Introduction: How to Build a Simple Sub Box

Building a box for your subwoofer is one of the best ways to put your peronal touch on a car's audio system. Whether it's a basic cubic box or a precisely designed kerfed / ported box, making sure your sub gets the appropriate enclousure for your needs can make a world of difference.

This step by step shows the process to build a basic sealed box.

In my case, I picked up a sub/amp/box combo on Craigslist for pretty cheap, however the box was slightly too big to fit into the trunk of my 1997 240sx (only a 15" opening). So, motivated to make my purchase work, I decided to build a box: Spending less than $50 for the materials.

Step 1: Planning: Figuring Out Cubic Feet

Before you start anything, you need to look at the sub you are using. Subwoofers recommend a min. / max. cubic feet enclosure. 

In this case, I was using an Infinity Kappa 12.1 which recommends a .75 - 1.25 cubic foot Sealed box. Reading some reviews, I chose to go with 1 cubic foot. (sealed vs. ported depends on your preferences. Sealed gives you a harder/punchier bass, ported gives you a wider range and can be heard from further distances. Read up more on sealed vs. ported if you're unsure on what you would like)

To figure out the cubic feet of the interior of the box you are designing, you multiply Length x Width x Height and divide by 1728
In this example, the exterior of this box is 10" x 16" x 16"
Keep in mind the thickness of materials -  3/4" on each side = 1.5"
8.5 x 14.5 x 14.5 = 1787 Divided by 1728 = 1.03 cubic feet

*Depth of the box should be judged by the depth of the subwoofer + 2" minimum*

Step 2: Materials

Materials you will need:

3/4" MDF boards - pretty standard unless you are using a much larger or multiple subs
Circular Saw or Table Saw -  or have Home Depot / Lowes cut them for you (make sure they stick to your measurements!)
Tape Measure
Straight edge / Ruler
1/2" - 1 3/4" Sheet Metal Screws
Wood glue or Liquid Nails
Silicone Caulk
Carpet - if preferred (found at auto parts stores / hobby stores)
Spray Adhesive - if preferred
Speaker Terminal Cup (found online / eBay)
Compass / Circle Template / screw, pencil & string combo
Box Cutter / Razor

Step 3: Marking Your Edges

With my design in mind and materials in place, I can begin marking my edges.

I used the boards to mark out a 3/4" edge on the piece I will drill through. This way I can stay within my limit and make sure all my screws are center.

Step 4: Pilot Holes

Using a drill bit that is reasonably smaller than my screw, I make straight pilot holes within my marked edges.

Step 5: Finding Center

On my top piece, I marker corner to corner so I can find my center. Depending on the size of your subwoofer, you need to make a circle big enough to fit your speaker and to allow the "lip"  hang over so it can be screwed in.

I needed an 11 1/4" diameter circle. So I also made cross marks between my "X" and measured half that from the center point.  I marked those measurements on my cross lines. (sorry, no picture)

Placing a nail in the center point, tying a string around the nail and to a pencil, I used those marked cross points to guide me as I created a perfect circle. (sorry, no picture)

*If you can find a template, this process will be much faster!*

Step 6: Cutting the Circle

Cutting the circle can be the harder part of this entire process, unless you use a Router or something better.

I used a jig saw. By drilling a large hole inside the circle I just drew, I can get my saw blade in and start cutting. It's helpful to have someone hold the board down or clamp it. (sorry, no picture)

In this photo, I have my bottom 3 boards already drilled together, with my top piece just placed on top for a mock up.
(If you notice the right side, I needed to pull that board back while I drilled in the top piece so it could line up perfectly. Not an issue)

The same process can be used to cut an opening for your terminal cup, or even a large circle bit for your drill.

Step 7: Assembled & Carpeting

Here you can see all my sides are drilled together. Some people put Wood Glue or Liquid Nails between the boards before they drill to ensure an air tight seal.

I lined the inside edges with the silicone caulk and let it dry (can take up to 24 hours)

Now for carpeting. I picked up a roll of grey carpet at an auto parts store nearby for $10. Luckily, it lined up perfectly so I could center the box on the mat and have the side fold up to the edge. A larger roll may be neccessary for bigger boxes.

Using 3M spray adhesive I sprayed the entire back of the box evenly, being certain to cover all the edges. I also sprayed the backside of the carpet that will meet with the box. Continued this process by spraying both box/carpet, then flipping the box forward. (having a large enough sheet of carpet makes this easy) Pat down the carpet and make sure its smooth as you go.

Don't worry about covering the holes, you will cut those out later.

When you get to the final side and the carpet edges meet, use a razor blade / box cutter and cut along the edge, trying your best to make it seemless. (this is where making sure you glued all the way to the edge is important)

Same process goes for the sides. You may have to cut some extra material off before you start, but make sure your material reaches the edge, spray the box and carpet then apply. Use your razor blade along the edge to cut off the excess.

Step 8: Finishing Up

Use the razor to cut out your holes for the sub and speaker terminal. Connect your wires from the terminal and to the sub. Center the subwoofer and screw that down

Step 9:

Now my tiny trunk can accomodate my $100 budget system. Enjoy!

Step 10:

Here's a set up I made for my old car. Same simple steps applied.
This one included side walls for the amp / capacitors as well as a floor board that had hinges so I could access the spare tire without removing the box.