isobaric bp box for 4 12w0s

the idea: 4th order bandpass box for 4 jl12w0s

I'm desigining this in WinISD right now, and I think i've gotten close to what I'm looking for.  This is the first time I've used WinISD, so I'm not sure if I'm missing something or not, so I figured I'd post where I'm at and perhaps get some feedback to avoid any potential pitfalls.

anyhow, the setup is 4 JL 12w0-4s  currently residing in a sealed box of about 5 cu/ft, not happy with this, I'm going with something that will give me a bit more volume and what I hope to be a better freq response in the range i'll be using it in...

the box i'm designing is in the blue, the orange is a "benchmark" and the yellow is close to what the current sealed box is...

I'm tuned at 44hz

rear chamber is 1.9 cu/ft

front chamber is 1.7 cu/ft

when i first looked at the port air velocity i was about to change my mind, but i came up with an idea to fit a very long rectangular port inside the box (kinda), doing so, i got the port air velocity down to below 17.....is this adequate? will a higher velocity be acceptable? this box is going in a very large car (vw vangon) and the i'll be sitting far away from the box...

am I missing something in this design? it seems very "too good to be true" that i'm going to get (what looks to be) good performance out of a box that small... i feel like i'm missing something...but alas i'm tired and can't concentrate anymore...

if you need any other specs lemme know....thanks in advance!

I agree that looks too good to be true.  Those chamber volumes for 4 subs?  First thing you could do, though, is adjust your db chart to show down to -12 instead of -29...it will simplify what you are seeing on the graph.  Is that the transfer function chart?

Check all your parameter input data in your driver database.  Now look at a vented box of about 5 cu ft net with a slotted port 3" X 12" X 28.6" L, tuned at 24 Hz.  That's a nice response for these four subs.  Very low port noise, no over-excursion above 20 Hz, smooth freq rolloff.  That's a big port structure, though, so this box would come out quite a bit bigger than your current box.  Experiment with the port opening size, try 3" X 10" and a shorter length for an overall smaller box.  Port noise won't be an issue with this smaller port, either.

difficult to build is part of the fun .... depending on what route i take and if there's interest i'd be happy to document it to share... i've built plenty of boxes including a couple horn designs, talk about a feeling of accomplishment when a very complex box comes to life and sounds good!  although i won't take full credit as a majority of the more complex boxes i've built have been from plans someone else has spent the time creating (IE Decware's "wicked one" folded horn box(and damn, what a box that was!))

i experimented with the vented boxes and it seems that there are definately some alleys i want to explore more thorougly...its very interesting watching all the values change with given inputs, learning more in a few minutes on WinISD than years of installing for a living (at least as far as speaker boxes goes)...

one thing thats really stumping me is the SPL charts, while i'm not really going for SPL over SQ, I'm curious as to why it seems the bandpass boxes seem to be falling far short of the current sealed box...obviously there is going to be the actual testing environment and cabin gain etc etc to take into effect, but i'd think that the bandpass enclosures would far exceed overall output volume of the sealed enclosure rather easily...at first i was thinking that perhaps i was losing spl due to isobaric mounting so i mapped a normal mounted 4 sub bandpass and had the same results as the isobaric design... is there a direct correlation between overall positive gain on the transfer magnitude function plot and overall DB on the spl plot? it seems to be the only way to break high 120s/low 130s db area with 4 jlw0 is to go over the positive 3db mark in the transfer magnitude plot...(which if i'm understanding correctly, starts sacrficing sound accuracy for sound volume)...

i think the "real life" outcome of this box is going to be somewhat hard to predict as compared with other vehicles due to what this is actually going in....the cabin of my VW Vanagon is so spacious there doesn't seem to be any sound pressure built up to really transform an enclosure into what it is capable of...perhaps more subs and more power is in order

thanks for the responses guys! keep em comin...

meant to attach this to the above post...its a graph of the spl chart...