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Second Biennial Hearing Aid Research and Development Conference

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September 22-24, 1997
National Institutes of Health
Bethesda, Maryland

Subjective Judgments of Clarity and Intelligibility for Filtered Stimuli with Equivalent Speech Intelligibility Index Predictions

Laurie S. Eisenberg, Donald D. Dirks, Sumiko Takayanagi, and Amy Schaefer Martinez, House Ear Institute, West Los Angeles, Veterans Administration Medical Center, and UCLA School of Medicine

The primary purpose of this experiment was to determine whether subjective judgments of clarity or intelligibility would be rated equal among conditions in which speech was equated for intelligibility (using the Speech Intelligibility Index, SII) but varied spectrally. Twenty normal-hearing listeners rated clarity and intelligibility for sentence material (Hearing in Noise Test) in speech-shaped noise at six paired high- and low-pass filtered conditions in which SII was equated for each pair. For three conditions, SII decreased monotonically (.8, .7,.6) but predicted intelligibility was held at a maximal level (95%). In the remaining conditions, intelligibility decreased as SII decreased (.5,.4,.3). Predicted intelligibility was estimated from the transfer function relating SII to intelligibility scores determined in preliminary experiments. Differences in ratings between paired high- and low-pass filtered sentences did not reach statistical significance for either clarity or intelligibility, indicating that both quality dimensions were rated the same at equivalent SIIs. For conditions in which the SII was equated and predicted intelligibility was maximized, intelligibility ratings remained essentially the same across conditions while clarity changed modestly. For conditions in which predicted intelligibility decreased, both clarity and intelligibility ratings decreased in a similar manner. Although high correlations were observed between clarity and intelligibility ratings, intelligibility ratings were consistently higher than clarity ratings for comparable conditions. The results suggest that normal-hearing listeners produce clarity ratings or intelligibility ratings for the same speech material and experimental conditions that are highly related but may differ in magnitude. Caution is required when substituting clarity for intelligibility.

[Supported by NIDCD and VA]

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