My acoustic research into 'labiodental' /r/ in adult speakers of British
English (various varieties) suggest very strongly that for 5 out of 6
subjects there is some degree of velarisation, though the constriction is
probably not as far back as for /w/. The formant structure for 'labiodental'
/r/ for these subjects is similar to that seen for the Russian barred /i/,
with F1 around 350 Hz, F2 at around 1400 Hz, and F3 around 2000 Hz. Given
the Russian parallel, this kind of /r/could be described as a high/close
central glide. Direct confirmation of the presumed velarised articulation is
still lacking.
The one remaining subject has formant patterns for /r/ which do not differ
significantly from those for /v/ (the average value of F2 is almost
identical in fact), so a truly labiodental /r/, but only in 1 case out of 6.
Sorry I can't furnish a more wide-ranging reference.
Mark
Mark J. Jones
Department of Linguistics
University of Cambridge
http://kiri.ling.cam.ac.uk/mark/
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