How do NAL-NL1 Prescription Principles Agree with Hearing Aid Fitting Objectives? Denis Byrne The principles of the NAL-NL1 non-linear hearing aid fitting procedure are reviewed in terms of how well they are likely to serve a range of possible hearing aid fitting objectives. The procedure's basic principle is to optimise speech intelligibility, as predicted by a modified version of the Speech Intelligibility Index (SII), while constraining the overall loudness of speech to be no greater than normal. The SII modification is a "Hearing Loss Desensitization" adjustment, based on NAL research showing that, as hearing loss becomes greater, there is a reduced contribution of audibility to speech intelligibility and the reduction is greatest for the high frequencies. Although not specifically designed to do so, NAL-NL1 tends to equalise the loudness of different frequency bands of speech and, in that respect, is similar to the NAL-RP linear fitting procedure. NAL-NL1 should maximise understanding of average or loud speech as it is designed to do so. It may not necessarily maximise understanding of soft speech because this is constrained to be no louder than normal. It may not necessarily optimise listening comfort as above average sounds are permitted to be amplified to higher than the preferred listening level. Sound quality was not specifically considered but has been evaluated indirectly for average levels in the validation of the NAL-RP procedure. The NAL-NL1 rationale does not consider optimising the detection, localization or externalisation of sounds. On theoretical grounds, those objectives might be better served by prescriptions having relatively more low-frequency amplification. This is examined in a limited way by comparing localization with NAL-NL1 prescriptions and those of another procedure. |