Here are a few ideas on how to build your custom box.
Types of boxes
--Free Air--
subwoofers are mounted under the rear deck or behind the rear seat of a car. Since the woofers use the whole trunk as a box, the trunk has to be as sealed as possible from the cabin. The drawback of free air subwoofers is that bass will not be very accurate, and more amplifier power will be required than with a regular box.
--Sealed--
These boxes will give the flattest frequency response and best overall sound quality. If a box is smaller than what it is supposed to be, the sound will be tighter, but more amplifier power will be required. If the box is too big, then the sound will be muddy.
--Ported--
boxes are usually bigger in size than sealed and have a tube 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 box design acts as a filter, cutting off lower frequencies.
--Isobaric--
this is a good way to get more bass in a smaller box. This is done by building a box and placing two woofers facing each other. Note that everything must be sealed, including space between woofers. When wiring 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.
--Band Pass--
consist of a woofer between a sealed and ported box. Bandpass boxes will yield more bass than sealed and ported boxes but over a narrower frequency range. Since the box acts as a filter, mechanically blocking lower and upper frequencies, a crossover is not needed in most cases. Bandpass 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 boxes that "breathe" through a moving membrane. Both the membrane and cone can not be in the same exterior space. The "box" has to be as small as possible, since it is used only for coupling the sub and membrane.
--Amplified Bass Boxes--
A good choice for small cars and for hatchbacks and pickup trucks. They usually take up very little room, putting out to fairly good bass. 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. 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 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.
