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The retraction of /s/ in /str/, eg street, is a sound change found in certain English dialects. Previous work suggests that /s/-retraction arises from lower spectral frequency /s/ in /str/. The extent to which /s/-retraction differs across English dialects is unclear. This paper presents results from a large-scale, acoustic phonetic study of sibilants in 420 speakers, from 6 spontaneous speech corpora (9 dialects) of North American and Scottish English. Spectral Centre of Gravity was modelled from automatic measures of word-initial sibilants. Female speakers show higher frequency sibilants than males, but more so for /s/ than /ʃ/; /s/ is also higher in American than Canadian/Scottish dialects; /ʃ/ is surprisingly variable. /s/-retraction, modelled as retraction ratios, is generally greater for /str/ than /spr skr/, but varies by dialect; females show more retraction in /str/ than males. Dialectal and social factors clearly influence /s/-retraction in English clusters /sp st sk/, /spr skr/, and /str/. 

To test the hypothesis that intraspeaker variation in vowel formants is related to the direction of diachronic change, we compare the direction of change in apparent time with the axis of intraspeaker variation in F1 and F2 for vowel phonemes in several corpora of North American and Scottish English. These vowels were measured automatically with a scheme (tested on hand-measured vowels) that considers the frequency, bandwidth, and amplitude of the first three formants in reference to a prototype. In the corpus data, we find that the axis of intraspeaker variation is typically aligned vertically, presumably corresponding to the degree of jaw opening for individual tokens, but for the North American GOOSE vowel, the axis of intraspeaker variation is aligned with the (horizontal) axis of diachronic change for this vowel across North America. This may help to explain why fronting and unrounding of high back vowels are common shifts across languages.