As you all already know, i use Vorbis for my artists' downloads pages.
The quality is 'hearable' (mono, 32000 Hz, q=0), but the size is still too mutch - remember that i put the whole album downloads (as chained oggs).

Any suggestion to squeeze (even little) more the size, keeping the 'hearability' ?
(in other terms, how use the << --advanced-encode-option lowpass_frequency= >> option ?)

Thanks in advice.

The lowpass frequency is set in all oggenc varieties in kHz, floating point; eg., 9.55.

In oggdropXPd, the input is in Hz and is converted internally to kHz.

oooh... a thread about my favorite switch

first thing... in my testing, using mono doesn't do any favors for making things sound more 'listenable' at lower bitrates.. actually, I have preferred how stereo sounds at equalavant bitrates (stereo seems to mask some artifacts according to my ears)

* moving the lowpass frequency above the default will add a few kbps
* moving the lowpass frequency above the default with less than -q 0 (-q -1) isn't all that great

if you want to reduce the size, lowering the quality parameter is the only way (or use a lower sample rate), I would try to keep it -q -.5 or higher (unless you use less than a 26khz samplerate)

I could be of more help if I knew what your target bitrate was
I'm not sure what the bitrate is around with -q 0 mono @32000hz.. so I don't know where to start