Most manufacturers don't provide you with a complete diagram for their recommended enclosures. Most will give you the net internal volume and the ports they recommend you use. However, other manufacturers may only give you a recommended net volume and tuning frequency. I've written this tutorial to cover all bases, but if you already know the ports you're going to use, you can disregard steps 2 and 3.
Here's a step-by-step process of how to get the proper dimensions when building an enclosure ...
1) Measure your trunk for max dimensions and note TWO dimensions that would be preferable. Subtract twice the thickness of the desired dimensions to get the desired internal dimensions (i.e. 3/4" MDF, 20" height = 18.5")
2) Figure out desired net volume, tuning frequency, and port area.
3) Figure out port length using the following formula :
Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)
Lv = Port Length in inches
Av = Port Area in square inches
Vb = Net Internal Volume in cubic feet
Fb = Desired Tuning Frequency
4) Draw out a diagram of your enclosure on a sheet of paper and figure out the displacement of the port.
5) Add up all displacements (port, driver, brace)
6) Add displacements to desired net volume and multiply by 1728
7) Divide by the two desired interal dimensions to get the third internal dimension
8) Add twice the thickness of the wood to get the exterior dimensions
Example ...
1) You measure your trunk and the max dimensions are 16" high, 36" wide, and 24" deep. You decide you want your enclosure to be 14.5" high and 30" wide. You're using 3/4" MDF, so the internal dimensions are 13" high and 28.5" wide.
2) Net volume - 2 ft^3, Tuning - 35 Hz, Port Area - 12.56637061 in^2 (4" round port)
3) 12.56637061*1.84*10^8/[2*1728*(35/0.159)^2] - 0.823*sqrt(12.56637061) = 10.89" (I'll round up to 11")
4) You measure the outer diameter of your port and it's 4.25". You subtract 0.75" from the length because the port passes through one of the enclosure walls. Volume of a cylinder = Pi*Lv*(D/2)^2 ... Pi*10.25*(4.25/2)^2/1728 = 0.084 ft^3
5) You just figured out port displacement. According to your subwoofer's manual, your subwoofer's displacement is 0.1 ft^3. You don't plan on using any bracing, so brace displacement is 0 ft^3. 0.1 + 0.084 = 0.184 ft^3
6) 2 + 0.184 = 2.184*1728 = 3773.952 in^3
7) 3773.952/(13*28.5) = 10.1861" (I'd round up to 10.25" in this case)
8) 10.25+(.75*2) = 11.75" ... Exterior dimensions = 14.5" high, 30" wide, and 11.75" deep
Now, in this example, the enclosure will be slightly larger and have a slightly lower tuning frequency, but it shouldn't make an audible difference.
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