U guys have come highly recommended

Hi, as has already been said i have been all over and this forum keeps being recommended, so i hope you guys can help me. 

i have 2 subs that need to go into 1 ported box.  this is where  i need the help.

1.  dimensions

2.  port size and placement

specs are as follows, for one sub.

jbl gt120-----http://manuals.harman.com/JBL/CAR/Boxes%20and%20Parameters/GT120_f.pdf

ported box-2.25cu.feet

port-30hz

the box cannot be bigger than 35" wide x 25"deep x 16.5"high

I am using 1"mdf and would really like to know the dimensions of the box, porthole size and placement.

i have tried to use many of the box building programs but keep getting large variances in answers.

i have been trying to get this info for about a week now, and have just become totally confused with all the different answers and info.

this seems like a great place to hang out.

PLEASE HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

kingpin111

As far as the dimensions go I'll give you a basic run down on how the math is calculated.

l * w * h = ft^3

12" * 12" *12" = 1728in^3

1728in^3 = 1ft^3

This is the constant to always remember. The next thing to remember is that the space you are seeking is an internal measurement so you must allow for dimensions of the materials used in the box. What you are after in your case is 2.5 cu.ft. of internal volume.

2.25 * 1728 = 3888in^3. Find any dimensions to fit this total. If you know that your trunk allows a box 34" wide, 16 inches high, then it is simple math to recalculate for the depth. Remembering of course that the material takes up space, in this case 1" materials are used for your box. So I would be left with a internal space of 32" wide and 14" high provided I had no internal dividers.

32" * 16" = 512

3888 / 512 = 7.69 inches.

The box would have a internal depth of 7.70 inches and a external volume of 9.70 inches. There is a great box calibrater on the side of the main menu under the Car Audio volume calculators section. This will allow you to input your known dimensions for length, width, height, material thickness and extrapolate any unknown dimensions for different box shapes. Algebra is a handy tool if you know it and sometimes the box you want is not what is going to be listed making it necessary to find out your volumes. According to your known dimensions, you have a box of approx 6.3 cubic feet, that's one huge beyatch. Trim some fat from that baby using the math I just gave you. Start with this first and then we can tackle the port. I'm outta here for the night so hopefully someone else can pick up where I left off.

hey thanks "forbidden"

stick with me here.

so far i have a box with the external dimensions of:

34"wide

16"high

24"deep

=4.66667 cu.feet

I would like to put a brace right through middle in between the two speakers which would be:

14"high  x  22"deep with most of the middle cut out (a piece of mdf 14x22 with a 12x20 piece cut out)

and how you guys figure out how to tune a port to 30 hz is a mystery to me.

SO CLOSE YET SO FAR.

HURRY BACK forbidden

thanks kingpin

With what Rob has said, I would add that the easiest way to do this is to use the diagram shown in the manual......and double it.  Make a box with 2 separate chambers, each with a 4" port.  All you would have to do is figure out the dimensions for one and then add another one to it.  Did you find the box volume calculator here?  Use 1" for thickness of baffles.

Just play around with plugging different numbers into the L, W and H.  Leave 1" thickness as a constant.  You know the absolute largest way the box can be in any dimension, so use numbers smaller or equal to the 35W x 16.5H x 25D limits.

For example, I put in 16.5W x 16H x 23.5D and got the volume you want for one:  2.52 cu. ft. (the manual recommended size includes driver/port displacement).  If you were to add another box onto it, the dimensions total would be:

double the width, add 1" for the separator baffle to the width and leave the other dimensions as is =  34 W x 16 x 23.5 outside dimensions

Each side (chamber) will have a round 4" port at the length shown in the manual (13.3")  Placement of the port is as pictured.  Do some research on bracing each chamber, and add a brace to each side between the sub and the port.  It might look something like this:

.............with the braces resembling something like this:

The bracing will add a little to your needed volumes, so use the formulas Rob gave to figure each brace's cubic inches (after you make them) and change to cubic feet.  Then add that volume to the overall needed cubic feet for each side.  Change one of the dimensions slightly until you come up with the right answer.

Looks like you got it nailed. 

Notes: the baffle that separates the two chambers will be solid like a side wall.  The braces as I showed have their strongest support at the centers of the side baffles, which gives greater support than a baffle with the center cut out.  They will also have room, because of the corner cutouts, to be moved farther toward the driver than my quick pic suggests.  Try to place them as close to center of each chamber as possible.

Port calculations look like this........ha ha which looks to me like some sort of Greek algebra......so needless to say, if you have the space in your car to use the port and volume recommendations in the manual you are one step ahead of the game.  You are lucky to have the space to work with.  The 30 Hz tuning freq is derived from the linked equation using box volume and port volume with an opening area determined by air velocity....done for you already by JBL.  So if you copy the port dimensions in the manual you will have it.  If, for some reason, you change the box volume in any way, you will have to recalculate port dimensions and displacement.

wow, you guys are good.

I have a lot of room in my trunk. 1988 oldsmobile cutlass. you can fit 3 bodies in their.

can i put the portholes on top of each other in the middle  with 2-3 " between them.

after looking at some measurements things are getting tight.

2x12.25(subs)+2x4"(portholes)=32"  plus i need at least 1-11/2 from the sides of the speakers which pushing close to 36". maybe this will work.

thanks

kingpin

How about trying this idea on for size.  Use the volume calculator for wedges https://www.the12volt.com/caraudio/boxcalcs.asp#wed2 and get the volume to 2.3 (whatever the manual says) in each chamber.  Cut your separator baffle to fit at an angle.  And relocate the bracing to a perpendicular position to what I first described ( behind the driver and port and side to side) since the enclosure is so deep.  You should have plenty of room for them.

Something like this:

As the old guy chimes in......why not eliminate the round ports and convert to rectangular slot ports, solves the problem right now. The center divider isolates both boxes and becomes one wall of the port. To calculate the slot port is simple. The length stays the same. It is all about the surface area.

pier^2 is the furmula for area of a circle.

So 3.1415 * 4^2 = 12.566 in^2

Find and two numbers that will now give you this measurement.

Say 1.5 * 8.37 This would be the internal measurement for the length and width of the slot port.