View Full Version : Custom Enclosure for 2012 WRX

06-27-2012, 07:27 PM
Hello everyone,

I thought is would be a smart idea to post my project before starting to get a green light from the experienced people here at CarAudioForum.

I just got my 2012 Subaru WRX and I want to put my sub in the back. The Rex is a hatch with a lot of contour and curves on the side of the trunk area. I plan to make a fiber-glass and MDF box so I can utilize all of the space available in the trunk. There is not a lot of room when the back seats up so my old box isn't a good option.

Here is some background information about the build. I am putting in an Apline 10" Typr-R. I have a 3D model of the trunk interior to get my volume almost exact. I used foam boards to trace and cut the contour of the car interior at different locations. Then I took photos, imported them into my 3D modeling program, scaled them, and traced them.

Here are the details and numbers:
-The fixed dimensions of the box are ~13" tall and ~28" wide. The depth is my only variable. I want a ported box with as little depth as possible (so as little volume, but still the correct amount). Alpine recommends a box volume of 1.1 cu.ft.

-I want to achieve a ported enclosure tuned to ~35hz. I can fudge the frequency a bit and I am open to recommendations, 35hz sounded like a good number to shoot for.

-I've never used a round port before, but I understand that they are more prone to whistling/chuff noise. I decided to shoot for a 4" Diameter port with flared ends.

-I used an free-online calculator at AJdesigner (Link HERE) to find the recommended volume and port length. Here are the numbers it gave me:
Volume (base on the sub's Qts=0.53 and Vas=21L) = 1.83cu.ft
Port Length (single 4" port, end correction of 0.5) = 13.12"

So here are a few questions I have about the build:

1)Alpine manual recommends a vent enclosure volume of 1.1 cu.ft, but the AJdesigner calculator recommends 1.83 cu.ft. What should I use? The 1.83 box extends about 12" into my main trunk space, 1.25 extends about 8", and 1.1 extends 7"

2)Will a single 4" flared port work okay for my this build? Do I need to go larger diameter and/or more ports?

3)Can you have a turn in a round port (similar to how a square vent makes a 90deg turn at the back of the box. Is this okay to do?

Please let me know your opinions and recommendations for this build so I can get started!

I really appreciate the help!

06-27-2012, 07:31 PM
So I looked at WinISD and it looks similar to the AJdesigner port length calculations. I cannot use winISD on my work computer durring lunch, so I adjusted my design using the AJdesigner and then matched the design in winISD at home. I will post the plot of this build soon.

Below are some pictures of the current box design with the port and driver locations modeled accurately. You can make out the internal lines of the box to see that everything has clearance. The front and the top of the box will be made from 3/4" MDF. The seat and door sides, back, and bottom of the box will be fiberglass. I might only do the back as fiberglass and the rest as 3/4" MDF, not sure yet. I figure that using fiberglass would result in thinner wall thickness which gives me more volume so I do not have to extend my box out farther into my trunk space. The top in fiberglass too I guess... But I might want to put stuff ontop of the box. If I go camping for example. Would fiberglass be okay to use on the top? I make an assumption that all the fiberglass locations will be 0.25" thick as a start. Does that sound about right?

I will be heat-forming the PVC port to creat a good flare on either end. I will either build a mold with a flare modeled or use a bowl. Any suggestions? I realize the model does not factor in the flares yet, I will probably refine the model and see how much it changes the volume. It also looks like the clearance at the end of the port might not work if there is a flare... I'll need to look into that, just thought about it.

Summary of Current plan:
Net volume: 1.211 cuft
Port Length: 20"
Freq: 35hz




06-27-2012, 09:23 PM
You don't want the inside opening (throat) of the port so close to the sub. See if you can do it differently.

06-27-2012, 09:34 PM
You don't want the inside opening (throat) of the port so close to the sub. See if you can do it differently.

this and you can get away with a 3inch port just fine with a 10. and i would shoot for the box to be a bout 1.3-1.4 cubes any smaller and the peak will be so sharp you wont pick up any lows or you will lose your higher bass range. this is why the program recommended such a big box

06-28-2012, 01:23 PM
Thanks for the advice guys!

I changed the design to use a 3" port which shortened the port length and increased the net volume to 1.3447cuft and the port length is now 9.46". I also swapped the location of the port and the sub and the clearance is much better. The port head it now on the other side of the box from the sub.

Now I am mostly concerned with the top, bottom, and sides of the box. I know the front will be 3/4" MDF and the back will be ~0.25" of fiberglass. What about the sides? I have been recommended to use MDF on all sides except for the Fiberglass back. I have also been recommended to stick with fiberglass for the added volume, but line the inside of the box with a deadener. I have 10 sheets of 10"x13" deadener material from Parts Express (HERE (http://www.parts-express.com/pe/showdetl.cfm?Partnumber=268-030)). I also have 1lb of poly-fill which I have been recommended to try from Parts Express (HERE (http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-317)).

So here is my favorite option so far. I will make all sides of the box fiberglass except for the front which will remain 3/4" MDF. This will bump up my volume to 1.40 cuft and a port length of 8.95" I will line the inside of the box with the Sonic Barrier Sound Damping Sheets. I will then loosely line 0.5-1.0lb of polly fill inside the box (recommended amount per cuft).