2.3.3. Dispersion and overlap of vowel categories in EFL and native AE

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Plotting the centroids does not tell the whole story. So let us also plot the spread of the vowel tokens and consider the amount of overlap between adjacent vowel categories. Figure 4 plots spreading ellipses around the centroids shown in Figure 3. These ellipses contain (theoretically) the 46% most typical tokens of the vowel category.
 

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Figure 4 . Centroids and dispersion ellipses for eleven American English monophthongs produced by monolingual and bilingual groups of EFL learners (/hVd/ items only), broken down by gender. Ellipses are drawn at ± 1 SD along the first two principal components of the scatter clouds. The right-most panels represent the control data produced by 10 male and 10 female native speakers of American English. See Figure 1 for details.
 

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The overlap between /i/ and /ɪ/ is substantial in all four panels A, B, D, and E. So is the (almost complete) overlap between /u/ and /ʊ/ (possibly, these contrasts will be upheld by a difference in vowel duration). There is a large difference in spectral distribution for the members of these long-short vowel pairs in the American control data, which are shown in panels C and F.

Jegyzet elhelyezéséhez, kérjük, lépj be.!

In the American native control data, there is no visible spectral overlap between /i/ and /ɪ/, nor between /u/ and /ʊ/. Also, /ε/ is clearly separated from /e, ɪ/, and /ʌ/ from the low-back vowel cluster /ɔ, ɑ/. The short/lax vs. long/tense mid vowel pairs /e, ɪ/ and /o, ʊ/ will be distinct by a difference in duration (see next section).
 
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