Wednesday, August 28, 2013

My Fiberglass Car Subwoofer Install (aka The Bass Boulder)

If you have ever completed a summer to-do list by the end of summer, please tell me and I will reward you with cake or some equivalent delicacy. Well, I can say I finished my biggest project and boy was it worth it. For those of you who have never considered fiberglass, yes it is as versatile as duct-tape, yes it is messier, and yes it can look smoother than my bumpy speaker box. Read more for some comments and how to get started on your own build!

Difficulty: Hard (hard-er the less careful you are)
Cost: $100-$200

Materials: Approx. 2 Gallons fiberglass resin, fiberglass mat, fiberglass cloth, big fleece sweater, fiberglass hardener, plywood, dowels, truck-bed liner spray-paint, painters tape, acetone

Tools/Aids Required: Paint roller, paint brush, rubber gloves, HEPA dust mask, goggles (optional), mixing pail, grinder, sand paper, drill, scissors, glue gun

So, why use fiberglass for a speaker build? Unlike wood, it can be molded and thus, can fit into odd corners of your vehicle to save space while not compromising sound. My 99' Honda Civic has useless space in the trunk wheel well, so I formed the odd spaces to a nearly 2 cubic foot box. Nice! It lets me have sound and my wife gets the trunk space she was afraid of losing with having a subwoofer!

This was my first adventure with fiberglass and it was extremely daunting at first, but only because the guides I read were so extensive with warnings and details. It turns out that it is not that bad. However, it is very messy and smelly. Resin does not come off of things easily and please wear cloths and tools you don't mind being used for anything else EVER AGAIN. If you are unsure of some materials or tools, read on for explanation.

First thing is first, tape the area you want to turn into a box. Over-tape it so that you can over-fiberglass the area and cut it back later. Here, you can see my taped area with the first layer of fiberglass.

To make things easy, cut your fiberglass mat into 2" x 12" strips and lay them down one at a time. Mix about 2 to 4 ounces of resin, roll a coat on, place your mat, then roll over top to make the resin penetrate the mat completely. We want no dry or bubbling spots. Force those areas with your brush or roller. I find a brush works for delicate jobs.

If you read the labeling, you will find that resin cures fast. So mix small amounts and be fairly swift. Being prepared beforehand will reduce stress and help you do a quality job. Do not waste resin, but be liberal without leaving pools of resin. If left in the Sun, you can add a layer every 20 minutes, otherwise wait a few hours or a even a day when it is colder. Warmer is better.

 Once you have two layers you can pop out the box and work on it in comfort. Use a well-ventilated area (such as a parking lot in my case) and build your layers from inside the box. I would make four layers total before doing the fleece.

When doing the fleece, be very liberal and roll it on HARD. You need the resin to penetrate to the bottom of the fibers to adhere to the rest of the box. This is always critical and probably the scariest part of the project. If unsure, look underneath the fleece carefully to see how deeply the resin penetrates.
After you add four more layers you should be thick enough. I used cloth exclusively, except at the beginning and found it easy to work with, contrary to many opinions found online. It is thick, which add weight and lends to the acoustic properties of the box. A rattling speaker box makes a speaker very inefficient.

Okay so now you want to trim off the excess edges to fit perfectly in your vehicle. Use a grinder, HEPA mask, and goggles. Glass dust can be dangerous and is itchy to the skin. Don't rub it in, just wash off when finished.

The next step is the speaker baffle, which can be seen in the opposite pictures. We will mount this ring with dowels inside the box.
Here is a tip: recess the speaker as in the picture of the side profile of the baffle with speaker inside. It keeps a lower and stealthier profile while keeping the speaker safe from damage.

