That is probably my biggest concern right now - will I get enough SPL out of running two Exodus DPL-15s at 250W each for this space. It's my current understanding that you don't need as much power when using IB subs (or semi-IB in my case) compared to sealed boxes as there is no air spring effect so I'm basically relying on the modelling software at this point and going from there. That being said, the modelling software projects that I will get an SPL of 112-115dB (from 20 Hz and up) when running each driver at 250W. This doesn't really mean a whole lot to me however as I don't really have any experience to compare this to. Is 115dB considered to be loud or is this something that is going to leave me disappointed? I don't want to be vibrating things off of shelves and knocking pictures off the wall - I just want to hear/experience all of the LFE present in whatever media I'm playing (movies/music/games).
Regarding the filter comment, I was planning on running without a HPF as these drivers do not exceed their Xmech (29 mm) until going below 3 Hz according to the modeling software (again, assuming a power of 250W). I'm hoping that I won't run into over excursion issues on the assumption that there were be little to no "audio" present at 3 Hz and below.
Cost is a limiting factor for me as well. Two Exodus DPL-15s and a Behringer EP2500 amp will only set me back around $570. I initially wanted to build the subs for a budget of $500 so this basically meets my original financial goals. I can be a little bit flexible but anything over $1000 is a little steep for me at this point considering that I also to have pay for an AVR, projector, screen, surround speakers, blu-ray player, etc.
Thanks for the reply and sorry for the long post (I have a habit of being too detailed and asking a lot of questions).
Just a few random comments....
The "air spring" of a closed box actually allows for more passband efficiency than having an infinitely large box. It stores energy in a way that comes back and helps the driver move (kinda like how you can make big waves in a bathtub by moving your hand back and forth, but waiting for the wave to come back before you push again....same analogy works when moving your feet on a swing set).
Adding a port is like adding yet another spring, which stores even more energy. The tradeoff is that you get a steeper rolloff...the same is actually true between a "sealed box" and an "IB". The Q of the box (which most of these modeling programs provide) will indicate how much gain you're getting from the stored energy.
Btw, keep in mind that the above is true when designing the driver. When you pick a driver off the shelf, the manufacturer has already chosen the passband efficiency....so the DIY approach becomes more of choosing the right cabinet to get the most out of the driver. In the end, it really comes down to choosing how the low frequencies roll off....you can go lower and steeper, or higher and slower. With a true IB subwoofer in too small of a cabinet, you'll notice a peak before the rolloff, which is the result of the cabinet adding some output that the designer of the driver wasn't designing in. The fact that your driver models the way it does indicates that you're close to an ideal sealed box scenario for that driver.
Just to throw some numbers out there, if you decided to go with a different driver that was optimized for porting in your current cabinet volume, then you could gain another 6dB of output for the same amplifier power. With a tapped horn, you should be able to gain another 10dB. If you ported with your current driver, then you could gain another half octave of deeper extension and possibly increase the power handling to allow for slightly more SPL (since you won't be as excursion limited).
For what it's worth, the dimensions you have to work with at first glance look very ideal for a tapped horn on each side. You could probably even fit a single full-sized folded bass horn too, which should be able to go even louder than the tapped horn.
Was there any particular reason you want to go with a sealed box?
As far as the SPL capabilities, you might be able to count on another 3dB of boundary gain, but at those SPL's you might be coming close to 3dB of power compression too. 115dB is not very loud at all when you're talking low frequencies. If you look at the "modern Fletcher Munson curves" (ISO226), you'll notice that 20Hz at 115dB is about the same loudness as 1kHz at 70dB (aka 70 phons):
Amplified speech (like a preacher at church or any kind of similar venue) will be talking at around 70-75dB depending on where you're sitting in the room. You would have to get over 100phons for things to start sounding very loud (which would be about 130dB at 20Hz).
90 phons should be enough to feel the bass, and I think most movie listening is done somewhere between 80 and 90 phons.
So is 70phons loud enough? If you're gonna watch your movies at the same level you might listen to a preacher speak, then that would be the max clean output of your system. You can always crank it up a bit more and rely on the suspension of your driver to behave like a compressor...you can usually get up to several percent distortion before it starts sounding super horrible.
Considering the size of your room, I think your proposed system is right on the hairy edge of being enough. If you ever want to crank it to impress your friends, then I think you might want to look into ways of gaining a few more dB.
Btw, our hearing at low frequencies is nonlinear....so a 10dB increase in bass is actually a 20dB increase in phons, which means it will have the effect of sounding 20dB louder. Another 10dB gets you up to around 90phons, and a tapped horn should be able to get you there....and probably with a cheaper driver too.
If you go the tapped horn or ported box route, then I would highly recommend a Crown XTi amplifier (about $300 used) so that you can take advantage of the built-in DSP.
FWIW, I built a single tapped horn with a pair of $100 12" drivers that can do just under 130dB at 24Hz, so I definitely know it's possible...and you've got more room to work with too.
Btw, what are you using for mains and what are you powering them with?