How to Make a Fiberglass Subwoofer Box




Introduction: How to Make a Fiberglass Subwoofer Box

Fiberglass subwoofer enclosures present some real advantages for a custom car audio setup. First, they can be shaped to fit a specific vehicle or space within a vehicle, taking advantage of space that couldn't be used with a normal rectangular subwoofer enclosure. Second, if properly executed, they can present a real custom look that will make your car audio system truly "one of a kind."

This tutorial shows you how to work with fiberglass to make a sub box located in your trunk behind your wheel wells. Although this tutorial is made to go into your wheel well areas, you could use this tutorial for anything, kick panels, racks, and other speaker boxes. If you have some knowledge of working with glass, then this should be a walk in the park.

What you'll need:

-1 Gallon Polyester Fiberglass Resin
-Fiberglass Matte
-Additional Resin Hardener
-MDF (For supports, and speaker rings)
-Dremel / Rotozip / Cutter
-A GOOD Respirator
-Disposable Paintbrushes Brushes
-Masking Tape
-Tin Foil
-Glue Gun, or Adhesive Spray
-Fleece Fabric
-Wooden Dowels
-Carpeting Material
-Packing Peanuts
-Wiring Materials
-Light Sandpaper

Additional Information

This project will not be easy, it will require about 30 hours of work, drying, and adjustment time. Expect at least 8 hours of working on the first day to get your car in order, and get a good pull of your trunk space. In the end you should have a show winning sound set up that will look as good as it will sound.

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Step 1: 1. Remove Everything From Trunk

You are now going to need to remove as much as you can from the trunk, everything you may have in there that will get in the way should be removed, even the trunk lid if it gets in your way. The carpet should remain, and you will need to get it as smooth and as perfect to the contours of your car as possible. Use a Vacuum to suck up any debris and dirt that will be in your trunk, you don't want to be dirtying up your box.

Step 2: 2. Tape Layer

Using a good quality masking tape, the blue painters tape works best, start creating a layer of tape in the area that you will be glassing. I recommend that you do 2 layers, and crisscross them to ensure you don't get resin seeping through. Resin will not come off of metal, clothes, carpet, or your dog, so be careful. Extend the tape layer a few (3-4) inches past where your box will go, and also glass out a few inches past that (but within the tapes area).

Step 3: 3. Tin Foil Layer

I have seen some tutorials do this, and some not, it is really up to you. it is an added layer of protection from the resin seeping though to your carpet, and I highly suggest it. Use squares of the foil, taping and overlapping them as you go. You should now be ready to start the glassing.

Protecting the interior is very important as this stuff will not come off. I suggest placing newspaper, or drop plastic all around your work area. Also, if you like you can also put a sheet of plastic between your trunk and cabin to prevent fumes from getting in and stinking up the car for a few days.

Step 4: 4. 1st Resin and Fiberglass Layer

This is where all the fun and time goes into this project, it will take forever, but you need to take your time. (NOTE: It is very important you wear a face mask, and a good quality expensive one, not the disposable kind, the resin, and fiberglass fumes will get in your lungs and cause problems for you breathing. Get a good quality one from Home Depot, and work in a well ventilated area.)

Prepare your resin by mixing the recommended amount of hardener and resin as listed on the package. You will not want to work with large amount, or change the amount of hardener (you can vary it depending on temperature). I recommend that you use no more than 2 - 4oz at a time. I suggest mixing a batch in plastic cups and working in small areas. You should get about 30 minutes of work time, and good area coverage with the right amount of hardener, and resin.

Once you have prepared your resin, you will need to prepare the fiberglass. This isn't that hard, just rip up the matte into manageable strips, about 1" x 4" long. (Wear your mask!, and some disposable gloves at all times!)

Now you can finally begin. The procedure I used is quite like paper mache if you have ever done that. Take your cheapo brush, and dip it in resin, spreading it over the area your are going to be working on. Then take one of your glass strips and place it on there. Then use your brush again to dab resin onto the strip until it turns more transparent. (Look out for bubbles, if you see white areas bubbling up you have air trapped. Use your brush and try to spread out the air to get rid of it. Air bubbles could make your box boom itself to pieces if its too weak.) Continue laying fiberglass in this fashion, crossing over the other pieces until you have a good layer that covers the taped area. (Remeber to make your shell bigger than you actually want, you will trim it down later). Wait about 2 hours until it has dried and become touchable. (You can use a hair dryer to help speed up the process!)

