Question
In simple terms, what is the real difference between the NAL-R and the NAL-NL1 fitting rationales? I would appreciate some clarification.
Answer
NAL-R (which stands for NAL-Revised) is a prescription procedure that indicates how to fit linear hearing aids to people with mild or moderate hearing loss. It's successor, NAL-RP (Revised, Profound, which you did not ask about) results in the same prescription for people with mild or moderate loss, but can be used for people with any degree of loss, severe and profound hearing loss. NAL-RP is also intended only for linear hearing aids, so at any frequency, the same gain is applied at all input levels. Despite this, some people have also applied it to non-linear hearing aids by using it to establish the gain-frequency response at an input level of 65 dB SPL, and then adding compression to raise the gain for lower input levels and lower it for higher input levels.
For non-linear hearing aids, however, we recommend the NAL-NL1 (Non-linear, version 1) procedure. The NAL-NL1 prescription was derived by finding the gain-frequency response that theoretically maximizes speech understanding while holding total loudness to be no greater than normal. The prescription are inherently non-linear, in that the gain prescribed depends on input level as well as on frequency and hearing loss.
The NAL-RP and the NAL-NL1 responses have three things in common. Both prescribe very similar gain-frequency responses for input levels around 65 or 70 dB SPL. Both are aimed at maximizing speech intelligibility, rather than restoring loudness at each frequency to normal. Both are well supported by background research as well as by empirical comparisons of their effectiveness relative to alternative procedures.
Dr. Harvey Dillon is the Director of Research of the National Acoustic Laboratories of Australia. He has been involved in various aspects of hearing aid research for the last 20 years, and lives in Sydney, Australia.