Hi Andrea,
welcome! Simple spectrum computations can be found on the documentation annexed with pd: look at the subfolder /doc/3.audio.examples of your PD distribution , for example E01.spectrum.pd. This uses simple fft computations.
Then there is the cyclone library that has a good
spectrum visualization example in the patch
spectrum-pm.pd for example. Note that cyclone only comes
with "extended" distributions by H.C. Steiner or
you have to install it yourself. If you have the
extended one the patch is in the
\doc\examples\cyclone
subfolder.
Another interesting example is here:
http://crca.ucsd.edu/~jsarlo/pd/
grab the "pitchnoise" patch, it contains another interesting example.
Hope that it helps,
Alberto Zin
http://puredata.org/Members/AlbertoZ
PS: by chance I got two books by you on my desk: your thesis on musical notation and the books on "Audio & Multimedia". Congratulations, nice works!