Build a Fiberglass Subwoofer, Start to Finish




    I'm a music enthusiast and have been interested in car audio for a while. I've seen many youtube videos on how to make a custom fiberglass body for a subwoofer. However, I needed to find many different videos to answer all of my questions. Hopefully this how-to will put them all in one place for you. I had acquired two 15 inch PYLE subs and needed to do something with them, so I started researching. I found a cool design from this site :

and went from there.

Here are the specs at a glace:
- Dual 15 inch Subwoofer Box
- Two PYLE 15 inch speakers
- 1000 watt peak each
- Dual voice coils/ 100 oz magnet
- Custom Fiberglass Front
- Dual port holes tuning box to ~34 herts
- Voice coils in parallel then speakers in series ( aka 4ohm)

- Height 21 inch (top speaker ring)
- Width 20 inch
- Length 37.5 inch

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Step 1: Materials List and Project Outline

There's a few stages involved in this particular project:
1) Research and have planned exactly what you want to build. This involves selecting size of speakers, how many speakers, color, port hole location, box tuning frequency.
     - No materials, just time and graph paper

2) Make the wooden body
     - 0.75" Medium Density Fiber Board (MDF Board) 
     - Wood glue
     - Clamps
     - Table saw (gives you straighter cuts than hand held)
     - Drill and different sized bits
     - Screws
     - Caulking (I used Liquid Nails)
     - Jig Saw (Most people use a router with a bridge that lets you cut perfect circles, I found my own way)
     - Port Holes

3) Add the fiberglass
     - Fabric Cloth (like Tshirt material)
     - Staple Gun
     - Fiberglass Resin
     - Gloves
     - Paint Brushes
     - Acetone
     - Plastic Containers for mixing
     - WD-40 optional
     - Fiberglass mats
     - Mask for your mouth (fiberglass resin has lots of fumes)
     - Good weather and open area to work in!

4) Add bondo and Sand to make a smooth face
     - Bondo and Hardener
     - Plastic Putty knives
     - Range of sandpaper from about 80 to 400 or more. (An electric sander here helps a ton!)
     - Rounded wood file (looks like a cheese grading tube)

5) The Cosmetics: Paint and add speaker carpeting
     - Spray Paints
     - Speaker Terminal
     - Speaker Carpeting
     - Spray adhesive
     - Exacto knife/ carpenter's knife

6) Wire the speakers and connect to the speaker jack
     - Speakers
     - Mounting Screws
     - Appropriately Gauged Wire

Step 2: Research and Plan

Here are some of the sites I used:

For the design concept which I went from

Port Hole Calculations

Speaker Enclosure Volume

Easiest-to-follow youtube video of a similar project

Step 3: Make the Wooden Box Frame

     All of my pieces of MDF board were cut from a 6'x8' piece. The pictures are straight forward, showing how each piece went together. I used 3 to 5 screws along each flat side of the boards to join them together. Make sure you make a pilot hole first or you will split the MDF board. Also, drill into to wood with a bit wider than your screw head to sink the screw in flush with the flat piece of wood. You don't want to see a bump when the speaker carpet goes on.
     The trickiest part of this step was getting the speaker rings in the position I had in mind. My advice would be to start with one and add another support to that one first and then tweaking the second one. This allows you to make the second one match the position of the first one without trying to get both of them symmetrical at the same time. I used boxes of screws and nails mostly to hold things in place while I used a nail gun to set the rings and supports.
     There's examples of all of this in the pictures.

Step 4: Adding the Fiberglass

     To set up for this step there needs to be something to apply the fiberglass too. For making sub woofer boxes with weird curvatures you need to use a cloth that can both be stretched and strong enough to hold the weight of the drying fiberglass. I found fabric cloth at a crafting store (at first I tried using an XXL Tshirt but it wasn't big enough). First staple the cloth to a ring mount and stretch it to one side and staple it along the edge. Then pull tighter in the mid section to give it cleavage. Then around the second ring, still keeping it tight. Then finish stapling around the last sides. Take a razor blade and cut the fabric away. I 1/4" staples because the MDF board is dense and 1/2" will not go all the way in. Lastly, I had to tap each staple with a hammer to make them sit flush. Because the staples were on the flat sides of the box I needed to make sure they wouldn't show as bumps when I put the speaker carpet on.
     My downfall was using a cloth that was too thin and I could only apply very small amounts of resin at a time. This also lead to lots of bumps and waves in the curvature when the fiberglass was set. I remedied that situation in the next step. I might have gotten around this by stretching is alot more when stapling it, but I would get a thicker material than just Tshirt material next time.
     Using fiberglass can be tough, and I would suggest looking up videos or reading about how to use fiberglass before you start mixing the compounds. At first i mixed too much hardener and the mixture reaction took place so fast that the soda bottle I was using melted in the presence of the reaction. Also, you usually apply this resin with paint brushes. If you want to reuse them each time by cleaning then with acetone keep the following in mind:
          1) clean them well before the resin even begins top harden
          2) don't use those cheap sponge type brushes I got. They might be $2 for a whole pack but the acetone disintegrates the glue holding that foam to that wooden handle. Your then left with a sponge and a stick.
     After 2 layers of fiberglass resin the surface should be strong enough to be able to work with. Now it was time to add the fiberglass mats with resin. When using the mats, rip 6" x 6" squares of the fabric to paste on with the resin. It's better to rip then to cut the mat because by ripping it with your hands the edges are full of strands of fiber glass. This makes for a stronger mold once things have hardened. 

