Audio Gurus, Please advise (sub enclosure)

[GSLENK]

So I am in the final stages of my subwoofer box design (my current box only fits with the rear seats folded down) My dimensions are pretty much permanant. The only 2 options left are:
-Decide where the sub will mount (a second option opened up when i finalized my design, the vent will remain where it has been planned)
-Decide the final length of the vent.

Here are the optimum specs from the manufacturers website:
Vent slot area 12x1 inches (mine will be 13x1, no other option)
Vent slot depth 24 inches (I assume that means approx, shouldnt matter if it is straight or "L" shaped...
Box will be 1.79 ft^3 gross volume (mfr says 1.8)

My main question is about sub placement.
My options are the original position on the green plane, or "vertical" on the blue plane?
Does it matter if the subwoofer baffle is on the same plane as the slot vent? ie. Does the flow of air in the vent have to be perpendicular to the plane that the sub attaches to like most traditional vented/ported setups? Will this effect efficiency/sound quality/distortion?

Second question: Should I extend the vent with the red piece labled 4.5? or ignore that added peice as the vent length is already 24" without the L-shape. In general, does a longer slot tune for deeper frequency or higher? (i would prefer the deeper freq, But since this is aimed for ~30-36Hz, I do realize most freqs below that may even be inaudible (subsonic... that is around 28Hz right?)

I have no preference on sub position, fit/woofer protection will decide as long as audio quality is a an insignificant factor. I can think up equal reasons why it should point up or foreward.

Looking at the drawing, it seems like the path of least resistance and starp turns and without air "pockets" would be without the L shape, and just a single straight vent (no red box piece) unless that pocket would help acoustics...

Thanks for any input.

 

[turbodude]

Hello, Nice design... Looks like you got some talent for sure. Well, i'm an MECP certified tech and have been installing and building systems for a while so here we go....

First off, the longer the vent, the lower the frequency you will get, i believe that red piece you have in an addition to the main port is unnecessary seeing how your main vent is already at 24". The opening size of the vent also plays a big role on the overall tuning frequency you will achieve. The bigger the opening, the longer the vent needs to be. Be careful as you want a good overall balance between length and size of opening. To small and it will sound too punchy, to large and it will be too sloppy. I personally feel that your vent is far to narrow at only 1" high? You should always aim for a 8:1 ratio when possible to give you the best balance and sound output from the port.

Check out this calculator below at this link to punch in numbers to see where your at, scroll down to where it says calculate vent length:

Car Audio - PORT Size Calculations and Formulas for WOOFER and Subwoofer BOXES

Once you remove that red add-on piece of the vent and adjust it's length and opening accordingly, I think that placing it on the green plane would be best as the internal air flow of the air being pushed by the woofer would be more free flowing to exit the vent and would not have as much interfering turbulence which is not good. Placing it on the blue plane would work but the vent design would have to be relocated a bit to increase efficiency for the air to escape.

I hope this helped, keep up the good work

 

[GSLENK]

I was pretty skeptical about vent height, although most boxes I have seen are about 1.5x12... Alpine's website stated the optimum vent has an area of 1x12" an 24 inches long. Thier math adds up too i suppose...
Vb (gross) 1.8 ft^3
Vb (net) 1.3 ft^3
Woofer dispplacement .29 ft^3
Vent displscement (excl. mdf) 1x12x24= .167 ft^3
Vent displacement (of .75" MDF) = .125 ft^3
And Vb (gross)~= Vb(net)+Wd+Vd+Vd

Or will the "thin" vent sound like an air tunnel?

heres the spec sheet:

http://support.alpine-usa.com/products/documents/OM_SWX-1243D.PDF

I think ill try it with one inch, but leave room to move the interior around a bit so that I could change it to 1.5" if desired. Mayble ill cut it down to 22"

Ill keep the curious updated...

I just finished the box eariler today. Put it together without silicone joints to sound sheck it. I think it sounds great. It is a little bit punchier than my last box... but not anywhere near punchy like a sealed box. also it fits frikkin perfect. there is ~1cm between the rear of the box and the back plastic of the car, 1 inch b/t the sub cone and rear seats (i did mount it in the green plane) and about a half inch between the top and the glass. the top is parallel to the glass, the sub baffle is parallel to the rear seats.

Ill post pics of the finished install after I carpet it, well maybe ill sneak some previews, the brown box will be easier to contrast for clearances etc.

If i were to do this again, which i might in a few years, to fit a double 12" bass system, ported of course, there is room for exactly one more box of this dimension... and there is enough internal sub clearance (but just barely) to make the port 1.5 inches. Which i will do next time, and make it longer. I made my port length 23 inches, the picture says 24.