The phonetics of phonological speech errors: An acoustic analysis of slips of the tongue

Acoustic analysis was used to examine whether speech errors involve lexical, segmental, or sub-featural errors in speech production. Nine participants produced tongue twisters that induced errors between /s/ and /z/ word onsets in contexts where the error outcomes were either words (e.g., sit to zit) or nonwords (e.g., suck to *zuck). Three measurements of the /s/-/z/ contrast were made: (1) percent voicing, (2) duration of frication, and (3) amplitude of frication. The tokens were also transcribed under careful listening conditions. Gradient and categorical errors were found for all acoustic dimensions. The errors might or might not be detected by careful listening, depending on the extent to which there were errors along all three dimensions. These data support previous articulatory studies that found speech errors at a sub-featural level. However, cases where /s/ and /z/ are realized with a categorical change in voicing are more common than would be expected if categorical changes in voicing were merely extreme examples of gradient voicing errors. Also, both gradient and categorical error rates were higher when the error outcomes were words. Thus, our study also provides evidence for the psychological reality of phonological segments and words as units in the speech production process.r 2002 Elsevier Science Ltd. All rights reserved.