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.
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