DIY Subwoofer enclosure questions

[Mishaiger]

Hello. i'm currently trying to figure out what subwoofer enclosure to build. I'll be building 4 same subs. I'll place them in corners of the room.
I've already built 3 different enclosures over past years. They are all sealed and small, because i lacked space for bigger subs. Now i finally got a little more space for subs, so i wanted to try vented/ported/bass reflex enclosure.

I have 4 "URAL Bulava 12 V.2" 12 inch subwoofer drivers. And 150W amplifiers for all 4.
Here is simulation of what i'm trying to achieve:
Basically, i'm thinking about port that is tuned to 14 hz. This seems to be ideal frequency to NOT add group delay or phase shift to 30hz and above.

But once again, i still have too little space in room for subwoofer bigger than about 130 liters.
Here is rough representation of what i'm dealing with right now:


I don't know if this is appliable to sub enclosure... I don't plan on stuffing sub with anything, so first resonance should be around 276hz, which is far from sub working range, so should not be a problem. Correct me if that is not how things work, because i'm not sure myself.
VituixCAD2 says i need port length about 107cm, but in this configuration above i'm missing about 30cm of port length.

So. here is the first bunch of questions/options:
1) If i fold a port to something like this to gain necessary length, wont it become transmission line or something? Will it introduce some problems, like chuffing sound or increased distortion maybe...
2) make port area (cm2) smaller. This will make port shorter, sure, but it will make velocity skyrocket:
Well, i can remove frequencies lower than 20hz with EQ, so this potentially solves the problem of velocity...


I think i should probably mention what i'm trying to achieve.
I need 115 dB spl from these 4 subs for movies. Even more, since i'll be redirecting bass from speakers to subs. And i had this with 4 sealed subs, but there was distortion starting from 30hz and lower, about 20%:

I have no idea if this distortion is audible, probably is, but since i have the opportunity to make subs enclosure better, i think i should.

VituixCAD2 tells me that bigger AND ported enclosure will boost:
20hz - 5dB
30hz - 2dB
40hz - 1 dB
compared to smaller AND sealed enclosure. This will reduce distortion and load on amplifier, exactly what i'm looking for.
But i also listen a lot of stereo music, so it must not have high group delay above 30hz.

So, in summary, what to do with port?
I'm also open to any other ideas, like changing box shape, although, i cant' make front any more or less than 40cm... or maybe tune port differently... transmission line?..

 

[audiofooled]

IMHO your main issue here is that the subs are seriously underpowered. Huge boxes and low tuning are hardly ever going to solve the problem regarding distortion. Drivers are simply power hungry, and are made for high excursion and SPL from small boxes (car audio?). For that you need power. For this driver, WinISD suggests vented box volume of 62 liters and 24.5Hz port tuning for optimal FR magnitude (green plot).

If you put it in a box of 130 liters and somehow tune it to 14Hz (red), you loose a lot of efficiency/output throughout the usable band and would require more power anyway, also DSP to flatten the response. So you're effectively trading less group delay for more THD and port air velocity/chuffing. DSP on the other hand would potentially add group delay and mess up your impulse/step response so again I see little to no benefit in a large box.

Bottom line, your SPL goals are certainly attainable, but perhaps if you go sealed (for the sake of discussion blue graph is sealed, the same 62 liter box), you need more power anyway but you have a better use of room gain and may flatten the response using DSP, with less adverse effects. You may even keep you current boxes because this driver is good for as small as 28 liter box for an optimal Qtc of 0.707.

Correct me if I'm wrong, but it seems to me that you need all of your amps, and then some, to drive just one of the subs (manufacturer claims 900W continuous, 1800w peak?). In any arrangement I would suggest you get rid of sub 20Hz frequencies in order not to kill your amp/drivers due to potential clipping.

Have fun!