Step 5: Bondo and Sanding

     This step takes more time than all of the other steps. The goal here is to create a surface which you can apply a high gloss paint to and make it look amazing. First work in the lowest areas, aka the big dips left from the fiberglass resin. Always keep in mind when you paint the final product, any imperfection will show.  I took a long flat hand sander to keep those speaker rings nice and consistently flat. If you took a piece of sandpaper and tried to make a perfectly flat donut-shape surface, it wouldn't work. This is a good time to cut the cloth out of the speaker and port holes. Any surface bubbles in the fiberglass resin should be cut and filled with bondo. You don't want to sacrifice the structural integrity of this box when you're going to load it with subs and pound on every joint of the thing. Nothing's worse than an annoying rattle when playing music.
      That being said, I suggest using crude sanding techniques first (I used a long thin cylindrical cheese grader like tool to take away high spots quickly). Then use sand paper around 50 grit. Move up in steps of grit until you get to at least 400. Using bondo and sanding it is as much of an art as with using fiberglass. I suggest researching tips and techniques before doing this if you aren't comfortable with it.

Step 6: Painting and Speaker Carpeting

     The next step is to apply the primer and get it ready for a gloss finish. This is where you use a really fine sand grit. Some people use a wet sanding technique (it saves your sand paper from building up too much primer and becoming unusable).  
     Use this step to add color to your speaker port holes (I sanded off the worn gold finish and put a flat black on it). I also blacked out the inside of the box. This just makes me happier to think the entire inside is one consistent color, and looks more professional. Also, I had to put the rest of the back wall on the box. I had taken it out to work with the fiber glass before. I cut a circular hole with the jig to fit the speaker terminal and spray painted the inside wall black.
     Once you are content with the primer, go ahead and apply the finish to the face of the box (I only had to paint the curved face because speaker carpeting would cover all of the flat sides). I'm not going to tell you how to spray paint here. I chose white gloss and used 3 layers to make sure the gray primer color wasn't able to show through any point of the face. 
     All set to add the speaker carpeting now! This video shows a good way to use speaker carpeting if you're not already familiar. I used the weight of the box to seal the glue and carpet. It's a good idea to use new razor blades, it makes for much cleaner cuts with the carpet. You do not want to use carpeting that is easily stretched and can fray easily. 

     The box is done, and the speakers need to mounted and wired. NOTE: This thing will reek of fiberglass resin, bondo, spraypaint, and glue for a long time. Let it outgas in the sun for weeks. I transported the finished product to my dorm room to use shortly after it was finished and it stunk up the car pretty bad. 

Step 7: Wire the Speakers

      There are two voice coils for each speaker. I put those coils in parallel and then put both speakers in series to get 4 Ohm overall impedance for an amplifier. NOTE: make sure the internal wiring isn't loose or it will rattle. Also, I was playing with this thing when it was on and it sucked a ping pong ball in through the port hole. 

Step 8: DONE!

     I see many comments on similar pages asking if the project is for sale. This particular cabinet I've made is for sale and all earning will go towards my tuition :). It has been used twice in it's lifetime to make sure it works. The first time was to check the speaker connection and the second time was to check the bass. The port holes sucked in a ping pong ball when the speaker cones hit a low note. Also, the lamp shades above the basement floor I tested it in shook back and forth.

      If you're wondering about the costs of materials, I kept track in the spreadsheet. I already had the larger tools readily available. To see the full size picture, click on the i on the picture and then click on "original size (1288x690) 77 KB". 

Feel free to ask any questions! I hope you liked the instructable!



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    21 Discussions


    3 years ago

    looks great, only improvement I would make is filling the cavity 1/4 full with some rockboard 80 to dampen any resonance going on in the box and improve the response coming out the bass holes.


    7 years ago on Introduction

    Fleece is what you want to use. Don't use cotton/polyester materials like t-shirt material, they don't absorb enough resin to solidify properly. It won't last long (my all fiberglass box is almost 10 years old).

    You can get Fleece from walmart for very cheap. old fleece pullover blankets, just make sure it's 100%. Fleece also soaks up WAY more resin than you think. at least twice what you would expect, then add another 50%.

