Looks like aiming for a slot cutout of the same area as the cone would be what to aim for in order to extend the top end of the rising curve but avoid too much of a peak, perhaps even a little larger if the subs are set back from the opening? (which I presume most of an 18" driver would be? lol)
That looks great! I'm guessing it would have been a lot larger without the isobaric layout? I've not even started to read up on Transmission Line speakers, my head is already too full of bits of info that don't always make sense... lol
I'm not quite sure where 'slot loading' turns into 'just a massive hole where the driver is located' but I think I will just have to design to the smallest plenum/slot/vent I can as I am so limited on space! Hopefully it will sound ok...
Yes and no. Transmission line design (I am just a novice) revolves around the rear line volume being determined by the surface area of the woofer (Sd) and the tuning frequency to match the natural resonance of the speaker. I did not work with a predetermined volume requirement. So I could have used a single 4 ohm sub and build the same size. I would expect the same performance but that is just a guess.
I am thinking that the overall enclosure volume might control excursion of the woofer but I have not deep dived into those aspect of enclosure design.