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WRX Wagon “stealth” Subwoofer Box

June 22nd, 2006 ydant

What follows is a general walkthrough/HOWTO for constructing a fiberglass subwoofer enclosure box that will nest within the spare tire well of a car. The car in question being a Subaru WRX Sport Wagon, although the principles will carry over to other cars equally well.

Goals:

  • Keep the spare tire
  • Allow the spare to be accessed with little difficulty
  • Sealed enclosure big enough for one 10″ subwoofer
  • “stealth” install - shouldn’t be obvious from an initial glance

Step 1 - Create the Base

To support my fiberglassing efforts, I need to use the spare as a mold so that everything
fits perfectly. Since I want the box to be flat across the top of the sub I use cardboard
to make a platform. The entire tire is then taped with duct tape to ensure no resin leaks
through.

The frame was created out of 1″x2″ board. The 2″ was too tall, so the boards were shaved
down partly. The goal was to have the top of these boards run level with the lip in the
styrofoam.

Step 2 - Protect the Car

I doubt I need to stress this, but protect your car. You don’t need much, but
cover the areas where the resin might go with plastic or SOMETHING. It will go everywhere.
Oh year, wear old clothes and shoes. Long sleeves help. Latex gloves are a necessity - you
will go through at least 20 doing just one box… Buy a cheap 3m mask - about $30.

Step 2b - Looseners

You want the glass to come out of the tire well, so line the tire well with aluminium foil.
This will come out super easy. But you don’t want the foil to stick to the glass, so tape it
down with painer’s masking tape (the blue stuff). You don’t want the glass to stick to the tape
so spray it down real well with silicon spray (or some other lubricant). Believe me, this will
give you decent results. Just aluminum foil will never come all off, even if you lube it.

Step 3 - Glass the Box

Just pictures of the glassed box. Fiberglass is easy to work with - tons of information
on the net. Just search around. I did one layer up the sides - don’t skimp on the resin, as
the fiberglass is held to the sides by ONLY the resin. Two layers everywhere else so it will
hold its shape after you pull it out. The goal is to do as little glass work in the car as
possible.

Step 4 - Remove the Glass and the Tire

If your work is stiff enough, take it all out of the car and let the car air out. Whew.
What a nasty smell!

Step 5 - Clean your glass

Now’s the time to expose the fiberglass for what it truely is. IF NOT STIFF ENOUGH, ADD MORE LAYERS.
Peel all of that foil and tape off! I used the 409 to cut through the silicon spray so it wouldn’t feel greasy.

Whoops - looks like I didn’t cover the entire bottom. See the holes? Fixable, though - just put a piece
of tape on one side and glass the other.

Step 6 - Finished Product and Strength

I followed up the original two layers of cloth with one more cloth layer and one layer of mat. I also used some
remaining resin to coat the bottom part. I filled some of the cracks between the wood and glass with resin to help
strengthen the bond. As you can see in the picture, it is very strong. Remember, the only thing holding the
glass and wood together is the resin.

Step 7 - Measure the internal volume

Subwoofers work best if they have the right size box. So we need to measure to see how much we actually
have. Water works best - the fiberglass is waterproof. If you get leaks, you need to seal them, or
your sealed box isn’t.

Other important measurements I didn’t do:

  • Depth - for top mounting
  • Clearance on top - don’t want the sub to hit the cover when it’s closed - may have to recess the speaker by building a lip inside the box lid…

I came up with two measurements:

Step 8 - Testing the fit

Now that it’s all together, test it to make sure it fits in the car. It does! Woo!

Step 9 - Paint

I got bored, so I painted the bottom with some spray I had lying around. I figured the yellow would help
make up for the performance hit of the extra weight…

Step 10 - Redo the top

I eventually managed to purchase a driver for my subwoofer, and realized a fatal flaw in my design. The sub is taller than I ever imagined it would be. I had to work out some way to solve this problem without having to redo the entire box.

Since the original design used a top of 1/2″ MDF, I realized I could gain 1/2″ of clearance by getting rid of the top. So I decided to replace the THICK MDF with THIN glass.

The first step is to make a support for the driver. I did that by cutting a ring out of MDF and supporting it using dowels. I tried to make the top of the support level with the top of the frame of the box.

As you can see, the dowels are enough to support the driver. With the extra support of the glass, there will be no support problems. I drilled 4 holes in the bottom of the box and screwed the dowels into the bottom to keep the whole thing from moving around.

Glassing the top is simple - the only unusual part is making something to lay the glass on. Take some old t-shirts and stretch them across the top - tight. Staple, tape, or glue the shirt around the edges. Soak the shirt in resin and let it dry. When it dries it will be hard and ready for layers of glass.

Don’t bother glassing over the hole. Cut the hole and the edges between layers to keep buildup from getting too bad.

The End?

I finally got my driver and amp. Driver is the Infinity Perfect 10.1d. Amp is the Rockford Fosgate 3001 running @ 300W RMS @ 2Ohms over the two voice coils.

Sound is exactly what I was looking for. Loud enough to feel inside the car, and barely noticable outside of the car. I usually run at about 3/4-1/2 power. I just wanted some bottom end in the system, and now I have it.

I still need to finish up the box - clean it up a little bit and then cover it. System is installed in the semi-finished state.

Please feel free to send me comments. subwoofer@ydant.com.

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