Vowels are "rhotacised" if they are accompanied by a retroflex
articulation, which normally causes the third formant to sink very
clearly. Without seeing the spectrogram of your rhotacised [a] I
can't say what really happens there. If possible, try saying "a-ar-a"
to see what, if any formant transitions can be observed between plain
and rhotacised vowels.

Retroflexion is difficult to achieve if the tongue is too busy doing
other things, which is why high vowels (requiring the back or middle
part of the tongue to be raised) are less likely to be rhotacised
than low or mid-low ones. American or Irish English "rhotacised
central vowel" (as in _bird_) is in fact a syllabic /r/.

Tongue-root retraction is characteristic of American /r/, and
especially of its "bunched" version (in which it's the dorsum of the
tongue, not the tip, that rises towards the hard palate). This tongue-
root gesture (combined with slight lip-rounding) helps to produce the
same (or very similar) acoustic pattern as that resulting from
retroflexion. The combined effect is a very low third formant:

http://www.umanitoba.ca/faculties/arts/linguistics/russell/138/sec4/en
hance.htm

(unwrap the URL before use)

"Rhotic" sounds are difficult to define; the class includes several
types of sound -- trills, flaps, taps, frictionless continuants and
even fricatives. Most show the lowering of the third formant, but
some do not. Perhaps the best definition is a negative one: rhotics
are non-lateral liquids (or segments equivalent to liquids in terms
of phonotactic functions).

A good overview of what is possible in human speech is this:

Ladefoged, Peter, and Ian Maddieson. 1996. Sounds of the World's
Languages. Oxford: Blackwell. (With lots of diagrams, spectrograms,
palatograms, etc.).

If you give me a little time, I'll compile a list of my favourite
handbooks.

Anyone wishing to do some acoustic analysis on their own can obtain a
copy of a good speech-analysis program by contacting Dr Paul Boersma
(University of Amsterdam) through the following URL:

http://www.fon.hum.uva.nl/praat/

Piotr

--- In phoNet@y..., m.v.ct@t... wrote:
>
>
>
> I understand that rhoticity and rhotic are terms
> applaied to consonant phones that become an [r] phone
> (rhoticity) and to vocalic phones that have added some
> [r] like sound. And it seems that articulatorily the English
> rhotic vowels are related to a retracted tongue root.
> Also i have read that rhotic vowels are acoustically
> characterized by a lower third formant, but as i have made some
> spectrograms with my own pronunciation of them -for example
> pronouncing a rhotic [a´]- the third formant is seen in what
> looks more a higher position than a lower...
> So my query is that if you know how are vocalic rhotic
phones
> accoustically defined please let me know.
>
> In another sense i understand that when speaking of
consonants
> the word "rhotic" is not used, an instead retroflex would be the
> proper. If you have any comment on this also would be wellcome.
>
> And the last, if you know a good manual of general phonetics
> (i mean by "general" not debvoted to the phonetics of just a
> particular language or familly of languages) that you think worthy
to
> recommend i please to tell me it.
>
> Yours cordially,
> mariano