Identification of synthesized [r-w] continua for adult and child speakers

A continuing problem in speech research is the comparison of phonetically similar but acoustically distinct sounds such as the comparison between adult speech and child speech. This problem is especially important in the remediation of speech disorders. In this study, we focus on this problem as manifested in the disordered production of [r] sounds. Stimuli which varied in F2 and F3 onset frequencies were synthesized to span continua from [r] to [w] for adult, child, and scaled adult (formants 1.5 times adult values) speakers. These stimuli were presented to 18 naive adults for identification as "R"or "W" at three listening sessions to test for stability of category boundaries. Significant shifts in category boundaries were found for all three continua between sessions one and two but not between sessions two and three. Category boundaries in terms of formant frequencies showed significant differences between the adult and childlike continua. Rescaling these values on a psychophysical dimension and adjusting for formant spread brought the category boundaries into close alignment. In free choice testing, synthesized [r] and [w] stimuli were identified by most subjects at least as well as natural [r] and [w] stimuli but synthesized child [r] stimuli were identified by most subject considerably better than natural child [r] stimuli.