Basic Subwoofer Box Building Information
Check with manufacturer or dealer for appropriate subwoofer box volume and design. Some subs can't be used in certain types of subwoofer boxes, and have very small tolerances for box volume errors. If a sub is installed in a subwoofer box larger or smaller than what is supposed to, it will sound bad and could be destroyed. Subwoofer boxes and subwoofer enclosures can be built in any shape, but it is difficult to calculate volume for complex shapes.
Materials needed to build subwoofer boxes and subwoofer enclosures
A subwoofer box has to be very rigid. Most common building materials are 5/8" or thicker particle board or medium density fiberboard.
If building a subwoofer box with Plexiglas, do not use anything less that 1/2 inch thick.
A common material used to mold complex shaped subwoofer boxes is fiberglass, but it is a real pain to work with, and several layers need to be applied for a solid finish.
Use glue at all joints (cheapest and most used product is Liquid Nails). Make sure there are no holes. Any leaks will degrade the performance of your subwoofers, not to mention the annoying noise air makes when being pushed out of a small hole.
Let glue cure for at least 24 hours before mounting the woofers. The fumes of some products will eat up rubber and other materials subs are made of.
Holding Joints Together
Screw joints (use 2" - 2-1/2" screws) every four inches or so. Pre-drill about 3/4" deep, so that screws do not split the wood at the edges, especially when working with particle board.
A box for Each Sub?
Even though it is not necessary to have two separate chambers for two subs, it is best to take this approach for two reasons: First, if one of the subwoofers dies, then the volume of the subwoofer box will be "twice" as big, as seen by the sub that is still working. This could cause problems and even damage the other sub. The second reason is bracing. building a subwoofer box with a divider in the middle will be much sturdier.
There are several way to build ports. If a pre-made port is not available, the most common material is PVC tubing. PVC tubing is very rigid, comes in different diameters, and is easily found at any hardware store.
Cut the tubing at the desired length. Consider the volume the port takes up when calculating the subwoofer box volume. Cut a hole in the box. Make sure the hole is as perfect as possible to minimize gaps between the box and the tube. A couple wood braces can be added for screwing the port to the subwoofer box. Seal the gaps using plenty of Liquid Nails or similar product.
Boxes that are more than a foot on width or length or height, should be braced (use a piece of wood maybe 3 or 4 inches wide across the box, so that the subwoofer box does not flex). It is a good idea to put wood blocks on the corners for reinforcement. Always consider that blocks, braces, neon lights, etc. inside a box take up space and should be accounted for when calculating internal volume.
It is advisable to put damping material inside a subwoofer box. Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin. This increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil. Polyfill also "fools" a sub into thinking it is in a bigger box. Play around with different amounts of polyfill until you get the desired results.
Finishing subwoofer boxes and subwoofer enclosures
Add wood filler to holes and sand the box to make a smooth surface. If you are painting the subwoofer box, It is a good idea to apply primer under the paint.
It is not necessary to sand the subwoofer enclosure or subwoofer box if you are using carpet or padding under vinyl, since the thickness of the material will cover any small imperfections. The best way to cut carpet or vinyl is with a good quality carpet knife. Blades wear out pretty quickly, so buy a handful. Cut a piece of carpet (or vinyl) big enough to cover the whole enclosure. Apply adhesive to both box and carpet (spray 3M adhesive 77 or 90 works great). Wait about a minute and place the fabric over the wood. For a good fit, stretch the fabric when applying it. The fabric should wrap around and end in a place of the subwoofer box that will not be seen. Do one side at a time, cutting excess carpet. If possible, add staples to hold the fabric at the ends.
Subwoofer Boxes and subwoofer enclosure types
A box ranges in complexity from the "plain vanilla box" (sealed) to band pass and even more exotic enclosures. Each enclosure has advantages and disadvantages and should be designed accordingly to the individual speaker parameters (the "one size fits all" rule DOES NOT apply to subwoofers and boxes).
Subwoofers need more amplifier power than everything else in the system. This is because human ears are less sensitive at lower frequencies, so a higher bass level is needed for everything to sound even. A low-pass crossover is required to block off high frequencies.
What type of subwoofer is better? A bigger subwoofer gives more bass, but needs a bigger subwoofer box. Since most people like to have a trunk, 10 and 12-inch woofers are most common. When buying a subwoofer always keep in mind that bigger size is not necessarily better. A good quality 8-inch sub will outperform a cheap 12-incher. Big subs (12", 15") have slower responses, yielding to boomier bass. Small subs (8", 10") have a tight and more controlled sound.
