Building a High-End Enclosure For a High-End Subwoofer System

Let’s face it: Installing car stereo equipment is no picnic. Cramming electronics and speakers into places they simply weren’t meant to be presents a challenge to even the most savvy of installers. And the more exotic the install, the tougher the task. While each aspect of system design brings a unique set of challenges to the installation party, building custom speaker enclosures is perhaps the most creative aspect of the installer’s job.

The box-building option is regularly exercised by enthusiasts in pursuit of smooth, deep bass – a tough nut to crack in the automobile environment. Two enclosure types that are practical for most automobile applications are the “sealed box” and the “vented box.” Although different in design and operation, both types utilize the same basic construction techniques. But before you grab a saw and carpenter’s square, you have to decide which box will best suit your needs.

Sealed Enclosures

Straightforward in design, the sealed box is nothing more than an airtight enclosure whose purpose is to enhance speaker performance. When a woofer is installed in one, the sound waves that emanate from the front of the speaker cone are separated from the rear-firing waves. This improves bass response, since opposing waves can cancel each other out when they aren’t isolated. Superb damping, good power-handling capability, and simplicity of construction make the sealed-box enclosure an ideal candidate for a variety of installations.

Vented Enclosures

Vented enclosures – also known as ported or bass-reflex systems – are more complex than sealed boxes, but the extra construction work required to install a vent has its rewards. Vented boxes are more efficient than their sealed counterparts, since they channel sound waves from the front and rear of the cone into the listening environment. Other virtues of the vented design include better reproduction of low bass, a reasonably flat response curve, and low distortion – provided the box is properly constructed.

System Design

The first step when designing an enclosure is determining how much space is available in your vehicle. If you own a hatchback, for example, calculate the cubic footage of the hatch area, keeping in mind that you’ll still need access to your spare tire. The space available will dictate the size and shape of the enclosure, which in turn will narrow your choice of drivers. For example, one 10-inch subwoofer typically requires an enclosure with 1 to 1.5 cubic feet of internal volume for optimum performance, while a 12-inch subwoofer requires 2 to 3 cubic feet. Figure 1 lists recommended box volumes for common-size drivers, but again it’s best to consult the manufacturer of the speakers you’re considering for recommended box volumes.

FIGURE 1 – Box Volumes for Common Driver Sizes

It’s important to note that any recommendation won’t take into account the volume that will be displaced by the drivers, crossovers, internal braces, and, in the case of a vented system, the port. To compensate for these components, you must build your enclosure slightly larger than your measurements indicate. As a rule of thumb, it’s generally wise to increase the volume by 20 percent.

When figuring out the shape of your box, keep in mind these “ideal” dimension guidelines: Depth should be 0.7 x Width and Height 1.4 x Width. Also, avoid constructing an enclosure with walls that have equal dimensions; this could result in a resonant peak in the midbass region.

If the enclosure is going to be irregularly shaped – as most are – it’s easiest to think of its interior as a collection of conventional shapes, calculate the volume for each, and add these volumes together. (See Figure 2 for formulas on calculating internal box volume.) For example, the volume of a box with a slanted front is most easily determined by adding the volume of the square or rectangular portion at the bottom to the volume of the triangular portion at the top. Also, it’s easier to measure in cubic inches and then convert to cubic feet than to work with cubic-foot figures alone.

FIGURE 2 – How to Calculate Internal Box Volume

• Area (rectangle) = Length x Width
• Area (triangle) = 1/2 Length x Width
• Area (circle) = 3.14 x Radius x Radius
• Area (circle) = .785 x Diameter x Diameter
• Volume (rectangular box) = Area (of one side) x Depth
• Volume (triangular enclosure) = Area (triangle) x Depth
• Volume (cylinder) = Area (circle) x Length
• 1728 Cubic Inches = 1 Cubic Foot
• 28 Liters = 1 Cubic Foot

For multiple-driver enclosures, you’ll get the best performance by isolating each driver in a separate chamber. This prevents variations in speaker tolerances and input power from restricting system output. If building separate chambers isn’t feasible, you can make sure that each driver receives the same amount of power by mounting two identical drivers in the same cavity and wiring them in parallel.

Given the extraordinary number of choices, selecting drivers can be a bewildering task. It requires an understanding of specs as well as a sharp set of ears. For some guidance, see the “Wire Service” column in the September/October CSR and talk to an experienced installer.

Once you’ve selected an enclosure type and the appropriate drivers, map out the design on paper. This will help you visualize the shape of the enclosure and may prevent unnecessary cutting. Before you take out your trusty saw, be absolutely sure that the enclosure will fit in your vehicle.

Material Selection

Fiberboard is the best material to use for the walls of your enclosure, since its high density minimizes panel vibrations and results in a more efficient enclosure. Don’t confuse fiberboard with particle board. Although particle board is similar in appearance to fiberboard, its tensile properties make it unfit for the car environment: Extreme temperature variations and humidity can cause particle board to chip, split, or warp.

Plywood is a viable option if fiberboard is not readily available. (Fiberboard is not always available in lumber yards.) Plywood is not as dense as fiberboard, but it has the distinct advantage of weighing only half as much. If you choose a laminated wood, make sure it’s free of defects; imperfections in lamination can cause your enclosure to buzz or rattle. As for thickness, 5/8-inch will suffice for enclosures with internal volumes of 2 cubic feet or less; for larger boxes, use 3/4-inch material.

To join the pieces of your enclosure together, your best bet is to use screws. Nails work, of course, but they can loosen over time – especially if you’re building a removable enclosure that will be subject to some abuse. Other materials you’ll need include carpenter’s glue, silicone sealant, spray adhesive, and various finishing materials (discussed below).

