2 12s and still use the passthru box design

Written by syc0path, Member 16645
Applies to the following vehicles: '97-'03 Grand Prix (all models)
Created 10/2/2003, Updated 1/20/2005

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IMPORTANT NOTE: becuz of an unfortunate incident wherein another member built multiple boxes using my design and sold them for a profit, I am copyrighting this design. U may build 1 box using my design, for your own personal use. Any other use of this design is a violation of copyright law.

A lot of GP’ers, myself included, were looking for a subwoofer box design that would fit 2 12s in the trunk without making the trunk unusable or blocking the pass-thru. So I designed one. I built my box for 12s; however, it was a very tight fit. It’s a somewhat complicated box to build and there isn’t much room for error with 12” subs. 10s would fit more easily. Make sure u know what you’re doing before u start, especially if you’re using 12” subs! The box yields about .90 cubic feet per side, but fiberfill can be added to increase the effective airspace if your subs require it. You’ll need the typical materials necessary for a subwoofer box:

¾ sheet of ¾” particle board or MDF, etc.
Wood glue or Liquid Nails
Nails or drywall screws
Silicone caulk
Subwoofers (duh!)
Recommended: speaker wire terminals, carpet

Circular saw
Tape measure
Flat edge
Drill w/ bits, hole cutter (if using terminals), and Phillips bit

Before u start building, keep in mind that these measurements aren't exact. Becuz of all the angles and curves in the trunk (not to mention production differences between individual cars) it's virtually impossible to give exact measurements. I recommend several trial fittings thru-out the building process so u can tweak as necessary. The measurements provided are intended to be a bit on the big side, becuz u can always take more off.

Refer to the blueprints to see the boards that need to be cut out of your sheet of particle board (note that once the box was done, I changed the design slightly – make the top board identical to the bottom board). I made this image very large to insure that it would be legible when it's printed.

If u have the right equipment to do it, the box will fit together a lot better if u cut the side and “tunnel” boards with about a 30* angle on the proper edges. Be especially careful when cutting out the openings for the subwoofers – the cutout size for a 12” woofer is about 11-11.25” and the front board is only 12.5”x12.75” so there isn’t much room for error. This pic shows some of the boards after they’ve been cut.

I then used a large elastic band to hold the boards in place for a trial fit.

Now would also be a good time to put the subwoofer where it will be mounted, to insure there is sufficient clearance for the magnet width and depth against the sidewall (my subs have 30lb magnets and they fit with about a ½” to spare). Once this is confirmed, line up the boards where u want their final position to be, and then use a pencil to trace them.

Then you’ll know where to apply the wood glue. Glue a side board to the bottom of the box and then the second – don’t forget to put glue on the front of the side boards and on the bottom board before u put the front board on.

It’s best to leave the back board off for now, becuz it will be easier to caulk inside the box later. Now put glue on the top of each board and glue on the top board. Once the top board is lined up, carefully measure where the screws should go. This pic shows the lines on the top of the box.

For most lines, I started the first screw about ½” from the edge and then put additional screws every 2” at 2.5”, 4.5”, etc. Take your own specific measurements before u start drilling! Note the missing screw in the bottom of the box.

This is necessary because otherwise the screw would stick up into the opening for the sub. Also note the missing screws on the face plate – this is necessary because the mounting screws for the sub will screw directly into the side board at those points.

Now that most of boards are glued and screwed together (except the back boards!), it’s time drill a cut-out hole for the speaker wire terminals. Some people like to cut a small hole, run the speaker wire directly to the sub, and then caulk around the speaker wire where it enters the box. However, I find that to look sloppy and to be inconvenient when moving the box in and out of the car. So I put in speaker terminals and caulk around the terminals from the inside of the box to insure an airtight seal. I like to use clear silicone as the caulk. Caulk all the seams between the boards. When u finish caulking and you’re sure u didn’t miss any spots (especially in the corners), you’re ready to put the back board on. It should be a tight fit – I had to use a rubber mallet to tap it into place. Glue and screw it just like with the other boards (of course u won’t have to skip any screws like in the front), then reach thru the opening for the sub and caulk the seams for the back board. Once everything is caulked, it’s a good idea to let it cure overnight. There’s some disagreement, but some people claim that the fumes released by the caulk as it cures can eat away at the surrounds of the subs. In any event, we’re building an airtight box, so the caulk may not cure correctly if the fumes can’t escape.

Once you’re ready to put the sub in place, tilt the box as shown.

This will make it easier to center the sub in the hole. Again, it’s extremely important to be accurate becuz of the very tight dimensions we’re working with. Mark the mounting holes for the sub and then DRILL THEM! Don’t just run the wood screws in with the drill, becuz chances are they won’t go where u want them unless u drill a pilot hole first. Here's the completed product:

This pic was taken thru the pass-thru to show how snugly the box fits under the rear shelf when pushed into position all the way forward.

Originally the “lip” on the top board was supposed to seal off the front of the box from the rear of the trunk, and another board was to be placed in the “tunnel” to complete the seal. This tunnel board would be removable so that the pass-thru could be used if necessary. However, the box actually sounded better (louder, tighter, and less upper-bass harmonics) when I turned it around and pointed the subs at the rear of the trunk.

The lip is no longer necessary and will greatly complicate the carpeting process, so I cut it off before I carpeted the box.

Once I confirmed the box was solid, airtight, and worked properly, it was time to carpet it. I used a little less than 3 yards of carpet that was about 54” wide. 48” probably would have worked though. It was pretty difficult to carpet becuz of the angles on the front and back of the box, and I had to cut seams to make the carpet fit. These pix show cutting out a flap that will wrap inside the tunnel, along w/ 2 flaps for the back of the box.

This shows how I laid out the carpet to be tight on the back. Note the little flap that I cut – that piece will be cut off later so the seams meet properly.

This pic shows the flap I cut off, along w/ another corner than I hadn’t cut yet. U can see how I folded the carpet over to deal w/ the angle.

I overlapped the seams a little bit.

Then I took my razor and tried to cut them so that the bottom wrapped around to the front a little bit, but not so much that the carpet overlapped (which would create a bump). Take a look at the sides:

I measured the center point on the side and drew 2 diagonal lines thru it. Then I used my razor to cut the carpet along those lines (I hadn’t taken out the terminals yet, but I did later). Then I glued the endflaps down.

Then I cut out the holes for the subs -- making sure to cut the carpet in a circle a little wider than the actual hole so that subs could have solid metal-to-wood contact to insure an airtight seal. I did a similar cut for the terminals, and then caulked the terminals w/ silicone before I put the subs back in. Here's the box back in the car…

The carpet doesn’t match quite as well as I’d hoped, but it’s pretty close. Here's a couple more pix w/ the neon at dusk and at night.

I think it turned out pretty well. I wasn’t happy about having so many seams in the carpet, but they’re really not as noticeable as I thought they would be. I had a hard time getting it in the car after I carpeted it becuz it was so tight between the strut towers. The first time I put it in, a wire got pinched and I couldn’t move the box forward any more until I pulled it back and moved the wire. I couldn’t push the box out! I got my brother and dad to help me and we still couldn’t move it. I finally had to jack the car up to take the stress off the strut towers! I cut a little bit of seam caulk off the strut tower and put the car back down. The box went in a little ezr, but it’s still very snug. I’m really happy with the results – it sounds great, looks pretty good, doesn’t slide around, maximizes available trunk space and still allows for use of the pass-thru. The car even handles better becuz the weight is pushed forward.

Copyright 2005.

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