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-----https://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.

kingpin111 wrote:

---

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

34"wide

16"high

24"deep

---

stevdart wrote:

---

=  34 W x 16 x 23.5 outside dimensions

---

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.

holy s#*t.  i should of stayed in school.

o.k. so lets see if i an=m getting this right.  i like the idea of one rectangular port in the middle, seperated by the baffle, cause this would also add strength.  is this right.  i am working on another drawing right now to show you guys cause i would like to be sure that i am on the same page as you guys.

thanks

kingpin

something like this.

kingpin(mike)

and this

kingpin

these are the measurements(outside) i have so far for each box:

16" high

16" wide

22" deep

=2.26852 cubic feet.

as for the size of the port. i am still a little iffy here.

jbl says 4"diameter x 13.31"  (30hz)

so if i make a port 1.5"wide x 8.9"(8 7/8)deep =13.35  is this correct. if so how high shoud the port be, or am i missing something here.

kingpin

Box size =

Port size =

The port length will remain the same at 13.31"

The port diameter tight now is 4". So using the formula I posted earlier shows this.

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.

alright round three.

i hope this is right, if not i am feeling really stupid.

and

kingpin

Yes!

wheeeeeewww    heeeeeewwwwwwww!!!!!!!!!!!

i should of came here in the first place.  thanks to everybody. 

and thank you "FORBIDDEN"

just one more thing.  here is my equipment list.

1" MDF

liquid nails and 2 1/2 inch flat head wood screws(pre drilling the holes)for assembly

1/4 weatherstripping foam when attaching the speakers

latex caulking to seal the joints inside the box

jbl says no insulation

and drilling a small hole in the back of the box to pass the wires through, which i will fill with caulking.

any tips or comments

kingpin

The port displacement is almost double what is spec'd in the manual because of the 1" MDF walls of the ports .  The JBL manual calculates the overall displacement of a standard thin-walled tube port and figures that into the box volume.  So, the difference is about +.09 cu ft displacement for each port.   Edge your internal volume in each chamber up slightly by about 160 in^3 to compensate.  The manual wants 2.25 ft^3 gross volume....you should go to 2.34 ft^3 to maintain the 30 Hz tuning frequency.

To figure it yourself :

The displacement of your port assembly will be determined by where you actually place the ports in the box.  By your diagram the ports occupy center stage, so there will be a 1" sidewall added to 3 sides.  If the ports were moved to the bottom, the displacement would change (2 sidewalls added).  Model your finished design and figure the displacement including the port walls (but not including the middle baffle common wall).  Compare that displacement volume to 0.107 ft^3, which is, by my estimation, the displacement of a 4" X 13 5/16"  tube port.  Whatever you get as the difference, add that volume to the internal volume of each chamber.

Use https://www.the12volt.com/caraudio/boxcalcs.asp#cyl and https://www.the12volt.com/caraudio/boxcalcs.asp#rec for help.  Note:  You can always use '0' as the value in "thickness" when you are calculating volumes.  That way, the calculator uses only your 2 inputted measurements to figure volume and doesn't add for sides.

And remember:  After you make the compensation for port displacement, you will be at gross volume JBL recommendation, as they included only the driver and round port.   Anything you add to the interior volume above and beyond, such as corner blocks or braces, must be measured and the displacement added to the volume.

and here i thought i was finished.

o.k.   so this is what i got.  the initial box volume is 2.25 cubic feet. here are the new measurements.

2.5 x 1 x 13.3(top and bottom sides of port)=33.25 x 2(top and bottom)=66.5cu.inches.

10.37 x 1 x 13.3(side of port)=137.92cu.inches

137.92(side) + 66.5(2 port sides) = 204.42cubic inches

204.42cu.inches/converted to cu.feet/ = .12cu.feet

.12cu.feet - .107cu.feet = .01

so i should add   .1cu.feet to the space.  is this right, it sounds wrong(self doubt).

can you please check if my math is correct.

thanks

kingpin

that should be

.12cu.feet - .107cu.feet = .01

so i should add .01 cu.feet to the space

You measured and figured the displacement of the MDF that surrounds the port.  It is better to measure the whole port structure (including air space) and compare it to what was allowed in the manual specs.  (Keep in mind the round plastic port has some volume in thickness of material, too.)  You didn't figure for the actual port opening in the above post, but you deducted what the specs allowed anyway.  You started with one approach, finished with another.  Result:  wrong answer.

When you do your math to figure cubic, see the object in 3 dimensions.  The port assembly is a box.  It consists of both the walls and the air space of the port.  Remember what Rob said in the beginning:

l * w * h = ft^3

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

1728in^3 = 1ft^3

So if you look at one of the port "boxes", you see an object that measures in 3 dimensions.  The measurements are 2.5 x 10.37 x 13.31 = ?

