Subjective judgements of clarity and intelligibility for filtered stimuli with equivalent speech intelligibility index predictions.

The purpose of this investigation was to determine whether subjective judgments of clarity or intelligibility would be rated equally among conditions in which speech was equated for predicted intelligibility (using the Speech Intelligibility Index, SII) but varied in bandwidth. Twenty listeners with normal hearing rated clarity and intelligibility for sentence material (Hearing In Noise Test) in speech-shaped noise at six paired low- and high-pass filtered conditions in which SII was equated for each pair. For three paired conditions, predicted intelligibility increased as SII increased monotonically (0.3, 0.4, 0.5). In the remaining paired conditions, SII continued to increase monotonically (0.6, 0.7, 0.8) but predicted intelligibility was held at a maximal level (> or = 95%). Predicted intelligibility was estimated from the transfer function relating SII to speech recognition scores determined in preliminary experiments. Differences in ratings between paired low- and high-pass filtered sentences did not reach statistical significance for either clarity or intelligibility, indicating that the spectral differences at equivalent SIIs did not influence the judgments for either of the two dimensions. For conditions in which predicted intelligibility increased, both clarity and intelligibility ratings increased in a similar manner. For conditions in which predicted intelligibility was maximized, intelligibility ratings remained the same statistically across conditions while clarity ratings changed modestly. Although high correlations were observed between clarity and intelligibility ratings, intelligibility ratings were consistently higher than clarity ratings for comparable conditions. The results indicated that listeners with normal hearing produced clarity and intelligibility ratings for the same speech material and experimental conditions that were highly related but differed in magnitude. Caution is required when substituting clarity for intelligibility.