The Acoustics of Word-Initial Fricatives and Their Effect on Word-Level Intelligibility in Children With Bilateral Cochlear Implants

Abstract

Objectives: Previous research has found that relative to their peers with normal hearing (NH), children with cochlear implants (CIs) produce the sibilant fricatives /s/ and /∫/ less accurately and with less subphonemic acoustic contrast. The present study sought to further investigate these differences across groups in two ways. First, subphonemic acoustic properties were investigated in terms of dynamic acoustic features that indexed more than just the contrast between /s/ and /∫/. Second, the authors investigated whether such differences in subphonemic acoustic contrast between sibilant fricatives affected the intelligibility of sibilant-initial single word productions by children with CIs and their peers with NH.

Design: In experiment 1, productions of /s/ and /∫/ in word-initial prevocalic contexts were elicited from 22 children with bilateral CIs (aged 4 to 7 years) who had at least 2 years of CI experience and from 22 chronological age-matched peers with NH. Acoustic features were measured from 17 points across the fricatives: peak frequency was measured to index the place of articulation contrast; spectral variance and amplitude drop were measured to index the degree of sibilance. These acoustic trajectories were fitted with growth-curve models to analyze time-varying spectral change. In experiment 2, phonemically accurate word productions that were elicited in experiment 1 were embedded within four-talker babble and played to 80 adult listeners with NH. Listeners were asked to repeat the words, and their accuracy rate was used as a measure of the intelligibility of the word productions. Regression analyses were run to test which acoustic properties measured in experiment 1 predicted the intelligibility scores from experiment 2.

Results: The peak frequency trajectories indicated that the children with CIs produced less acoustic contrast between /s/ and /∫/. Group differences were observed in terms of the dynamic aspects (i.e., the trajectory shapes) of the acoustic properties. In the productions by children with CIs, the peak frequency and the amplitude drop trajectories were shallower, and the spectral variance trajectories were more asymmetric, exhibiting greater increases in variance (i.e., reduced sibilance) near the fricative-vowel boundary. The listeners' responses to the word productions indicated that when produced by children with CIs, /∫/-initial words were significantly more intelligible than /s/-initial words. However, when produced by children with NH, /s/-initial words and /∫/-initial words were equally intelligible. Intelligibility was partially predicted from the acoustic properties (Cox & Snell pseudo-R > 0.190), and the significant predictors were predominantly dynamic, rather than static, ones.

Conclusions: Productions from children with CIs differed from those produced by age-matched NH controls in terms of their subphonemic acoustic properties. The intelligibility of sibilant-initial single-word productions by children with CIs is sensitive to the place of articulation of the initial consonant (/∫/-initial words were more intelligible than /s/-initial words), but productions by children with NH were equally intelligible across both places of articulation. Therefore, children with CIs still exhibit differential production abilities for sibilant fricatives at an age when their NH peers do not.

Figure 1

Mean durations of productions of /s/ (black) and /ʃ/ (gray) for each group of children (large circles), shown with ± 2 standard error bars. Mean durations for individual children are shown as backgrounded smaller circles.

Figure 2

Peak ERB number trajectories of /s/ (solid, black) and /ʃ/ (dashed, gray). Points and 1062 error bars denote the mean and standard errors of the data, respectively. The lines and ribbons denote the median predicted trajectory and the 95% prediction interval, bootstrapped from 1000 replicates.

Figure 3

Variance trajectories of /s/ (solid, black) and /ʃ/ (dashed, gray). Points and error bars denote the mean and standard errors of the data, respectively. The solid lines and ribbons denote the median predicted trajectory and the 95% prediction interval, bootstrapped from 1000 replicates.

Figure 4

Excitation drop trajectories of /s/ (solid, black) and /ʃ/ (dashed, gray). Points and error bars denote the mean and standard errors of the data, respectively. The solid lines and ribbons denote the median predicted trajectory and the 95% prediction interval, bootstrapped from 1000 replicates.

Figure 5

Mean intelligibility scores for each group of children (large circles), shown with ± 2 standard error bars, for /s/-initial words (black) and /ʃ/-initial words (gray). Mean intelligibility scores for individual children are shown as backgrounded smaller circles.