The duration of three infants' vocalisations were examined during a six-month longitudinal study. In contrast to most other infant research, this study included in its analysis all vocalisations including those deemed vegetative or those having non-modal voice quality. All three infants produced vocalisations which decreased in duration in the initial months. However between the 3rd and 5th month a significant increase (p<0.001) in the duration of vocalisations was found (from mean 207ms to 431ms). When vocalisations were analysed using perceptual voice quality categories, all were found to have significant differences (p<0.05) in duration relative to modal voice. Non-modal voice qualities showed initial decreases in duration before increasing in duration in later months. In contrast those vocalisations produced using modal voice showed a positive linear trend and had the greatest linear rate of change across the study. These findings highlight the importance of including a wide variety of infant vocalisations including those with non-modal voice quality in infant linguistic developmental studies.
Bibliographic reference. Gregory, Adele / Tabain, Marija / Robb, Michael (2013): "Duration of early vocalisations", In INTERSPEECH-2013, 1258-1262.