Speaker size is the first determinant of box size
A powered subwoofer is the most dramatic addition that can be added to a home theater or listening environment. It is the subwoofer that delivers the thunderous and/or stomach-rattling thrills in movie theaters. A powered subwoofer is the most efficient type of sub, because the amp is perfectly matched to the loudspeaker. Though it is a challenging undertaking, there is abundant help on the Internet for building a powered subwoofer, and the cost savings are 90 percent less than buying an equivalent manufactured subwoofer.
Instructions
Speaker Box Preparations
1. Choose a 12-inch or 15-inch loudspeaker. The loudspeaker is the main determinant of subwoofer box size.
2. Choose a plate amplifer. The plate amplifier generally mounts inside the speaker box.
3. Decide on the type of subwoofer box. The type of subwoofer box is the number two element in determining its size. An enclosed or sealed box is the easiest to make and delivers the tightest bass response; a ported or vented cabinet is more efficient and will produce more low end response, but is larger in box size.
4. Get the Thiele/Small parameters that dictate the enclosure size for the chosen loudspeaker. Thiele/Small, aka, T/S parameters, are provided with every good quality loudspeaker. They are also available at the Thiele/Small website among its database of 5,000 speakers. (See References)
5. Get a software program to aid the subwoofer design. WinISD is among the most popular speaker design programs, and is a free download from its website for Windows users. (See Resources)
6. Plug the T/S parameters and speaker size into your design software, and print out the subwoofer construction plans.
Building the DIY Sub
7. Buy the wood for the cabinet. Use 3/4-inch birch plywood. Birch plywood was the design material for the two most famous speakers in existence, the "Voice of the Theater" from Altec-Lansing, and the Klipschorn from Klipsch.
8. Cut the wood to size for the box panels. Do it yourself, or have the store where the the wood was purchased cut it for a small fee. The panel sizes are obtained from the software-generated plans.
9. Use PVC pipe for a port. The port or vent enhances the reproduction of low frequency sound by extending the low frequency range response of the box. The software design program will establish the size and length of the port. (Skip this step if building a sealed box for the subwoofer.)
10. Glue the two sides to the bottom piece and the bracing piece. A bracing piece the size of the bottom piece is used in all subwoofer boxes with speakers larger than 10 inches in diameter. Test fit all cabinet surfaces before applying glue. If also using screws, counter-sink them. Use furniture clamps to bind the box. Let it dry for eight hours.
11. Make the cut out for the amplifier in the back panel. Test-fit the panel to the side panels first. Then, glue-screw-clamp as above. Allow eight hours for this joinery to dry completely.
12. Attach the top panel. The top panel can be the same size as the bottom and the brace, or be larger and used as a table top. (Bass frequencies are non-directional, so the sub can be placed anywhere.) Secure with glue and wood screws. Clamp as in previous steps, and allow to dry for eight hours.
13. Make the cutouts in the front panel. The cutouts are for the loudspeaker (and the port if building a ported sub.) Caulk all box inside seams with silicone caulk prior to attaching the front panel with glue and screws. Then, apply silicone caulk to the front panel seams through the cutouts. Clamp again with furniture clamps, and leave to dry for 15 hours until the silicone caulk is fully dry. Drying caulk can damage the speaker surrounds.
14. Construct the speaker port. (Skip this step if building a sealed box.) Use PVC plastic pipe for the port. Flare the outside end of the port to minimize distortion. Glue the port to the back of the front panel. After allowing the glue to dry, apply silicone caulk around the port. Allow the caulk to again completely dry. Absolutely no air should escape a ported box except through the port.
15. Sand the entire box. If screws are to be permanent, fill all countersunk screw holes with wood putty. Sand all seams again.
16. Install the plate amplifier. Be sure that amplifier RCA connections face out from the box. Check for amplifier clearance with the bracing. Cut out a section of bracing if there's not enough clearance. Secure the amplifier to the back panel with glue. After the glue is dry, apply silicone caulk around the inside perimeter of the amplifier. Allow enough time for the caulk to thoroughly dry.
17.Attach the cables for the amp-speaker connection. Connect the wires to the speaker terminals on the loudspeaker, and attach the loudspeaker to the front panel. Use wood screws and a non-drying caulk around the speaker frame to make an airtight seal.
18. Apply a finish to the box. Stain, paint or wood veneers are good finish choices.
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