The preferred number of channels (one, two, or four) in NAL-NL1 prescribed wide dynamic range compression (WDRC) devices

Objective: The recently introduced NAL-NL1 rationale for fitting WDRC devices prescribes a relatively high compression threshold and prescribes compression ratios lower than those prescribed by loudness normalization rationales. The aim of this study was to investigate whether the compression characteristic prescribed by NAL-NL1 is most effective in a single-channel scheme or in a multi-channel scheme. Design: Twenty-four subjects with flat or steeply sloping hearing loss participated in the study. One, two, and four channels were implemented digitally in the laboratory and evaluated on the basis of a paired-comparison test and a speech recognition test. The test stimuli consisted of speech and noise presented at average input levels, and speech and noise alternating every 3 sec among different input levels. The single-channel and 2-channel NAL-NL1 prescriptions were also evaluated in individually selected everyday situations in the field using a digital 2-memory device. Results: The three compression schemes produced no significant difference in speech recognition scores. Most subjects showed no preference for either scheme in the paired-comparison test. Those who did mainly selected the single-channel scheme. These preferences can be explained on the basis on audibility and quality. In the field all subjects with a steeply sloping loss, but one, preferred the 2-channel scheme. Among the subjects with a flat loss more preferred the single-channel scheme than preferred the 2-channel scheme. Statistical analyses showed that those who preferred the 2-channel scheme were fitted with significantly greater differences in the compression ratio in the high frequencies, and those who preferred the single-channel scheme were fitted with significantly greater differences in the high-frequency gain for a 65 dB input. Conclusions: Multi-channel compression prescribed according to NAL-NL1 in up to four channels showed no adverse effects on speech recognition relative to a single-channel scheme. The pairedcomparison test showed a small, but explainable preference for the single-channel scheme. The field test revealed a preference for the 2-channel scheme by subjects with steeply sloping loss. When using the NAL-NL1 rationale it is recommended to use at least two channels when fitting clients with sloping loss, whereas for clients with flat loss the number of compression channels is probably unimportant.