Comparison of Two Non-linear, Multi-channel Fitting Algorithms Victor Bray The National Acoustic Laboratories new fitting algorithm (NAL-NL1) for multi-channel non-linear amplifiers has many similarities, and some differences, to an independently derived algorithm for a new 9-channel DSP hearing aid. Similarities are the utilization of audiometric thresholds to predict the dynamic range, adjustments for device style, and display of multiple frequency-gain targets with respect to different signal input levels. Differences are the fixed configuration of 9 compression channels and use of multi-channel Expansion in the proprietary algorithm. Both algorithms prescribe gain based on a target of the long term average speech spectrum combined with equalization of loudness across frequencies, but the two algorithms accomplish this through different mechanisms. Side-by-side examination of the two algorithms, including bench-top analysis and field-trial results, reveal the similarities and differences between the two formulae. Results of the comparison will be presented along with recommendations for future fitting algorithm development. |