Step 5: 5. Additional Glass/ Removing

You will want to add a few more layers of fiberglass now using the same technique in the previous steps. After a few layers it may be strong enough to remove from the vehicle, it is much easier to do the work outside of the car! You could have problems keeping the contours of the trunk in your box if it is not strong enough, so make sure it is strong once you remove it.. You will need to add about 4-5 (more is better) layers total of glass on the inside, so whether you want to do it inside the trunk or remove the box and do it outside is up to you.

When you do remove your box from the car it is now a good time to also remove all the tape from the box, and inside of the car. Its ok if some of it sticks to the box, you won't notice it. You should now be doing all your glassing outside of the car. Just make sure everything is dried before you place it back in the car

Step 6: 6. Trimming the Excess

After your 5 (or more) layers of fiberglass have been laid you should not have the cleanest edge on the out side of the box. Hopefully you added more than you wanted because you will need to trim off the excess anyway's to get a good thickness. Place your box in your trunk (it should be snug) and draw a line where you want the edge to be. Then using a dremel, rotozip, jig saw or other cutting device work your way along that line. You should then have a nice edge all the way around, with the same thickness throughout.

Step 7: 7. Working Around Obstructions

On my vehicle there is an moving part of the trunk that would come in contact with the box when you close the trunk. It is very important that you note and obstructions and create a clearance in your box. Now is a good time to do it, before you begin to install your speaker rings. Create a necessary jig out of MDF to create a pocket in your box. Then use wooden dowels and hot melt glue to secure the piece in place where it encounters the obstruction. Once it is in place you can fleece and fiberglass the area around it (step 10), or wait until your speaker rings are also in place. It is up to you what you do, but I recommend working on the obstruction area first because it will be easier to lay fiberglass on the inside.

Step 8: 8. Speaker Rings

will not go through this process very much. You will need to construct your speaker rings out of MDF preferably 3/4". Your subs should come with some kind of diagram on the sizes, cut as to their specifications. If you would like the added look you can make a second ring to countersink your sub in the box more. Just secure the 2 rings together, and go with that, it will look very good. If you are using carpet, and counter sinking, it is important to make your ring bigger than normal so you can put carpet underneath the subwoofer. You may choose to paint your rings black if they are visible. Additional information about speaker rings can get found elsewhere on the net.

Step 9: 9. Positioning and Volume

You should now take into consideration how much volume you will have in the speaker box itself. If you are just now figuring this out, then you may have problems with having too little space for the desired sub you are using, which could be very bad. A method that has been proven to be an easy and cheap way for finding volume is to use packing peanuts. Take your subs desired volume and fill up your box with that around of packing peanuts, hopefully you took the volume and wheel well volume size into consideration before you started.

Take your speaker ring, and place that where the packing peanuts stop, you will want to be a little bit further out than that due to your subs coil taking up space. (Remember, its better to have your box built with too much room rather than not enough.) Hover your speaker ring in that area, and remember about where it was. You can now decide if you want to angle it towards anything as well, the front of the car, and other side of the trunk, or upwards.

This is now a rather tricky part you will need to take your hot melt glue gun, and dowel rods and some how make your speaker ring float in the area where you want it to be. There are other methods with using plumbers metal positioning wire to hold it up, but I feel that's just more work. Make sure the ring is firmly in place, you don't want it to come undone or collapse.

Step 10: 10. Fleece

You will need enough Fleece, or some other cotton/polyester material to stretch over the big opening between the fiberglassed parts and your speaker rings or MDF. Using some hot melt glue attach the fleece to the top of the fiberglassed area. You will need to then stretch the fabric over the MDF to the other side of the fiberglassed part, securing with more hot melt. You should now have a fabric covering everything that is not covered in fiberglass. Make sure you get every single wrinkle you have out of the fabric, it is very important, make tiny cuts if you have to, but hopefully it will stretch around everything cleanly.

