Thank you so much AllenB,

I realise I missunderstood the Dickason book about it. the shematic is not so clear and I understood. I thought it was the lowest frequency till the shortcut that tilted up... and it's quite the oppositt... Great step up for my understanding

I thought it was a bobsleigh curve and it's a launching ramp instead

I didn't give you the values for two reasons : my original question was about to understand the concept for any crossover and refering to the crossover I study I haven't the inductances values nore the exact capacitances. I just see it is in the midle of a LR4 cause there are 2 shunted inductances.

Ok for the phase, curve is more important first, so forgett the phase if you need to correct the curve.

So the idea to choose the capacitance as a starting frequency (first order serie cap) is good but the curve will tilt up towards the highest frequency. the higher the Z of the resistor the stiffer the launching ramp and I assume the spl bump at the end is the independance curve at the frequency level + the Z of the resistor ?

Am I right know ?

Sorry to have not a LCR to give you filter values for a proper more precise illustration, but I haven't a LCR yet...and frankly it doesn't matter for my understanding. I still have to build a diy box for that to use with a Arta or Rew to measure in real life and see the result in a real filter... one step at once

Thank you again AllenB, your help is much appreciated. I wanted first to understand the mechanism of this serie R//C. So the behavior is near a L-pad ... sort of.