Hello,
I noticed that the object [spectrogram~] won't show anything in the array in my pd-extended.
I'm using ubuntu 11.10 with pd-extended installed from the apt repository http://apt.puredata.info/auto-build/2011-12-07/ which seems the only place where I can find a compiled package.
In the system's official package "puredata" the object works fine.
I opened [spectrogram~] and using the [print~] object I noticed that [sqrt~] outputs only zeros, and the multiplications above shows some non-zero results.