Thanks to you Dean70 ! It's a really good idea to test equlization using tone (pink noise ?) and analyser view; Even if it could have been more accurate, it gives some ideas about the real effect of filters...
As a result, shelving filters seems to work like that :
Frequency = central frequency (not like crossover cut-off frequency as I was wondering before)
Bandwidth = not used ( constant filter slope )
Gain = total gain/attenuation between lowest and highest frequency
Actually filter slope is 3 dB per octave, so you could adjust the real band width by setting the correct gain value; For instance a gain value of 12 dB will give a four octave band width
Example : F= 140, B=NA, G=-12 => sheving filters with 3 dB slope from 35 Hz to 560 hz, -12db after 560 hz
Could anyone from dev team confirm or not ?