As recall, Vb is to be tuned to speaker free air resonance 25hz.....At the time I made the
PVC duct, I made it long to be trimmed if needed....It sounded so good I left it....... later
I added A coupling / splice to extend the duct to facilitate testing / tweeking. I should
test the Z and check the two resonance peaks...I will take photos..You will enjoy you
cabinets what is the Vb ??? ........GC