Typical relative frequencies of the first four formants measured in dF units (average spacing between formants, or formant dispersion) above or below schwa based on estimated VTL in American English, from Hillenbrand (1995), who measured F1-F4 in ~1.5K recordings (139 speakers, 12 vowels from each). Audio and formant measurements are freely available online: https://homepages.wmich.edu/~hillenbr/voweldata.html. The dataset below is the result of modeling Hillenbrand's data with brms: mvbind(F1rel, F2rel) ~ vowel + (vowel|speaker). It shows the most credible location of each vowel centroid in the F1Rel-F2Rel space.
An object of class
data.frame with 12 rows and 5 columns.
A dataframe of 12 observations and 5 columns: "vowel" = vowel (American
English), "F1Rel" to "F4Rel" = formant frequencies in dF relative to their
neutral, equidistant positions in a perfectly cylindrical vocal tract. See
schwa - this is what schwa() returns as $ff_relative_dF
Hillenbrand, J., Getty, L. A., Clark, M. J., & Wheeler, K. (1995). Acoustic characteristics of American English vowels. The Journal of the Acoustical society of America, 97(5), 3099-3111.
plot(hillenbrand$F1Rel, hillenbrand$F2Rel, type = 'n') text(hillenbrand$F1Rel, hillenbrand$F2Rel, labels = hillenbrand$vowel)
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