Step 11: 11. Resin

Using the good mixture and the resin and hardener (as mention in step 4), take your cheapo paint brushes and start painting the fleece with resin. Cover the whole area, except where your sub will go, that isn't necessary to do. Saturate the fleece with the resin until it becomes soaked through pretty well, it will need to dry, but once it does it should be pretty solid.

Step 12: 12. Fiberglass

You are now on the last leg of your fiberglassing work. Add 4-5 layers of fiberglass to the fleeced area following the steps as seen in step 4. Make sure you don't add glass to the area where the subs go, those will be cut out later.

Step 13: 13. Trimming the Fleece

You can now cut open the sub hole and trim the fleece. You should use a dremel tool or similar to open up the area and trim it as close to the MDF speaker rings and possible. You can also remove a bit of material from the back now, if there still is some.

Step 14: 14. More Fiberglass Support

If you are noticing any spots on your box where there may be some weakness, due to air pockets, or thinness. Now would be a good time to add another layer of fiberglass. Try to do it on the inside of the box to keep the outside nice.

Step 15: 15. Sub Wiring

There are several different ways to wire your subs, no I'm not going to talk about series or parallel, or ohms, information on how they should be wired can be found here. The main problem you will face is how to get power to the subs once they're in a sealed box. Most pre made boxes have a connected you simply connect your wires to. But if you didn't plan to have something like that, it won't be very easy to install one now. (If you would like a little pod to wire into look at step 7 about obstructions). Other than a simple pod another easy way it to just drill a hole and stick the wires through sealing them up afterwards. There are also other connectors which go through the fiberglass, and connect on the other side, it is really up to you, just remember the holes must be air tight.

Step 16: 16. Sanding

If you made this right, making sure all the fiberglass was laid good with no bubbles and the fleece was wrinkle free you should not have to sand. But chances are there a few humps, or places where it needs to be sanded a bit. Just lightly sand the area, (wear your face mask!) until the area is filed down. If you need to have a place filed in, use more fiberglass, or use bondo. If you are carpeting the box it isn't too important to make it smooth, but vinyl or painting requires it to be smooth.

Step 17: 17. Finishing the Box

How you finish the look of your box is up to you, there are several choices you can do. Paint the outside of the box in the color of your choice, add a layer of vinyl over is all, or carpet it to match your interior. For this tutorial I will talk about carpeting because that is what I choose to do. Automotive carpeting should be pretty easy to find, some hardware stores actually sell carpet that is a good match for your interior, just go to your home center and look. For mine, however, I wanted the exact carpet that was used in my car, so I went to the local junk yard and pulled the old carpet interior out of a junked car. Same stuff, you will just need to clean it up a bit. Attaching the carpet to the box is simple, once you have it cut out to fit and have made sure that it does fit, you just need to use some 3M adhesive spray to adhere the carpet to the box.

Step 18: 18. Volume Check

Using the packing peanut method once again, check the volume again. Hopefully your box is larger than the recommended size, if not the recommended size exactly. If it is too small, I don't know what to tell you other than live with it, or start over. For those of you with boxes larger than recommended (which I hope is most of you), all you need to do is add some Polyfill to your cracks and weird sides of the box to lower volume.

Step 19: 19. Crank Up the Beats

You can now install your sub in the hole, wire it up, and install the box into your car. It should be a perfect fit, nice and snug. If it isn't, I don't know what to tell you. If its close, you can go ahead and place some screws into the box and into the frame of the car to secure it. You may also want to think about purchasing some sound deadener to place in your trunk and behind the box. It will prevent any rattling of loose objects around your box. Run the wires to your amp, and CRANK UP THE BEATS!

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    34 Discussions


    Question 9 months ago on Step 11

    Do you glass the outside of the fabric, or inside?

    Brad h
    Brad h

    Question 1 year ago on Step 2

    Does the fiberglass release off the blue painters tape ? Or do you use a release wax on top of the tape ?


    2 years ago

    Polyfill Increases volume to the speaker not the other way around. This is well documented in all speaker and subwoofer design books. If your box is too big you Port it. If it's too small you polyfill it.

    Honestly the Best thing to do is polyfill and dynamat the inside of the box if it's too small this will increase the effective volume by up to 10%.


