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      01-10-2012, 04:48 AM   #1
IrishCoffee's Avatar

Drives: '11 E92 M3, '14 328i, '14 1LE
Join Date: May 2011
Location: San Diego

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DIY: Fiberglass Subwoofer Enclosure

Hello Everyone

I just built a fiberglass enclosure for my E92 M3 and thought I'd do a DIY for those interested.

I have the premium audio and think it sounds great. I just wanted a little more low-end to round it out. I decided to add a JL Audio 10W6-V2 in a small sealed enclosure in the left corner of the trunk. Trunk space is important to me so it needed to be out of the way and not cause any problems with accessing any areas of the car. Also, I wanted to make sure it was easily removable for days at the track or if I really needed the extra room on a trip.

I worked in a car audio store as a teen and have done a couple of these before. They aren't necessary difficult to build, but there is a lot of work involved, and a lot of chemicals. You need to have an area available to you that is fairly well ventilated and allows for a mess to be around for a few hours at a time. Don't do this in your house or on your balcony... your wife/girlfriend will kill you due to the mess and the fumes.

Overall this project took me about 20-25 hours of labor. There is a lot of time waiting between coats of fiberglass and you need to make sure you do things properly otherwise it will be a mess. You need patience for this.

A few words about fiberglass work if you've never done it before. Avoid contact with fiberglass resin and cloth/mat. It has a tendency to get all over you hands and arms and is a pain to clean off. I wear a sweatshirt and latex gloves when working with the stuff. Also, I recommend a dust mask at a minimum for fumes and while sanding. Acetone will clean up any resin that gets on your hands, but rubbing acetone all over your hands several times a day causes some pretty serious irritation to your skin.

Please use safety glasses when cutting and sanding. I've had shards of wood and fiberglass hit my glasses before. I can't imagine losing an eye over something so stupid. Just do it.

Hardware Store Supplies ~ $125 total
  • 1 small piece of MDF for the baffle. No need to get an entire sheet. Don't use crappy particle board. Get the MDF. I think the small sheets at home depot are 2'x4'
  • 1 small piece of 3/8" or 1/2" MDF for the baffle cover.
  • 1 gallon fiberglass resin
  • 1 small can of Bondo
  • 1 pack of fiberglass cloth
  • 2 packs of fiberglass mat
  • 1 medium container of Acetone
  • 5-10 disposable paintbrushes
  • 1 bag of ~50 disposable gloves
  • 1 pack of disposable masks
  • 1 pack of sandpaper 80 grit
  • 1 large roll 3.5 mil plastic
  • 1 large roll of masking tape
  • 1 putty knife
  • 4 medium disposable mixing cups/containers
  • 4 paint stirrers
  • 1 roll of tin foil
  • 1 bottle of wood Glue
  • 1 pack of basic 1.5 wood screws
Upholstery Store ~ $35 total
  • 2 yards of backless trunk liner to closely match the BMW fabric. I used Keyston Brothers trunkliner TL04
  • 1 can of spray adhesive

Tools needed:
  • Jigsaw
  • Power Sander of some sort
  • Safety Glasses
  • Razor
  • Scissors
  • Tape Measure

OK, let's do this. First I pulled the left trunk trim panel out. There are a couple of black clips that need to be popped out and one torx screw near the rear seats. After that the whole thing can be worked out with a little force.

First I make some templates out of scrap cardboard to fit the trunk liner bottom and side profile. Then I trace them on the MDF and cut them out with the jig saw. Here they are during a test fit.

After the baffle is done I use the 3.5 mil plastic and cover the entire piece using masking tape to hold it down. This is difficult and may require using several layers and a lot of tape. This will be frustrating trust me.

Once completely covered I add aluminum foil and more tape as a easy release layer. It also adds extra protection to the trunk liner. If you get resin on the trunk liner it's pretty much done for. I also slipped the bottom piece in for a test fit.

Now, it's time to get messy. First I cut the fiberglass cloth into strips about 4 inches wide by 10 inches long. Then I use spray adhesive to adhere them to the inside of the tin foil. This allows them to stay in place better while adding the resin. Then I mix about 12 ounces of resin in one of the disposable cups and stir it up with the catalyst. Use one of the disposable paint brushes to lay it onto the fiberglass cloth evenly.

Repeat this process several times allowing it to dry. I use the fiberglass cloth first because it's thin and easy to manipulate. After the first layer I use the mat because it's thicker and builds the enclosure up quicker. Once you've applied several layers and it has dried you can take it out of the trunk liner. Remove the plastic from the trunk liner and hope that no resin leaked onto it. If it did then I guess you'll be going to Tischer to order another because there is no way to clean it off. Here it is popped out of the mold from the front and rear.

Then I use the jigsaw to cut the excess off as square as I possibly can. This takes a few passes and some eyeballing. I then sand the excess resin off the bottom piece in order to prepare to attach the baffle.

Using wood screws and a countersinking drill bit I attach the baffle to the MDF base. I then use more resin and fiberglass cloth to seal the gaps around the edges.

Test fit... looking pretty reasonable

Next I add an extra baffle ring to add some additional mounting depth for the speaker. I also attach the second baffle (made of the 3/8" MDF) to original the 3/4" piece. This top baffle protrudes about a half an inch around the edges of the enclosure and really just fills the gaps around the edges. It is purely aesthetic.

I then add Bondo to areas that need a little filling in and areas that I want to smooth out. Bondo is great stuff and is very moldable. You'll be tempted to use your hands to mold it. Use gloves. This stuff is really hard to get off your hands.

I then use my palm sander to sand all the rough areas smooth. I then hand sand it until I am happy with the result. It doesn't have to be perfect; the fabric will cover up small imperfections.

Test fit

Next I use the spray adhesive and a razor blade to apply the finishing fabric. I also add a ring terminal for easily disconnecting and reconnecting the speaker. A bag of poly fiber fill is added to the enclosure for acoustic purposes.

I then screw in the sub and the grill

And it fits! The fabric is about a 90% match which is excellent.

I added a JL Audio XD 600/1 to power the sub. I am in the process of finalizing the install for that and the Audio Control LC2i and will post pics when finished. I hooked it all up and it sounds great. It looks stock and is easily removable so I am happy.

The weight and the snug fit alone keep it in place really well. However, I am looking into adding a small bracket of some sort to make sure it stays in place in case of a hard left turn or something worse. I'll update the thread when I finish that too.

Many thanks to everyone who has posted other audio installs for ideas and info. Particularly, thanks to Technic for his hundreds of useful posts regarding wires, pin configurations, etc. These are really what forums are great for.

Also, I did something very similar in my 2008 Infiniti G37 there are more pics and instructions for that project here: