Installment No. 2
This month we focus on the creation of the ported subwoofer enclosure and its integration into the back hatch of the car. Keep in mind that these specifications are simply a guideline - not all drivers and enclosures will conform to this every time. Use your best judgment when building your own boxes.
A base reflex enclosure was best suited for the Sony drivers. To decide the correct enclosure for the driver, I first had to find the efficiency bandwidth product of the driver, or EBP. EBP is equal to fs divided by Qes. Fs is the driver free air resonance in hertz. This is the point at which the driver's impedance is maximum. Qes is the driver's Q (a measurement of the control coming from the speaker's voice coil and magnet) at resonance. The "fs" and "Qes" values were found using the Thiele/Small specifications provided by Sony. The general rule of thumb is that an EBP of 50 or less equals a driver best used in a sealed enclosure. An EBP of 50 to 90 has flexible enclosure options. An EBP of 90 or higher is best used in a ported enclosure. The Sony driver had an fs value of 20.3 at 2 ohms and Qes value of 0.35. When applying the EBP formula to these values the EBP for this driver is 58. So plans for a ported enclosure worked out perfectly.
The driver we chose is an XS-LD125P5 12" component subwoofer. We used three dual voice coil subwoofers (dual 4 ohms) wired in parallel and presented a 2-ohm load at the amplifier. The amplifiers are XM-D1000GTR - one amp per sub wired at 2 ohms - which gave us a power output of 900 watts by 1 RMS.
This car's stereo was built for dual purposes: as a display at car shows and for sound system quality. Keep in mind that when you're planning your subwoofer enclosure placement within your car, think about how it will perform when the car is open and on display. Try to place the subwoofers where they'll have a reflective surface to output against even when the car is open.
Next month we'll continue with the third installment of the project with the design and construction of the center gauge pod and the fiberglass rear quarter panels.
The rear half of the car was prepped for building the subwoofer enclosure. 3M 233+ green tape is an excellent choice as it won't leave a sticky residue that will damage the vehicle. Make sure to buy it in several different widths. | 
Two 31/44" MDF strips were cut and glued together to form spacers, which allowed clearance over the hatch-mounted battery. These strips were eventually attached to the bottom of the enclosure. | 
The floor and rear panels of the box were cut out of 31/44" MDF and glued together with superglue. Always use a square to ensure that they are joined at the right angle. |
The sides were cut and glued, and this framework was test fitted inside the car. Notice how the car is protected from the wood. Always make sure that you are level in each step. | 
Next. I needed a perfect template to cut the opening. One of the subs was traced onto MDF and flush routed to match the outside sub circumference. The piece, or jig, is taped up and an outer ring is roughly cut about 31/48" larger than the jig. Use body filler to match up the outer ring to the jig to yield a perfect template.. | 
The Rage Gold body filler is mixed and applied evenly and neatly within the circle gap. Then, I sanded the filler flush to the wood and popped the center jig out, which left me with a perfect circle. |
The template was used to space the subs on the top of the enclosure and the openings were traced onto the MDF in preparation for rough cutting. This piece was then test fitted and appraised inside the car. At this stage it is important to see the end look of your design and anticipate all factors regarding placement of the subs. | 
The rear of the enclosure was examined in anticipation of routing the subwoofer 12-gauge wiring to the amplifiers. The top amplifiers are the subwoofer amps, which keeps the wire runs as short as possible. | 
The openings were roughly cut with a jigsaw within 11/416" of the traced line. The template was double-stuck in place and the openings were flush-trimmed with a laminate flush bit to the edge of the template. This had CNC-type accuracy for my openings. |