    3 years ago

    Does anyone have any recommendations for getting it out after the resin sets? There shouldnt be any leaking through my tape layers.


    3 years ago

    Damn this is one of the best write ups I have seen in a while. I am looking to start doing my setup real soon, so this will help my setup and I will also be posting up pics as mine as well. Thank you for taking the time to post this.


    5 years ago on Introduction

    Great tutorial, but one important note. Fiberglass is usually sold with wax in it (check container) if so, you must get all the layers built up before it dries. If you can't do this (it dries say after two layers) you MUST sand the hardened layer to remove the wax before you add additional layers or you will get layer separation. Also, when you are finished with your fiberglass layup, you must sand the final surface (again, to remove the wax) so that paint, carpet glue, etc. will stick to the fiberglass surface. Other than that, great article.


    8 years ago on Step 19

    This is a pretty sweet setup. I would love to have something like this in my car. Do you know anything about a sound deadener?


    Reply 8 years ago on Step 19

    Go to Lowe's and get stuff called "seal & peel" it comes in a roll thats 6" by 25' and it cost less then $20, it works about as good as dynomat or better but it's way cheaper.


    Reply 5 years ago on Step 19

    Peel and Seal is the aluminum liner... Seal and Peel is a caulk compound that you put around windows and doors that you aren't going to use in the winter. Very different products.


    5 years ago on Step 3

    Newspaper will do next-to-nothing against resin. Go with a plastic layer just to be safe.


    6 years ago

    Excellent instructable I just finished installing my sub and box that I built entirely using these instructions and it came out amazing. I have wanted to do this for a long time and the step by step process here made it simple thank you

    photo-2013-08-14 9:07 PM.jpgphoto-2013-08-14 9:07 PM.jpg

    10 years ago on Step 18

    Polyfil makes your box appear to be larger to your sub. It will help your sub if your box is too small.


    Reply 8 years ago on Introduction

    True. With fluffy fillings one can make the speaker "think" that the enclosure/box is bigger than it really is, virtually increasing the echoroom (not sure if proper word). In ported boxes it is more important to have a properly measured enclosure than in enclosed ones (those without ports) but it doesn't hurt to try different setups.


    Reply 6 years ago on Step 18

    The sub doesn't "think" anything... It doesn't have any special electronics in it measuring the pressure behind it. The reason polyfill makes it *sound* like it's in a bigger box is because it slows the movement of air inside the box.


    i was wondering, do you take out the wooden sticks after its all fyberglassed? or leave them in? :)


    8 years ago on Introduction

    Using fiberglass really helps to make an enclosure that isn't shaped geometric, like a box. An ideal shape for any subwoofer box would be ball-shaped, if not horribly mistaken. Even a small angleshift on a box will make the subwoofer sound better, removing "standing waves"(or something like that) using more of its possible frequency range so you can hear/feel everything that the bass is capable of producing.


    9 years ago on Step 10

    Also if you need the fleece glassed into the recessed area of the speaker ring you can take a staple gun and staple the inside corner of the countersink to get it glassed in the corner.


    9 years ago on Introduction

    I must admit I'm not into sound systems, however; you did a very professional job and the result deserves a loud GREAT JOB!!

    I am quite envious of this instructable. I didn't know about this site back when I did my own custom boxes. I have to say though, I absolutely love the job that you have done. Here is a finished picture of what mine look like.
    My finished product
    Instead of carpeting the subs, I went for more of a retro look and polished off the boxes and completely removed the floor of my trunk, adding an MDF floor with hinges (for easy access to the spare tire without removing subs or amp), and a plexiglas window for the amp to be mounted below for viewing pleasure.  I ran into a bit of a cooling issue with no air flow by the amp, so I installed some computer fans below the floor to circulate air across the heatsink of the amp.

    I'm running an alpine MRP M1000 with two Alpine R12's and cut the cost nearly in half by doing it all my self.

    I finished them about two years ago, and it was my first experience with fiberglass. I must say if you don't know what you're doing it makes things a lot harder, but you learn quick. Your instructable definitely clears a lot of things up and has great tips!

    Once more, awesome instructable, keep up the good work!