NAL-NL1 Procedure for Fitting Nonlinear Hearing Aids: Characteristics and Comparisons with Other Procedures

TLDR

The goal is to maximize speech intelligibility while keeping loudness at normal or lower levels. The study introduces the National Acoustic Laboratories' nonlinear fitting procedure, version 1 (NAL‑NL1), for fitting nonlinear hearing aids. NAL‑NL1 predicts speech intelligibility using a modified Speech Intelligibility Index that accounts for reduced performance with greater hearing loss, especially at high frequencies, and compares its prescriptions with those of desired sensation level, FIG6, and a threshold version of the Independent Hearing Aid Fitting Forum procedures. NAL‑NL1 yields prescriptions similar to the established NAL‑Revised, Profound procedure, but generally prescribes less low‑frequency gain for flat or upward‑sloping audiograms, less high‑frequency gain for steeply sloping losses, and less compression, while all procedures differ markedly for some audiograms.

Abstract

A new procedure for fitting nonlinear hearing aids (National Acoustic Laboratories' nonlinear fitting procedure, version 1 [NAL-NL1]) is described. The rationale is to maximize speech intelligibility while constraining loudness to be normal or less. Speech intelligibility is predicted by the Speech Intelligibility Index (SII), which has been modified to account for the reduction in performance associated with increasing degrees of hearing loss, especially at high frequencies. Prescriptions are compared for the NAL-NL1, desired sensation level [input/output], FIG6, and a threshold version of the Independent Hearing Aid Fitting Forum procedures. For an average speech input level, the NAL-NL1 prescriptions are very similar to those of the well-established NAL-Revised, Profound procedure. Compared with the other procedures, NAL-NL1 prescribes less low-frequency gain for flat and upward sloping audiograms. It prescribes less high-frequency gain for steeply sloping high-frequency hearing losses. NAL-NL1 tends to prescribe less compression than the other procedures. All procedures differ considerably from one another for some audiograms.