# How do you calculate Sub Box



## djbizz1 (Nov 26, 2005)

Allright, can someone show me how..


I have a box dimension of H 15.5' L 27' W 31'
I did the calculation here: Which gives me 7.5 cubes for that dimension
http://www.transchool.eustis.army.mil/twvr...eetininches.htm


I'm trying to figure out if i can fit two 12's BTL's in that box dimension.
https://ssl.perfora.net/www.ficaraudio.com/...view.shopscript


So for example, FI BTL says:

Ported Box: 1.75-3cuft
Displacement: 0.18




So how do you calculate that? 

7.5 Cuft Total box dimension
- .18 Displacement
- .18 Displacement
------------------------------------------
7.14 Total cuft after Displacement???

So now what? how do you calculate the Port area?


7.14 - Port area = Total Cuft?


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## 79 cutty (Sep 27, 2005)

> _Originally posted by djbizz1_@Jul 19 2010, 01:05 PM~18083471
> *Allright, can someone show me how..
> I have a box dimension of H 15.5'  L 27'  W 31'
> I did the calculation here:  Which gives me 7.5 cubes for that dimension
> ...


In order to know what your port area is you have to know what kind of port you are running, whether it be an aero port (circular port) slot port, etc.


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## djbizz1 (Nov 26, 2005)

> _Originally posted by 79 cutty_@Jul 19 2010, 01:21 PM~18083641
> *In order to know what your port area is you have to know what kind of port you are running, whether it be an aero port (circular port) slot port, etc.
> *





Imma go with the Slot Port. Tuned @ 32 Hz if that helps.. Thanks..


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## brian84corvette (Nov 22, 2009)

slot port is eaziest to map out to calculate your real air space of the box. 
remember when doing the math to use the inside of the box measurments - or just subtract the thickness of your wood your using.

ok I just drew up this crappy picture

pink box #1 and #2 are your actual cubic air space
the yellow area is all port and does not count as cubic air space for your sub









after you figure out how wide and long your slot port is going to be, draw up your design on some good graph paper in a scale that fits on the page - but using a ruler calculate your drawing by 3 or 6 inches per little block on the graph paper...

do a good job so your semi acurate...

then just figure out the area in box #1 and box #2 and add them together
then subtract your sub displacment - and any extra bracing you are going to give the box = will basicly give you what the air space will be for your subs to see give or take a little tiny bit because your not measuring the actual box you built ya know ? but it gets you close enough to at least design the box to fit in your viechle and still have enough air space for the subs....

I had to play this game fittin a REsX 18" in the rear hatch of my old corvette.
RE had posted on its website that the sub wanted 5.5 to 8.0 cubic foot box ported @ 35/34hz but my corvette was tiny in the rear hatch and I could only build a box that was 5.2 cubes after port and displacment for the sub...
the benifit of the small box = more power handling capability
the sub I had would only hold 1K rms to it if it was in a 8.o cube box before it started psyicly reaching max excursion and making ugly mechanical noise
but in the smaller box it was taking 1400w all day long because the smaller air spring was harder for the sub to move in and out to its full potential.
only problem I had with it was extended play times the sub would get hot

just something to think about...

I also hated being tuned at 35hz
and will never again build a box tuned above 33 because I like all that old school bass mekanic / dj billy e type stuff and there were alot of low notes that I was missing in that system due to having tuned so high.


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## djbizz1 (Nov 26, 2005)

:0 nice fukkin' model car bro! But shit still confusing... is there a formula to this? lol


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## djbizz1 (Nov 26, 2005)

Well, anyways, the question is, do i have enough air space to fit 2 12's FI BTL in that Box dimension. H 15.5' L 27' W 31'


How much Airspace to i have left after, Slot Port, displacement, .75 wood thickness and tuned at 32HZ..


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## brian84corvette (Nov 22, 2009)

by my calculations - your box of 15.5 / 27 / 31 inches made of 3/4" wood
will have an internal air space of 6.09 cubic ft.
using 14" of port per cubic ft of box
would give you 85" of port area to fill
wich would basicly be the shape of an L inside your box that is 4.5" high by 18" long. the wood to make this port and the area its taking away from the box measures 1.06 cubic ft. and then take away .18 for each sub and you are left with a big ass box with no internal bracing for streingth - that yeilds 4.67 cubic ft air space for both subwoofers to share

what does FI recomend for them to have each sub ?


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## brian84corvette (Nov 22, 2009)

http://www.rockfordfosgate.com/rftech/box_wizard.asp


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## djbizz1 (Nov 26, 2005)

> _Originally posted by brian84corvette_@Jul 19 2010, 04:05 PM~18084970
> *by my calculations - your box of 15.5 / 27 / 31 inches made of 3/4" wood
> will have an internal air space of 6.09 cubic ft.
> using 14" of port per cubic ft of box
> ...



FI BTL: 1.75 - 3 cuft
i was hoping to get the maximum cuft per sub.. fukk so only 2.33 per sub!! Damnit! 


So i might be better off with the 12's DC LEVEL 4 XL ??????
it requires: 1.75³-2.75³ ft


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## BIG DIRTY (Jan 31, 2002)

HEIGHT X WIDTH X DEPTH / 1728


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## 79 cutty (Sep 27, 2005)

You can always try messing with these and see what you get:

http://www.reaudio.com/speaker_box/LPort_Box_Calc.html

http://www.carstereo.com/help/Articles.cfm?id=31


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## djbizz1 (Nov 26, 2005)

> _Originally posted by 79 cutty_@Jul 20 2010, 05:39 AM~18090536
> *You can always try messing with these and see what you get:
> 
> http://www.reaudio.com/speaker_box/LPort_Box_Calc.html
> ...



The RE is cool, but too bad it only calculate for 1 sub.


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## djbizz1 (Nov 26, 2005)

> _Originally posted by BIG DIRTY_@Jul 19 2010, 06:35 PM~18086211
> *HEIGHT X WIDTH X DEPTH / 1728
> *



Thanks


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## 79 cutty (Sep 27, 2005)

> _Originally posted by djbizz1_@Jul 20 2010, 08:44 AM~18091418
> *The RE is cool, but too bad it only calculate for 1 sub.
> *


I am trying to find the link to the other calculator I had as well. See what I can come up with for you.


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## 79 cutty (Sep 27, 2005)

Here ya go:

http://www.caraudio.com/forum/enclosure-de...calculator.html


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