Effects of coarticulation, prosody, and noise freshness on the intelligibility of digit triplets in noise

Background: In a number of European countries, a functional self-test to screen for hearing impairment is available via telephone and the Internet. The tests estimate speech-reception thresholds using an adaptive procedure in which digit triplets are presented at varying signal-to-noise ratios. In different languages, the stimuli were created either with or without coarticulation; and some implementations use fresh noise samples, while others do not. Purpose: The present investigation concerns the influence of coarticulation, prosody, and noise freshness on measured thresholds. Study Sample: We performed a laboratory study using 12 normal-hearing listeners. Research Design: In a blocked design we compared speech-reception thresholds for conditions with and without fresh noise tokens. In each block we used three types of triplets: with coarticulation and prosody, with neither, and without coarticulation but with prosody. Data Collection and Analysis: Thirty-six thresholds were recorded per subject, and they were analyzed using analyses of variance. Results: The results showed no significant differences among the three triplet conditions. The freshness of the noise did not affect thresholds when, at least, a fresh noise token was used per threshold estimate (23 presentations). Scores dropped significantly when a whole experimental block was performed with a single noise token.