When designing a Subwoofer enclosure, there are many aspect you must think about; from the placement of the box all the way to painting and airbrushing. One of the first things to think about is placement and shape of your subwoofer enclosure.
Where would you prefer to have your subwoofer enclosure? Most people keep them in their trunk, and some will install them in the cab of their vehicle. It's best to have an idea of where you would like to contain the enclosure so that you can find the correct design and structure of your enclosure.
Once you have an idea of where you would like it to be placed, then you can decide what shape you want your subwoofer enclosure to be. There are a few types to think about, such as a plain box, a triangle, a trapezoid, or a beveled box. Different shapes are specifically for looks and appeal to the owner, outside of the basic box shape, the other designs are used more in car shows.
After you have decided on where you want to place your box and what shape you would like to use, you will need to decide if you will need a ported box or a sealed box. If you are looking for something with more bass, or if you listen to more rap and hip hop, you would want a sealed box. However, if you listen to alternative, rock or country, you would be best with a ported box. The difference between the two is the sealed box is completely enclosed around your subwoofer, whereas the ported box had an airflow opening that allows the sound waves to pressurize into the chanel and come through the open port.
Next, you need to figure out what cubic space you would need for the inside of the of the subwoofer enclosure for optimal sound. This is determined by length times the width times the height. Remember that the measurement needs to be the inside of the box and not include the wood. The box that your subwoofer comes in will tell you what cubic space is required for the optimal performance and from there you can experiment with numbers to figure out the measurements.
The very first measurement you want to obtain is going to be the width/depth. This is because you need to make sure that the subwoofer is going to fit into your box. The most simple way to do this is to go to http://www.the12volt.com/caraudio/boxcalcs.asp, where you can enter your numbers into a calculator to determine the correct measurements for the cubic space needed for your box.
Once you have decided where your box will be kept, which shape you prefer to use, and you have your measurements needed (including the space in which you plan to place your subwoofer box, you don't want to build the box and find out it does not fit where you want it), then you can start building your box.