Tools Required

You won’t need any special tools to build a speaker enclosure, just a tape measure or yardstick for measuring, a couple of pencils for drawing the cut lines, and a compass for drawing the speaker and vent cutouts. To make a straight, accurate cut, your best bet is a table saw; a circular saw is the next best choice, followed by an ordinary hand saw. To cut circular openings for the drivers and possibly a vent, you’ll need a jigsaw. An electric drill will come in handy for drilling screw holes and can double as a power screwdriver. Other required tools are a hammer, a standard screwdriver, a razor knife, a caulking gun, and safety goggles.

Caution

To prevent injury, please follow these precautions.

• ALWAYS wear safety goggles
• NEVER operate any power tool unless properly qualified
• ALWAYS observe precautions in the operator’s manuals

Construction

Draw out each piece of the enclosure before cutting so you don’t waste material. Once individual sections have been cut, temporarily assemble the box to make sure the pieces fit snugly together. One option to consider at this juncture is the use of internal braces, which, depending on the overall size of the box, may result in a sturdier enclosure. Usually, 1.5 inch x 1.5 inch strips of material will suffice. Generally, you’ll need to add braces only for boxes that have internal volumes of more than 5 cubic feet.

If all of the pieces fit and the edges match up, the box is ready for permanent assembly. First make sure all joints are free of debris. Then slowly run a bead of carpenter’s glue along the line where you’ll be making the first joint. Carefully assemble the two pieces and secure them using screws or nails. Use one screw every 6 inches or one nail every 3 inches. If you’re using screws, drill pilot holes to prevent the wood from splitting. And for a more polished look, countersink the screws. (Note that the type of joint that you select is not critical as long as the box is strong and airtight.)

Continue assembling the box but don’t attach the baffle board – the front panel that holds the speakers – until you’ve made sure the enclosure is sealed. To do this, fill a caulking gun with silicone sealant and run a bead along the inside of each joint. When you’re finished caulking the box, secure the baffle board in place.

With the box assembled, you are now ready to cut the speaker openings and the vent, if you’ve chosen a vented design. I’ve always found that it’s easier to work on the wood once the box is assembled. The vent tube can be made of plastic, cardboard, or – for a square or rectangular vent – wood. First, calculate the center of each hole and draw a circle of the appropriate diameter using your compass. Make 3/8-inch starter holes with a drill and then cut out the holes using a jigsaw.

Once the speaker and port openings are cut, put some silicone sealant on the tip of your index finger and reach into the enclosure to seal the inside joints of the baffle board. It’s very important for the enclosure to be completely airtight; air leakage reduces the system’s power-handling capacity and causes distortion. Once the joints are sealed, I recommend covering three adjacent inside walls – back, top, and bottom, for example – with 1 to 2 inches of polyfill (available at fabric stores for about $1.50 a bag). The use of filler will help minimize midbass resonances.

Finishing

The physical appearance of your enclosure is strictly a matter of personal taste. Although you may not think it’s important, most people want the cabinet to complement the vehicle’s interior. Phoenix Gold (800- 445-9229) and Rockford Fosgate’s ACI division (602-967- 3565) sell fabric for this purpose. Phoenix Gold’s 4 Season carpet is available in eleven colors and costs about $10 per square yard. Perfect Interface’s Overdrive is a cross between carpet and felt that’s available in twelve different colors; it’ll run you about $12 per square yard. Your local installer might be able to help out here, too.

Typically, spray adhesives are used to bond fabric to the surface of an enclosure. (3M Spray Adhesive sells for about $8 a can.) You’ll need your razor knife to trim away excess material and cut around the speaker openings. You also can use paint or Formica to finish your enclosure. Although Formica looks good, it’s very difficult to work with. Paint, on the other hand, is easy to apply but can make your box look rough.

Mounting the Drivers

Before mounting the speakers, you’ll need to drill a few holes. First make a hole at the bottom of the enclosure’s rear panel for the speaker-cable connector. Then place the drivers in their respective holes and mark the location of each screw hole with a pencil. Remove the speaker and drill pilot holes. Before proceeding, remove all wood shavings. Snake a generous length of speaker cable through the hole in the rear of the enclosure. Then mount the connector of your choice (Radio Shack sells many different types) and, using your finger, seal the opening with silicone. Next, hook the speaker wire to the connector. If you don’t use connectors (in the case of a permanent installation, perhaps), tie a knot in the speaker cable at the point where it exits the enclosure.

Now mount your crossover devices, and wire the cables to the speakers. (Don’t forget about polarity.) With all wires securely in place (I recommend soldering), secure the drivers to the baffle board, using extreme caution – the last thing you need at this stage is a screwdriver-size hole in the speaker.

Covering the speakers with some type of grille may be a good idea depending on the location of the box in your vehicle. Grilles are available from numerous sources, including speaker manufacturers. Two accessory manufacturers that sell an assortment of grilles are American International (800-336-6500) and Ampersand (800-423-5167).

Securing the Enclosure

Once your box is complete, the next step is to secure it in the vehicle. A careful inspection of the area surrounding the enclosure will help you determine the best and safest means for doing so. L-brackets usually suffice, or you can remove the drivers and bolt the box directly to the vehicle. If you choose the latter approach, be extremely careful when drilling holes. It’s amazing how much damage can be done to a vehicle if you don’t look where you’re drilling. (Don’t overlook the gas tank.)

The final step: Connect your new creation to its power source and hit the play button on your head unit. Now you can give yourself a pat on the back for enhancing the performance of your auto sound system. I’ve built many enclosures over the years, and with each one I’ve picked up a new trick or two. Patience is key, but it is even more important to talk to manufacturers and experienced installers. Following their directions makes for smooth sailing on the road to good sound.