aid Technology
• Speech Perception

Background:

• Traditional hearing aid fitting procedures may not optimally balance speech intelligibility and loudness perception.
• The Speech Intelligibility Index (SII) is a key metric for predicting speech understanding, but requires modification for varying hearing loss degrees.

Purpose of the Study:

• To introduce and describe the National Acoustic Laboratories' nonlinear fitting procedure, version 1 (NAL-NL1).
• To compare NAL-NL1 fitting prescriptions with existing methods like DSL [i/o], FIG6, and IFF.
• To evaluate the impact of NAL-NL1 on gain and compression across different audiogram configurations.

Main Methods:

• Development of the NAL-NL1 procedure based on maximizing speech intelligibility (using a modified SII) and constraining loudness.
• Comparative analysis of NAL-NL1 prescriptions against NAL-Revised, Profound, DSL [i/o], FIG6, and IFF threshold procedures.
• Evaluation of prescribed gain and compression ratios for various audiogram shapes (flat, upward sloping, steeply sloping high-frequency).

Main Results:

• NAL-NL1 prescriptions closely resemble NAL-Revised, Profound for average speech input levels.
• NAL-NL1 generally prescribes lower low-frequency gain for flat/upward sloping audiograms and less high-frequency gain for steeply sloping losses compared to other methods.
• The NAL-NL1 procedure tends to recommend less overall compression than the compared fitting strategies.

Conclusions:

• The NAL-NL1 procedure offers a distinct approach to hearing aid fitting, potentially optimizing audibility and comfort.
• Significant differences exist between NAL-NL1 and other common hearing aid fitting protocols, particularly in gain and compression settings.
• Further research may be warranted to validate the clinical efficacy of NAL-NL1 across diverse patient populations.