Using the dowels, position the baffle where you want the speaker in the box. You will stretch fleece over this and the box to create the front of the box.
Another tip: drill four holes in the baffle to fit the dowels tightly. Slide the dowels to change the position of the baffle, then screw the dowels in place to hold a permanent and structural place. Some tutorials tell you to hot-glue the dowels, which lends next to no support to a free floating speaker being held only by the resin. Imagine what a speaker does to a box if it can turn your car into a slinky.
 BUT, hot-glue the fleece to the box and make sure it stretches well with no sags or creases. It may sag with resin in it, but that is okay. Once the first layer is dry, build another 6 layers or so. Don't fiberglass over the hole the speaker goes, but please glass the whole baffle (so some over-glassing will happen). When you are finished, grind and trim away and sharp corners or fiberglass that went over the edges.
I found that the first layer didn't adhere perfectly in every area to the rest of the box. Also, there seemed to be some gaps in the layers at the edge. Fill these with resin and fiberglass 2 or 3 layers of mat over weak areas. Weak spots will crack and show up over time when you use a subwoofer.

With that finished, stuff your box with insulation and keep it fluffy. You want it to cover most every surface inside the box. Though you want volume in the box, insulation tightens the sound and simulates larger volume inside the box. Both are a good thing, though stuffing the box seems counter-intuitive.

 Before painting, drill a hole, run your wires, and test the sub at your highest volume before distortion. Feel the box and put a few layers of just resin over spots that vibrate. A very small amount is fine, but any vibration will hurt speaker sound and efficiency.

I painted mine with truck-bed spray for durability. It is awesome stuff that can just be touched up when scratched, unlike vinyl, carpet, or paint. It has a pebbly finish which looks very nice.
You can grind your box smooth, but I didn't want to remove any contributing portion of the overall structure for the sake of looks. Plus my brother remarked on its boulder-like appearance. Hence, the name "Bass Boulder." I thought that had geek appeal. Anyways, contact me for comments or questions, or look to the tips below if you think I missed something. Overall, this was a very satisfying build that I would do again in the far future.

Just added this one to show it with my 3/16" steel grill.


-For a second opinion, consult this good guide that I used.
-Keep acetone handy to clean brushes and rollers. They will harden and be useless, be prepared to buy a few brushes and roller replacements. I'd go with cheap brands.
-Clean up dripped resin immediately to avoid a permanent or painful-to-remove blotch.
-Adding too much hardener speeds up the curing process and can even melt your container. The reaction makes the resin very warm and can get so hot it burns and melts plastic, if you add too much hardener.
-Take your time and be careful. That sounds a bit contradictory, but the 10 minutes or so that you get to use up mixed resin is actually a lot of time. Mixing small portions lessens the amount you need to use.
-Bubbling may occur and will not compromise structural integrity. If you are worried, poke a hole in a bubble or grind it away and fill it with resin. As you can see, I let bubbling happen. This was because I didn't always use small strips of fiberglass. It saved time at the expense of quality.
-Once you set the dowels, insert the speaker to make sure it fits. I left room for a full sized 6" deep speaker, thought you may need a shallow-mount speaker.
-Putting a grill on the speaker can add to stealth, aesthetics, and protection.
-Don't know how to measure ounces? Measure water into a measuring cup with ounces and pour into a pail. Mark the pail with a marker and remember how many ounces that is. I used a 30ml (1oz) Pepto-Bismol cup to measure my ounces. You can reuse the pail by squeezing it to crack out dried resin. Old dairy containers are wonderful for this.

I hope you enjoyed this half attempt at a tutorial, but mainly the adventure that was my experience with this box. Cheers to my Dad and brother for their input and to my wife for putting up with the mess, I cannot wait to have a real shop some day.


  1. Do you remove the tape after you pull it out of the truck or did you apply resin to the other side to

    1. Yes and no. The tape and/or aluminum foil is the surface you brush resin on and lay your fiberglass correct? So most of it will stay adhered to the first layer or resin. When you pull out the box the tape should unstick from where you made the mold, but stay stuck on the backside of the box. This is why you use painter's tape, which does not leave residue. For more ease, put down tape, then aluminum foil, then another layer of aluminum foil, then tape again. This makes it even easier for the box to come out since there is no real adhesion due to the two aluminum foil layers side by side.

      Hope this helps! Good luck on your project, if you are doing one!