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Discussion Starter · #1 ·
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?

I really appreciate the help!



1,687 Posts
I use WinISD pro alpha for my simple modeling. its free as well. A little bit of a learning curve and you have to really put new speaker data in correctly but it works very well.

Yes you can put bends in round ports.

When modeling your enclosure woofer combo, model them under power available.

I use a 1500 watt amp. Thing is, do to impedance rise, power compression etc, the sub never actually sees that 1500 watts. It's closer to around 1000 watts. So model your sub with say 2/3's of the power your amp says it produces at the impedance you are wiring the sub at.

look at port velocity if your modeling program has that capability. You want to remain under 30m/s at full power and really try to be around 20m/s

to reduce port velocity, you either have to use a bigger port (area) or multiple ports which is like using a bigger port. Problem is, it makes the port longer for a given frequency.

Another problem is you have to account for the volume of the port in the box. The bigger the port the more volume it takes up. It can become a headache.

So if you have the room, the AJ enclosure might actually be including the port volume into the final enclosure volume. Unless it specifies, the alpine design probably does not. I would have to do the math to determine.

Sorry I would run the numbers for you but the only moddeling i CAN DO is Unibox. I am on the road for work and the work laptop will not let me run some software.

Can you send me all the T/S parameters for the sub. I will model it.

4 Posts
Discussion Starter · #3 ·
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 jig 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

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