Developmental weighting shifts for noise components of fricative-vowel syllables.

Previous studies have convincingly shown that the weight assigned to vocalic formant transitions in decisions of fricative identity for fricative-vowel syllables decreases with development. Although these same studies suggested a developmental increase in the weight assigned to the noise spectrum, the role of the aperiodic-noise portions of the signals in these fricative decisions have not been as well-studied. The purpose of these experiments was to examine more closely developmental shifts in the weight assigned to the aperiodic-noise components of the signals in decisions of syllable-initial fricative identity. Two experiments used noises varying along continua from a clear /s/ percept to a clear /[symbol: see text]/ percept. In experiment 1, these noises were created by combining /s/ and /[symbol: see text]/ noises produced by a human vocal tract at different amplitude ratios, a process that resulted in stimuli differing primarily in the amplitude of a relatively low-frequency (roughly 2.2-kHz) peak. In experiment 2, noises that varied only in the amplitude of a similar low-frequency peak were created with a software synthesizer. Both experiments used synthetic /a/ and /u/ portions, and efforts were made to minimize possible contributions of vocalic formant transitions to fricative labeling. Children and adults labeled the resulting stimuli as /s/ vowel or /[symbol: see text]/ vowel. Combined results of the two experiments showed that children's responses were less influenced than those of adults by the amplitude of the low-frequency peak of fricative noises.