With 5.1 Channel amplifiers in such abundance, there's a growing need for sub-woofers. Here I'll show you how to make your own for less than $30.

Step 1: Tool & Materials

Tools

Tablesaw
Jigsaw
Chisel
Hammer
Nails
Wood glue
Soldering iron
Drill
Divider
Screwdriver
Tape Measure

Materials

Particle board or furniture grade plywood
Woofer
Terminal post
Wire
Screws
Nails

Cut list

4 ea. 14" x 8" x 13/16" particle board
2 ea. 14½" x 14½" x 13/16"" particle board
1 ea. 13" x 13" x ¼" plywood
8 ea. 13/16" x 13/16" x 7¾" particle board

A note about woofers in general. When it comes to speakers you generally want to look for three things.

Big cone - The bigger the cone, the more air you're going to move. Especially in a woofer, you want to move a lot of air. I went with a 12" because this will be mounted between floor joists under my sofa. I got mine on clearance and they're out of stock now, but this is close.
Heavy magnet - I'm not sure why a heavy magnet is good, I just know that it is.
Rubber surround - The surround is the part that attaches the outside edge of the cone to the frame. If this part is made of foam, it will rot and tear in just a few years. Rubber is the best.

Step 2: Cut the Sides, Top and Bottom

I usually refrain from giving precise measurements to allow you to use available materials and fit the project to your application. Since this project is designed to fit under the floor between the joists, I'm giving dimensions.

The raw material I'm using here is 13/16", particle board with Formica bonded onto it. People say particle board is cheap, and it is, but its also dense. And dense is what you want for a speaker cabinet. The size I'm shooting for is 14½" x 14½" x 8". This will fit between the floor joists under the floor and the woofer will blast up directly under the front of the sofa. The speakers in the room and the video monitor are in a fixed location so the sofa isn't going to move.

For ease of construction and durability, we'll use rabbet corner joints. Begin by cutting your particle board to 14" wide. Next cut four 8" wide pieces and taking several passes on the tablesaw, rabbet in a 5/16" deep by 13/16" wide notch along one 8" edge of each. Use a chisel to smooth off the surface. This notch will be the basis of a glued corner joint.

Also cut two 14½" x 14½" squares, one 13" x 13" square and eight 13/16" x 13/16" x 7¾" particle board.

Step 3: Glue and Nail Corners & Bottom

Lay out the four sides and spread a good amount of glue into each rabbet. Insert the edges without a rabbet into the adjoining rabbet. Drive three nails through the rabbet edge and into the end of the adjoining side. Do this on all four corners. This will form an open ended box.

Apply a good amount of glue to one open end of the box and nail one of the 14½" x 14½" squares onto it. Now you have a box with one closed end.

Finally, glue and nail the 13/16" x 13/16" x 7¾" particle board pieces into all of the corners and lengthwise along all of the edges. This will reinforce the corners and provide a resting place for the plywood which will be the speaker mounting surface. The 13"x13" plywood should rest flush with the top of the box.

Now is as good a time as any to install the terminal strip.

Step 4: Mark and Cut the Plywood

Draw an X diagonally from corner to corner across the 13"x13" and the remaining 14½" x 14½" squares. By aligning the speaker mounting holes with these diagonal lines, you can easily see that the speaker is centered. Trace a circle around the speaker frame onto both pieces of plywood.

Using a divider, measure from the outside of the speaker frame to where the frame cone begins. Transfer this measurement to the 13"x13" plywood. Cut this smaller circle out of the smaller piece of plywood. Also cut the full sized circle out of the larger piece of plywood.

Step 5: Finish the Enclosure and Install the Speaker

Apply glue to the plywood mounting surfaces and set the smaller piece of plywood into its resting place. Apply glue to the entire back side of the larger piece, lay it in place and nail down the perimeter.

Once the glue dries, solder the wire from the terminal strip to the speaker. I use 16 gauge speaker wire. Now there is lots of science out there that says monster cable is better. AC (which is the kind of voltage traveling on a speaker wire) conducts over the surface of the wire, so the more surface area the better....right? Here's the thing. I've used that phenomenally expensive wire before and not once have I heard the difference or had the wire heat up. 16 gauge works fine.

From there, set the speaker down into its resting place and screw it to the enclosure. I've opted for no grill cloth because I'm going to apply weatherstripping and seal the front of the enclosure to the floor of my living room, running screws through the floor joists to secure it in place. Enjoy the surround sound!