Shifting the box size to the larger end of the range causes the Q to fall. A Q of 0.7 is considered "optimum" where the frequency response is the flattest and the overall performance of the box and speaker combination is most balanced. As you make the box larger, the Q becomes smaller, entering a range where the combination is considered "critically damped" as the Q hits 0.5. This is rarely a good place to be in car audio, since by the time you hit a Q of 0.5, your power handling is getting low. Sure it might sound really nice, but there's not a lot of pressure in the box to act as a shock absorber for the speaker.
Here's the deciding factor: If you were to build one cabinet at 0.7 cubic foot, and another at 1.0 cubic foot and played them while switching between them, you'd be unlikely to hear any difference. Test gear would measure a clear difference, but your ears are the target judges and they won't be able to tell the difference at all. So it becomes a matter of "what can I fit in?" If you're limited in available space, then the smaller end of the range will be fine. If you have the room, I'd tend to go for the top end of the range, since I can always de-volume the box if it was too large. You should also note that cabinets tend to "shrink." By the time you install bracing and the speaker, you have used up some of the internal volume of the cabinet, so having a bit of wiggle room is a good thing.
My instructor that teaches our "Hot Metal" and "Inked" evening classes is part of Alpine's show car fabrication team, so I had a chance to ask him about this. He confirmed my thoughts that an internal volume of 0.85 cubic foot is designated for a Q of 0.7. He also revealed that you could build a vented cabinet with a 1.7 cubic foot internal volume. This volume accommodates the space taken up by a 1-foot-by-1.25-inch rectangular port 21 inches long. If you had the room, you could start with a vented cabinet, knowing that if you didn't like it, de-voluming the cabinet to less than a cubic foot is pretty easy. If the ported option is to your liking, you'd be experiencing a 2dB boost at around 34Hz, resulting in a bit of extra "free bass" caused by the natural resonance of the design.
The cabinet material choice between MDF and plywood is a balance of differing characteristics. MDF is denser, which makes it the common standard for sound quality. In other words, MDF is stronger than plywood in terms of density. Plywood is stronger than MDF in terms of resiliency. Due to the layers of grained wood veneers in plywood, it's much more likely to survive bending loads, but the dense nature of MDF makes it better able to survive vibration loads. Since we are mostly concerned with the sound, I believe MDF is a better choice, since it's strong enough at the thicknesses we normally use. Sam, I do think that a sandwich of 3/4-inch plywood and 1/2-inch MDF would be a waste of time and money unless you were using thousands of watts of power, and had a damping membrane between the two laminations.
Sorry we can't turn out magazines any faster, but if you need an extra fix of troubleshooting, check out my podcast "Cause Stock Sucks"on iTunes.
Q I'm currently in the process of building from scratch a stereo system in my little '93 Ford Ranger single cab, short box. Should I replace the factory 6x8s with aftermarket 6x8s or step up to 6x9s? With it being only a single cab, I'm not sure what I can get away with for a sub (size and how many) without going too bassy. I want it to thump but not drown out the mids and highs. Keep up the great job you guys do.Thanks,"Fat Joe" Benjamin
AJoe, I am disappointed in you. Have I not taught you anything? Keep the factory 6x8s? Repeat after me: 'Cause stock sucks! Now let's start looking for aftermarket replacement speakers before I come after you.