A box ranges in complexity from the “plain vanilla box” (sealed) to bandpass 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 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
Sealed – is the most common box and easiest to build. These boxes will give the flattest frequency response, and best overall sound quality (especially at lower frequencies). The box internal volume should be as close as possible to the recommended by the manufacturer. 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” (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 box itself acts as an amplifier, yielding to more bass than a sealed enclosure (3 to 4 dB). Ported boxes do not have a linear frequency response. If the box is not built according to specifications, it will not sound good. The box design acts as a filter, cutting off lower frequencies.
Isobaric – configuration is a good way to get bass in a smaller box. This is done by building a box about half the volume of a sealed 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. Bandpass boxes will yield more bass than sealed and ported boxes (especially at lower frequencies), 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. These enclosures are usually big, and very unforgiving when precise volumes and port sizes are not followed. 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. 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.
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 box, but then again, you still have a full trunk.
Amplified Bass 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 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.