18" reflex sub ports

so basically i've positioned the diamond shaped plate further forward thus cutting the baffles from the rear...that gave me more volume to play with.

now i've always wondered whether theres a difference in performance with front vs rear mounted drivers?

cos if i can mount them outwards, that'll save some hassle of making sure they can actually fit inside.

i was thinking about that last night...to make the diamond shaped panel a port panel! I'll need to measure it carefully though as for now it's just a plate.

I actually like the look of the 1 box so i'd prefer to keep it as 1 and just extend the height to 60cm and see what the volume ends up being.

Space in front of the baffle doesn't count towards the enclosure size..but as mentioned it will have some impacts on the sound reproduction which are harder to predict. But will probably be positive in terms of the character of the sound and perceived impact delivered by the box. Appearances are important too and can have a slight placebo effect. It looks loud and punchy.

Looking at the height of your subs, and considering a midtop could easily be designed to be between 35 and 55cm, even if your kick section was 65cm you'd be looking at somewhere between 200 and 225cm overall stack height, depending on whether the kick is in two sections and how tall your midtop is, which is typical, if not optimal, for a system like this. If it's at the high end of that number and the kicks were in two sections you could adjust the height to 190cm for low ceilings, cosy/intimate gigs, or gigs where the system is going to be raised on a stage already, and you could definitely design the midtops to be pole-mountable so there wouldn't be any situations you couldn't work with.

A close commercial equivalent of what you're trying to achieve is the HK Audio Linear 5 system. 'big venue pack'. http://hkaudio.com/products.php?id=400

That's 233cm tall. Their smaller 'rock pack' is 201cm. That's fairly typical. If anything the 'rock pack' is a little shorter than a typical compact ground stack.

Another benefit of designing your own midtops is you could give them that unusual 57.5cm wide form to match the rest of the stack. A set up with the TH18 and then 3 matched dimension boxes, all 57.5 x 34cm from the front (with the midtop box being not quite so deep), could look very neat. You'd get a good visual balance of two sections 'the bass', and 'the rest', each approximately 1 metre tall.

Something like this ie: 

And in 'short stack' mode:

It's worth considering that for the 65-125Hz frequency range, the 12" drivers really don't need to be facing anything like straight forwards. The resulting look of a design that works might not look like it will work well at all, so if you can only see one way that they could physically fit within your size requirements, and it doesn't look like it'll work, try it anyway. Hopefully someone can tell just from looking at it.

There are a few other unusual loading types for lots of drivers in a small space like push/pull designs, but don't know how they work exactly or if they would be appropriate.