Answer is 345 cu. inches.  Convert to cu. ft.:  345 / 1728 = 0.199 ft^3

The JBL specs already allow for .107 ft^3. (remember, we're allowing for some thickness of the plastic walls, too).   The new larger displacement is .199 ft^3

.199 minus .107 = .09 which is a hair away from one tenth of a cubic foot.  So as I said in the post above:

So, the difference is about +.09 cu ft displacement for each port.   Edge your internal volume in each chamber up slightly by about 160 in^3 to compensate.  The manual wants 2.25 ft^3 gross volume....you should go to 2.34 ft^3 to maintain the 30 Hz tuning frequency.

You have to add that one-tenth of a cubic foot to each of the two chambers.  If it were a lot less than that you wouldn't be worrying about it.  But .09 ft^3 is 160 in^3...when you look at it like that you can better see how much space that is.  It's enough to throw off the tuning frequency, not by much but I'm thinking you want to be able to knowingly say your box is tuned to 30 Hz.  Am I right?  You have the links to the calculators and read again the notes I made about using them.  You've got the basics that you need to get the volume right. 

And one other thing....I would not build a box that is that deep without adding an internal brace.  I advise using the bracing I described earlier (it's easily cut out of a single piece of MDF).  And as said earlier, the bracing displaces volume, too, so you have to compensate for it.  Placing it to brace front-to-back is best, but if room is lacking side-to-side is what you want to do.

A brace might look like this:

After you cut it out, you would measure it by, again, looking at it as a 3-dimensional object.  To do this, you would divide it up into manageable sections like I've numbered in the pic.  1" would be the common depth (for the MDF thickness).  Add them up and you've got the displacement that you have to add to the overall volume.

A lot of people just say hell with it, it's good enough.  But the best enclosures come from those who are precise with their measurements.  And it's especially important when dealing with a vented enclosure. 

alright, i got what you are saying now.  i was a little confused at your earlier post. after reading it over a couple times and reading your latest post i understand things better.

i took your measurements of 2.34cu.feet and added another .03cu.ft(brace) = 2.37cu.feet.

17.25 wide  x  16 high  x  21.25 deep = 2.378cu.feet.

thanks

kingpin

Good job.  But by the displacement you posted for a brace, I would guess that they might be a little too narrow for the strength you want.  I would go about 3" width, maybe up to 4".  I'm thinking you should have .05 to .06 ft^3 per brace.   (And I'll take your word for it on those dimensions.  You know what the max OD can be.)

on the volume calculator:

15.25  x  14  x  19.25  inside dimensions(0.0 material thickness) = 2.3784cu.feet

17.25  x  16  x 21.25 outside dimensions(1.0) material thickness)=2.3784cu.feet

"And, I just caught this:  you are figuring internal dimensions now for volume.  You won't be able to go 16" H on the inside.  Nor can you go that width.  You'll have 2" total for height to add for OD and 3" total in width.  You'll have to push it to the rear."

you have totally lost me with this.

i appreciate you sticking with this thread and helping me.  as i like to do things properly.  i can say with all honestly that you guys have earned my respect as this is not as easy as it looks to get it done properly.

sorry if i am trying your patience, i just dont want to do things half assed. 

i am feeling pretty stupid about now.

kingpin

hey STEVDART

did you just edit your post.  am i missing something here.

kingpin

Yeah, sorry.  At first I thought you posted inside measurements, then I checked with a calc.  Then I edited.  lol

 STOP ME!  (You'll get a button some day.)

cool.

thanks a lot stevdart.

i guess i am ready to go on  with the build. 

thanks again

kingpin(mike)

stevdart wrote:

---

The measurements are 2.5 x 10.37 x 13.31 = ?

---

I had a virtual reality thought later on this.  The port opening will extend to the outer face of the front, so the 1" front baffle will count for 1" of the 13.31" port length.  That means the measurements in the quote above should be tweaked to account for that, taking the 13.31 down to 12.31 length.  Very slight adjustment in overall volume but a critical adjustment in actual port volume.  Just be sure to measure the inside port size when you are building.  Good luck.

point taken

kingpin

Avolon, a couple of things:  "...but when manipulating the port size, I thought it was only important to maintain the volume..."  Depends on what you are doing.  In the case of this thread, the port was being changed from round to slotted.  If the length remains the same, the area of the circle only has to be calculated and a rectangle formed instead.  But in the case of changing port diameter, and increase also demands a longer port length.  Volume, then, is not maintained... it increases.  "manipulation" can be different things.

In this thread, the ports are aligned along the common wall where support is not needed.  The space across the front is not wide enough to allow a port closer to the driver.

And for bracing, the most critical brace is from front to back, centered (as close as possible) on the enclosure chamber.  Next critical place is center top to bottom.  A slotted port used in place of a brace would not give you the front-to-back support because of the slot opening on the inside.