Subwoofers are a key part of any car stereo system; without them you are missing out on much of the low note sounds and the thumping bass found in many songs. But an improperly installed and mounted subwoofer can lead to rattling, poor quality sound, and even damage to the subwoofer and components.
Many people choose to build their own subwoofer box; which can save money, allow for custom designs, and ensure a perfect fit for the subwoofers you own. If you are reading this you are probably well aware that a good subwoofer box needs to secure the subwoofers, allow air to flow through them, and be the proper size. But before you build the sub box you need to figure out what kind of wood to build the sub enclosure out of. The key to picking out the perfect building material for your box is to find material that is as non-resonate as possible. You don't want sounds from the subs to bounce all over the box causing vibrations, muddy sounding bass, and distortion. You want a wood that only allows the pure bass sounds to come out of your subwoofers.
With all that taken into consideration, the most commonly used wood for building sub boxes is plywood. You'll want birch plywood as it is dense and sturdy. Most audio system experts suggest that 3/4 inch thick birch plywood is sufficient. Some people choose to use thick 1 inch plywood, but this will add bulk and weight to the subwoofer box. Another material to consider is a material that only looks like wood. MDF fiberboard is often used to build audio speaker enclosures because unlike real wood, it resonates less. Fiberboard also does not warp or expand like real wood; making it possible for your sub box to withstand a wider range of temperature and weather conditions. If you choose fiberboard, the 3/4 inch thickness rule still applies. Too thin and you'll risk having a cracked box or a lot of undesirable distorted sounds.
Once you've chosen the wood or material from the above choices you'll want to make sure you build your subwoofer box to perfectly fit your subs. It is recommended that each subwoofer has it's own box and that each box be built to hold the subwoofer securely inside. Air holes need to be made in the box to allow air flow or you'll get muffled sound and risk damaging both box and sub. These air holes may vary in size and number depending on how powerful your subwoofers are. Since this article is primarily about the box material and not the box design, that is about as far as I'll go into box sizing and proper fitting.
Back to the materials; you'll want to make sure you measure out and size everything out in advance. Cuts should be made clean so the box parts sit flush against one another. Well made sub boxes won't require many nails and screws as well cut pieces can be super glued together. You can also use a product such as Liquid Nails. Braces may be considered as they can keep the box held tightly together and also for securing the box to the vehicle.
Overall you'll want to use either birch plywood or MDF fiberboard that is 3/4 inch thick. Limit or eliminate nails/screws and use a strong adhesive to put your box together. Take your time cutting and measuring the pieces of your box and consider making an individual enclosure for each subwoofer. Your main goal in designing and building your own sub boxes is to minimize resonating sounds and vibrations as well as protect your investment.