3.5 Way .5 Additional Volume

[Einric]

I am looking into adapting my two way DC28f + RS100-8 into a 3.5 way.
I will be looking to add a pair of Aura NS6-255-8A for the 3 & .5
My question is how much extra box volume is needed for the .5?
I would be looking at a .8 cuft sealed enclosure with the Aura crossed in at baffle step.
The second Aura will be added at 80hz to boost the low end.
How much extra volume should be included to account for the second driver?
Wouldn't the second driver totally destroy the response of the first driver when it kicks in at 80hz?
Should I model sealed with two and with one then take a midpoint on the smaller side of half?

Thanks

[Tytte71]

The normal way of dealing with .5 systems are to use the second driver to compensate for the baffle step diffraction loss and then let both drivers run in parallel downwards. Ideally both drivers would have equal volumes.

Easiest solution IMO would be to make it as a 3-way with both drivers running in parallel over its working range.

If you choose to make a 3.5 way, each of the woofers should preferably have about 6dB higher sensitivity than the RS100-8. Using the two added drivers in a 3-way, each of the drivers should preferably have equal sensitivity to RS100-8. Avoid selecting added drivers that in sum have higher sensitivity than the RS100-8. The Aurasound NS6-255-8A have some 7,5dB higher sensitivity than the RS100-8.
I don't know much about the Aurasound driver, but I would rather select one or two woofers that in sum have similar or lower sensitivity compared to the RS100-8 and that are made to play bass (lower fs, higher xmax, etc).

The 3,5 way idea could work but care should be put into tuning the cabinets (Q) and high-pass filtering the .5 driver (fill-in driver).