    Fleece (and t-shirt material) isn't strong enough on its own to support the pressure in a box. A pair of 12's such as JL W7's can blow apart a properly made 3/4" MDF box that's been glued and screwed, Fleece will be obliterated. Pile on the fiberglass matte until it's roughly 1/2" thick or more. My box is 3/4" thick completed.

    I lightly sanded between each layer to knock down any peaks and you MUST get a fiberglass roller to get rid of air pockets. Air pockets will kill your box.

    Take extra care putting on the last layer of fiberglass. the more care you take here the less "bondo" and the less sanding you have to do later. I did a rough sanding and put a smoothing layer of bondo over the box. I then put a thin padding material and pulled cloth over it.

    Also, the mask for your mouth? don't be fooled. you CANNOT use one of those 99cent things that you see doctors use to keep germs out.

    you have to use a full respirator, something like

    1 reply

    Reply 5 years ago

    it's hard to sand the bondo smooth... i got tired at mine to.... , flece works the best


    6 years ago

    Very nice job


    6 years ago

    This box looks amazing! Hoping to get one built for my car once I save up some monies.


    7 years ago on Introduction

    Hi , the box looks incredible and I'm gonna build the same one right away.
    But there is one small problem. In our country theres nothing called bondo. So is there like an alternative putty I could use? What happens if I mix talc with resin and then add the catalyst as a hardner? Will that work?

    1 reply

    Reply 7 years ago on Introduction

    Hmm there must be another name for it. Bondo is typically used on automotive body repair. So find out what the name of that putty is. Hope that helps!


    7 years ago on Introduction

    Oh, and I use auto carpet from Wally World. It stretches a bit and looks good. Comes in rolls and a variety of sizes. And it is inexpensive.


    7 years ago on Introduction

    Great job!! Cardboard, like from big boxes, can be cut out and used to figure out sizes and shapes. Then the cardboard can be a template to cut out the MDF parts. I find having a hunk of stuff in the space I am putting the speaker box helps me with the vision of the final product.


    7 years ago on Step 4

    I've heard that fleece is a very good choice of material. It can stretch and will hold the fiberglass. I've used stretchy material before with so so results and haven't tried fleece yet.


    7 years ago on Introduction

    ooooooh!......iam speechless.....outstandind job

    Fantastic, Its about time someone did a good instructable on this. I used to make ones just like these in the past - pre/instructable time period, but I've had others ask how, now I can point to yours, excellent 5 stars!


    7 years ago on Introduction

    Pretty cool looking project.

    I guess I ought to clue you in on the #1 secret to working with fiberglass. Acetone takes it right off you. I don't go near fiberglass resin unless I have some acetone around to get it off me. Then I don't wear gloves, I just go with the fur factor. I don't think I'd want to contaminate glass I was working with with WD-40.

    One of my more disturbing nightmares is in fact trying to work with fiberglass resin without acetone. I just wouldn't do it.

    1 gallon LDPE jugs cut in half are great to mix in too. Milk jugs, water jugs, those sorts of bottles. You can even reuse them, once the resin hardens in them you can break it right out. Unless you really scratch the plastic up resin won't stick to it at all.

    Oh, and for brushes use bristle brushes. I use what I call chip brushes, they're the China bristle brushes? They come in packs of 3 dozen for the 2" size. I don't even bother trying to clean them out after a session I toss the things.

    4 replies

    Reply 7 years ago on Introduction

    Hey thanks for the advice! You also clued me in on a mistake in the writing. I meant to say use acetone, NOT paint thinner. And yes I eventually just used up all of the brushes and didn't care about reusing them.


    Reply 7 years ago on Introduction

    heh yeah I don't even open a can of fiberglass resin unless I have a liberal supply of acetone. That is one of my immutable rules in life. Sometimes acetone is considered paint thinner, but usually when someone says paint thinner I think of either lacquer thinner or mineral spirits. I don't think either of those is nearly as effective removing fiberglass resin as acetone is. When I have werewolf hands I don't reach for anything but acetone so I don't really know. Other than on me, I usually don't try to clean fiberglass resin up ever.


    Reply 7 years ago on Introduction

    I had actually confused the two when I went to buy materials and was pretty upset when the thinner wasn't working at all haha. I hope to do this project again but incorporate LEDs which will be synced to the music!


    Very well done. The one recommendation that I have is to add a sound dampening material to the inside of the box. This wil reduce interference from the sounds sent back into the box from the speaker. I have put fiberglass, dense fabrics, or even some soft ceiling tiles on the inside walls. That would increase the quality a bit.

    1 reply

    Hey that's a good thought! I would put insulation in only when the box stops smelling like chemicals. Otherwise the insulation might absorb the smell. Just a thought. It certainly would add a quality factor to the trained ear!