Piotr and David, thank you for the biblography.

Piotr it is enought for me with just the manual you mention,
I have searched it in amazon.com and I think it is what I was asking for.

Thank you for your explanations and the link you gave, i have visited
it and there I have found also the kind of physical explanations of
the acoustical traits that i wanted to know.

[you wrote]
"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).
Well, i see that there are more questions that need to be answered.
A problem that i have with rhotics is that I have not yet understood
why are they +vocalic, because the voiced component -wich is called also
svarabatic component- that i think might be the reason to consider
them +vocalic is not always present, as for example in "ere" when
the rhotic is not a trill and is in between vocalic phones (?).

Thank you -once more- for the link to the Prat program. I use
the "Speech Anylizer" of the SIL (that can be found at www.sil.org )
and is a good program, but one thinks that never is enough to know
what one has if there is something else that one might try, so that i have
asked for the Prat also.

Yours cordially,