RX-8 Subwoofer Box Design - Half Rear Deck - 12 Inch Speaker - 1.2 cf
 

So, I have two 12 inch subwoofers that I wanted to put in my RX-8. The boxes I have won't fit and I and couldn't find commercial boxes with the proper fit and internal volume that these speakers require. So, I designed my own. My plan is to build one this weekend and test it out.

As background, these speakers are the long-discontinued 12" Rockford Fostgate Punch Z (RFZ3412) that I have migrated to 4 different cars over the years and they just keep going. 25-200 Hz @ 150 watts RMS (400 peak). Honestly, I love these subs.

Well, after I ran all of my wiring for the amp, I attempted to put my bandpass box in the trunk and quickly realized that there is no way in hell that the box is going to fit through the trunk opening. So, I pulled out an old sealed truck box that I had laying around and put that in there for now. Well, the box isn't the correct volume for these speakers, so I've been looking at what it would take to design a box that would work with these subs in this car.

Each of these subs has an optimal sealed box volume of 1.25 cf (cubic feet). I started looking around online and most of the boxes for the RX-8 have .75 cf of internal volume per sub. You could technically use a dual box and just have one hole cut into it, but what a waste of space. So I decided to get out my tape measure to find out if it was even possible to fit a 2.5 cf box under the rear deck area.

After some time, I figured out that the deck area is a roughly trapezoidal prism shape about 13" tall, 16" deep on the bottom, 10" deep on the top, 30" wide in the rear, and 35" wide in the front. These measurements give us a little over 3 cf of volume. So, factoring in 1/2" MDF, we get a bit over 2.6 cf of useable intenal volume. It can be done!

I set to work on a simple design to fit the space and came up with with a single chamber box that would fit in the space and provide the internal volume these speakers need. Unfortunately, when doing a test fit with a cardboard box, I realized that I was back to sqaure one. An object with a 35" x 12" face will not fit through the trunk opening. No way. Back to the drawing board.

After discussing options with a friend, I decided to make them two separate boxes that would fit side-by-side. I ended up with a box that is 12" x 17.5" at its widest point which passed the test fit just fine. Because of the extra walls between the two boxes, I lose a little bit of internal volume and each box comes in at about 1.2 cf rather than the target 1.25 cf. I think I'm willing to live with that :) Also, the added benefit of two boxes is that I can run just one sub if I need some extra cargo space or want to reduce the weight.

So, this is the design that I came up with:

Attachment 203897

It almost perfectly fills one half of the rear deck. If you haven't noticed, the speaker actually fires down in these boxes. I chose this orientation because of the ~13" height limitation of that space. With a 12" sub, you should really have more like 14" of space to mount it in. Well, the bottom piece of the box is perfect. In my design, the speaker deck sits 1/2" off of the floor of the trunk and the speaker is flush-mounted in that deck. That combined with the concave design of the speaker, the cone has about 1.5" of travel available to it.

I'll let you all know how it turns out.

Seems feasable to me.

1/2" clearance between the deck face and floor? Hell no. You should allow a minimum of 3", if not 4", to allow for proper wave propagation.

Have you considered corner mount boxes with the proper volume. Or you could use the rear section behind the back seats and vent it through the pass through. A friend of mine wants to do that to mine but I am going for a performance car not a loud (audio wise) car.