Types of boxes
Free Air - subwoofers are either mounted under the rear deck or behind the rear seat of a car. This configuration will not work very well for hatchbacks. Holes have to be cut where the woofers are to be mounted. Since the woofers use the whole trunk as a box, the trunk has to be as sealed as possible from the cabin. Trunk can be isolated usually by putting particle board under the deck and behind the seat. The drawback of free air subwoofers is that bass will not be very accurate (especially at lower frequencies), and more amplifier power will be required than with a regular subwoofer box, but then again, you still have a full trunk.
Sealed - is the most common subwoofer box and easiest to build. These boxes will give the flattest frequency response, and best overall sound quality (especially at lower frequencies). The subwoofer enclosure internal volume should be as close as possible to the recommended by the manufacturer. If a subwoofer box is smaller than what it is supposed to be, the sound will be tighter, but more amplifier power will be required. If the subwoofer box is too big, then the sound will be muddy.
Ported - boxes are usually bigger in size than sealed and have a "tube" (port) that lets some air out of the box. The idea of a ported box is that the speaker port pushes (or pulls) air at the same time as the woofer, reinforcing bass. The subwoofer box itself acts as an amplifier, yielding to more bass than a sealed enclosure (3 to 4 dB). Ported subwoofer boxes do not have a linear frequency response. If the subwoofer enclosures are not built according to specifications, they will not sound good. The subwoofer box design acts as a filter, cutting off lower frequencies.
Isobaric - configuration is a good way to get bass in a smaller subwoofer box. This is done by building a subwoofer box about half the volume of a sealed subwoofer box, and placing two woofers facing each other. Note that everything must be sealed, including space between woofers. A spacer between both woofers must be used in most cases to avoid subs hitting each other. When wiring, make sure that woofers are out of phase: Wire one of them backwards (negative to positive, and positive to negative), so that both pull or push at the same time. An isobaric configuration will NOT put out much more power than a box using a single woofer. Its main purpose is to reduce box size. Another drawback is that since one of the subs is exposed, it is more prone to damage.
Band Pass - enclosures consist of a woofer between a sealed and ported box. Band pass subwoofer boxes will yield more bass than sealed and ported subwoofer boxes (especially at lower frequencies), but over a narrower frequency range. Since the subwoofer enclosure acts as a filter, mechanically blocking lower and upper frequencies, a crossover is not needed in most cases. These subwoofer enclosures are usually big, and very unforgiving when precise volumes and port sizes are not followed. Band pass subwoofer boxes also tend to mask distortion. If you can't hear distortion and turn your stereo down in time, you could damage your subs.
Aperiodic - Very small subwoofer boxes that "breathe" through a moving membrane. Both the membrane and cone can not be in the same exterior space. Either the membrane part has to be isolated by cutting a hole in the car so that it is outside, or the subwoofer has to be isolated from the rest of the trunk in a similar fashion to free air woofers. The "box" has to be as small as possible (ideally the membrane should be right up against the sub), since it is used only for coupling the sub and membrane. Aperiodic membrane configurations are very hard to design and tune, but give good frequency response and respond faster to transients, giving accurate and tight bass as opposed to boomy sound. They are not ruled by Thiele-Small parameters like other designs, so any woofer would work with the membrane.
Amplified Bass Subwoofer Boxes
A good choice for small cars and (ideal) for hatchbacks and pickup trucks. They usually take up very little room, putting out to fairly good bass. The most known manufacturer is Bazooka® for it's Bass Tubes®. Their design is a ported box. The woofer has to be close to a wall or, better yet, to a corner. To fine-tune, the bass tube is moved either closer, or farther form the wall or corner.
It is convenient to get an amplified tube, since amplifier, crossover and subwoofer are all integrated in a small package. If you buy the components separately, you will end up spending more money. Another good feature of tubes is the fact that they can be easily and quickly installed and removed.
If you decide to get one, keep in mind that even though they all look the same, cheaper brands will not sound good. A decent tube will run in the $300's (amplified), and in the $100's for a non-amplified.
Custom Bass Boxes
Many manufacturers such as JL Audio and MTX are making custom boxes (with subs included) to fit in center consoles, under seats, or in other small spaces. Although these boxes do cost a lot of money, most give superb performance and integrate easily in a car without taking up too much room.
Also see: How to Select the Right Subwoofer Box
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