No abstract

Naive listeners arc readily able to differentiate spontaneously produced speech from speech produced from text. The prior studies have employed lexically, syntactically, and thematically identical pairs of natural sentences extracted from brief fluent monologs (< 40 s in duration), finding relatively high levels of performance in tests of perceptual differentiation. To determine which attributes of the speech signal contribute to the perceptual differentiation of spontaneous and prepared speech, the present study manipulated several likely acoustic parameters employing techniques of speech synthesis. One condition reduced the frequency variation of the synthetic copies of the utterances to a monotone. A second condition removed the segmental attributes (consonants and vowels) from the sentence pairs by low-pass filtering of the synthetic signals, leaving metrical and fundamental frequency variation intact. The final condition neutralized both the segmental and phonatory attributes, leaving only metrical properties available to perceivers by which to differentiate the sentence pairs. Although systematic perceptual effects were anticipated, in fact these acoustic conditions modulated the differentiability of individual sentence pairs in different ways. Evidence of this kind indicates that perceptual analysis of spontaneity takes place at the level of the sentence, and comparisons across the set of conditions prove that no single acoustic emblem of the speech signal conveys spontaneity to the listener.