NAL-NL2 empirical adjustments
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NAL-NL1, the first procedure from the National Acoustic Laboratories (NAL) for prescribing non-linear gain, was a purely theoretically-derived formula aimed at maximizing speech intelligibility for any input level of speech while keeping the overall loudness of speech at or below normal loudness. The formula was obtained through an optimization process in which speech intelligibility and loudness were predicted from selected models. Using updated models and applying some revisions to the derivation process, a theoretically-derived NAL-NL2 formula was obtained in a similar way.