How to Build Yourself a Sub Box

One reason to choose a simple sealed enclosure for your sub box is that you can easily build one yourself if you’re handy and have the right tools. Building your own enclosure can potentially save you money, and you can build it and finish it the way you want. Plus, you’ll get the satisfaction of doing it on your own and you’ll learn a lot along the way.

Make sure you follow the subwoofer manufacturer’s instructions on enclosure size. Otherwise, if you try to squeeze a 15-inch subwoofer into a box that’s too small, or you place five 10-inch subwoofers in a box that takes up the entire bed of your pickup, you likely won’t be happy with the quality of the bass!

Building the proper size enclosure means making sure it has the amount of air space inside as required by the subwoofer, per the sub manufacturer’s specifications. While this takes some basic mathematics skills, you can take the guesswork out by using one of the online subwoofer-box calculators.

(See the “Measuring air” sidebar earlier in this chapter.)

This basic step-by-step guide can’t possibly cover everything you may encounter when building your sub box, so you may want to have someone with box-building experience help you. You can also find some great

Measuring air 

How do you calculate the volume or amount of air space in a subwoofer enclosure? You can try recalling some of the basic math from high school, or, if you like doing things the easy way, just use one of the subwoofer box calculators online that even takes into account the volume of the sub. Point your browser to Basic Car Audio Electronics {www.bcael.com/spboxnew2.htm) or The 12 Volt.com (www.thel2 volt.com/caraudio/boxcalcs.asp) and you'll be on your way.

resources on the Web, such as CarStereo.com (www.carstereo.com), Basic Car Audio Electronics (www.bcael.com), and Car Audio Help (www.caraudiohelp.com). Several online forums also provide D1Y installation advice and tips, including rec.audio.car, www.mobileaudio.com, www.caraudioforum.com, and www.sounddomain.com.

To prevent injuries while building your box, be sure to follow these precautions:

• Always wear safety goggles.
• Never operate power tools unless you have experience.
• Consult the owner’s manual and instructions for any tools or materials you use.

Gathering your toots and materials

To build a simple sealed enclosure, you’ll need

• A tape measure for measuring the wood
• A pencil for drawing cut lines
• A compass for marking the speaker cutout
• A table saw, circular saw, or hand saw for cutting wood
• A jigsaw for cutting the speaker opening
• An electric drill for drilling holes
• A screwdriver for installing screws (you can also use a drill) 
• A caulking gun for sealing the seams of the box 
• A razor knife for trimming the finishing material

Fiberboard is the ideal material for building an enclosure, and many installers favor medium-density fiberboard (MDF). Don’t mistake particleboard for fiberboard, although they are similar in appearance. The temperature extremes that occur regularly in a car can cause particleboard to warp and split. Particleboard also doesn’t have the tensile strength (the measure of the force it takes for a material to break apart) of fiberboard. Therefore, it can’t stand up to the extreme vibrations caused by a subwoofer and can add resonance or vibrations that effect the performance of the subwoofer. You can also use plywood for your enclosure. It’s less dense than fiberboard, but has the advantage of weighing less. 

Wood at least 5/8-inch thick is recommended, but thicker is usually better, up to a point. Unless you’re constructing an enclosure that will house several subwoofers, you shouldn’t need to use wood more than one inch thick.

Use screws to join the pieces of wood together instead of nails because nails can loosen over time with steady abuse and vibration. You’ll also need carpenter’s glue to initially attach the sides of the box and silicone sealant to seal the seams.

If you don’t want a rough wooden box, you’ll need to choose some sort of finishing material, such as carpeting (trunk liner carpeting usually works best), and use spray adhesive to attach it the box. You’ll also need a terminal cup, which is a piece of hardware that fits into the box. The subwoofer’s speaker terminals attach to it via speaker wire, and the terminal cup in turn attaches to an amplifier’s power leads. A terminal cup can be found at Radio Shack or any other electrical supply store.

Building the box

Trace and mark each part of the enclosure on the wood before you cut it. After you’ve cut each section, temporarily assemble the box to make sure the pieces fit together. If everything looks okay, you’re ready to assemble the box:

1. Make sure all of the joints are free of debris and then carefully run a bead of carpenter’s glue along the line for the first joint. The type of joint you use isn’t as important as making sure the box is airtight.
2. Attach the first two sides of the box and join them with screws every 6 inches.  Drilling pilot holes before you insert the screws is a good idea to keep the wood from splitting.
3. Assemble all sides of the box except for the baffle, which is the panel that the subwoofer will attach to.
4. Before installing the baffle, use a caulking gun to run a bead of silicone along the inside of each joint for an airtight seal. Air leaks can reduce power handling and cause distortion. After the box is sealed, you can attached the baffle and then cut the hole for the speaker.
5. Determine the center of the hole for the subwoofer and sketch a circle of the appropriate diameter using a compass. 
6. Make a series of 3/8-inch starter holes with a drill and then cut out the hole for the sub using a jigsaw.
7. After you’ve cut the speaker openings, reach into the enclosure and, using your index finger, seal the inside joints of the baffle with silicone.
8. (Optional.) Some installers line the inside of a box with 1 to 2 inches of polyfill, which you can find at fabric stores.

This makes the sub think that the box is larger by absorbing or slowing down some of the sound waves inside the box if you inadvertently built it smaller than the sub’s specs call for.

Mounting the subwoofer

Before mounting the speaker to the baffle, you need to create a hole in the enclosure’s rear panel, typically at the bottom, for the terminal cup. After cutting the hole to the exact size of the terminal cup with a jig saw, slide the cup in and seal the area around it, inside and out, with silicone.

Now you’re ready to install the subwoofer:

1. Place the subwoofer in the hole in the baffle and mark the location of each screw hole with a pencil.
2. Remove the sub and drill pilot holes.
3. Be sure to remove all wood shavings and any other debris from inside the box before securing the subwoofer.
4. Connect speaker wire to the subwoofer, paying close attention to the polarity to make sure you connect the positive wire to the positive terminal and the negative to the negative.
5. Secure the subwoofer to the baffle with screws, being careful not to run any of them through the cone of the subwoofer!
6. (Optional.) At this point, most people cover a sub box in an automotive-style carpeting, which is available at an auto-parts store or online.

Auto Carpet Direct at www.automotivecarpet.com is one such source; Installer.com at www.installer.com, is another. Carpeting isn’t your only choice of material for covering your subwoofer enclosure. You can get as creative as you like and use vinyl, Formica, or even mirrored Plexiglas. Spray adhesive is typically used to bond the carpeting to the enclosure, and a razor knife can be used to trim excess material. 

Securing your sub

Whether you use a grille over the subwoofer is purely a matter of personal preference, although it’s usually a good idea to protect the sub’s cone. You’ll also want to secure your enclosure so that it doesn’t go flying when you make a sudden stop or a sharp turn. L-brackets, a piece of hardware that’s shaped like the letter L with screw holes on each side, are great for securing a sub box. Attach one side of the L bracket to the box and the other side to the car. Just be careful when drilling holes into your car!