Distributional characteristics of VOT in children's voiceless aspirated stops and interpretation of developmental trends.

Numerous researchers have measured voice onset time (VOT) in children. Authors attempting to trace developmental trends in consonant voicing have frequently framed their hypotheses in terms of how children's VOT means and/or standard deviations compare to adult norms. However, data from previous studies suggest that children's VOTs may not be normally distributed. Specifically, rightward skew is observed in the voiceless aspirated stops, such that mean values exceed the medians. The current work presents detailed distributional analyses of VOTs in /p, t/ from 7 five-year-old children and 14 adults. Distributional non-normality was common in both the adult and child data, as measured by Shapiro and Wilk's W statistic. The children showed an insignificant but consistent tendency towards higher values of skew than the adults and greater differences between VOT mean and median values. The results suggest that theories of VOT development should not be based solely on means and standard deviations, but need to address the distributional characteristics of the data more fully.

Laryngeal factors in voiceless consonant production in men, women, and 5-year-olds.

Voicing control in stop consonants has often been measured by means of voice onset time (VOT) and discussed in terms of interarticulator timing. However, control of voicing also involves details of laryngeal setting and management of sub- and supraglottal pressure levels, and many of these factors are known to undergo developmental change. Mechanical and aerodynamic conditions at the glottis may therefore vary considerably in normal populations as functions of age and/or sex. The current study collected oral airflow, intraoral pressure, and acoustic signals from normal English-speaking adults and children producing stop consonants and /h/ embedded in a short carrier utterance. Measures were made of stop VOTs, /h/ voicing and flow characteristics, and subglottal pressure during /p/ closures. Clear age and gender effects were observed for /h/: Fully voiced /h/ was most common in men, and /h/ voicing and flow data showed the highest variability among the 5-year-olds. For individual participants, distributional measures of VOT in /p t/ were correlated with distributional measures of voicing in /h/. The data indicate that one cannot assume comparable laryngeal conditions across speaker groups. This, in turn, implies that VOT acquisition in children cannot be interpreted purely in terms of developing interarticulator timing control, but must also reflect growing mastery over voicing itself. Further, differences in laryngeal structure and aerodynamic quantities may require men and women to adopt somewhat different strategies for achieving distinctive consonantal voicing contrasts.

Time normalization of voice signals using functional data analysis.

The harmonics-to-noise ratio (HNR) has been used to quantify the waveform irregularity of voice signals [Yumoto et al., J. Acoust. Soc. Am. 71, 1544-1550 (1982)]. This measure assumes that the signal consists of two components: a harmonic component, which is the common pattern that repeats from cycle-to-cycle, and an additive noise component, which produces the cycle-to-cycle irregularity. It has been shown [J. Qi, J. Acoust. Soc. Am. 92, 2569-2576 (1992)] that a valid computation of the HNR requires a nonlinear time normalization of the cycle wavelets to remove phase differences between them. This paper shows the application of functional data analysis to perform an optimal nonlinear normalization and compute the HNR of voice signals. Results obtained for the same signals using zero-padding, linear normalization, and dynamic programming algorithms are presented for comparison. Functional data analysis offers certain advantages over other approaches: it preserves meaningful features of signal shape, produces differentiable results, and allows flexibility in selecting the optimization criteria for the wavelet alignment. An extension of the technique for the time normalization of simultaneous voice signals (such as acoustic, EGG, and airflow signals) is also shown. The general purpose of this article is to illustrate the potential of functional data analysis as a powerful analytical tool for studying aspects of the voice production process.