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Chupa 15th August 2004 02:55 AM

MDF vs fiberglass
im having to make myown box for my 04 silverado ext cab for subs under the back bench seat. The problems with using MDF is its hard to get a lot of air space because there are so many curves, and nooks, and cranies, that if you dont make the box so that it uses each one of them your not gona get the air space you need. Im just barley able to make .8cu ft., and that box i made was too big so i have to start over.

If i used fiberglass to make at least the bottom it would save me a lot of effort and i would be able to easily get the required airspace. But i had a firend tell me that if i use fiberglass it wouldent sound so good because of the acoustic qualities of fiberglass.

what should i do? is my friend right? could i get away with makind it fiberglass and then just like put MDF pices in it?

any advice is much apreciated. thx

getafix 15th August 2004 03:43 AM

I'm not sure about the acoustic quality of fiberglass.. but since this glass shell is actually sitting/molded on your car's chassis, which I presume is steel, you'd also get the benefits of a rigid surface. Besides a lot of SQ setup incorporate fiberglass in their auto installation.

hoped that helped

theChris 15th August 2004 04:21 AM

fiberglass has been used exensively in car audio. here are the key points:
1.) use a laminating resin. general purpose and finishing resins often have short working times and a wax compond included. the wax dries nice and glossy, but new fiberglass layers will not bond to it. an iso resin is ideal becuase it has more tolerance to heat.
2.) fiberglass will not bond to duct tape or aluminum foil.
3.) fiberglass is an irritant. if you get it on you, well, unhappy until you can get to some:
4.) acetone will help get resin from your hand.
5.) curves are important. move curves means more strength. the flat points of a fiberglass structure will be the weakest.
6.) keep sharp angles to a minimum. fiberglass will not take kindly to going around 90 degress edges.
7.) use 2 pair of latex gloves. this stuff will eat through 1 layer without much problem.
8.) try for 4-6 layers of mat. cloth is expensive and you never seem to get done because it adds little mass. 4-6 layers should be as sturdy as MDF that is 2-3 times as thick.
9.) strength depends on the geometry, number of layers, amount of air-pockets (bad), the ratio of resin to glass, and the number of free ester chains in the resin. vinyl ester has more and is stronger then polyester, but with 4-6 layers it should not matter.
10.) VERY IMPORTANT -- all surfaces in the car must be covered. covered with plastic or with foil or with duct tape. this stuff will not ever come out of your carpeting

the typical fiberglass install is 2 parts -- part 1 is making the boundary mould. this is where the fiberglass molds into the vehicles contours. use a few layers are you will not be able to add layers later without losing form.

part 2 involves the speaker mounts (speaker rings) and amprack mounts. you can build a wooden tray for an amplifier is desired. make sure to have room to run wires or run wires through pipes... for woofers, a wooden support frame is usually made for large projects. for smaller projects stiff metal mesh is used along with picture frame tape (metal strip with holes) to make a form for the inside. some users will opt to make this structure from MDF instead of fiberglass. for kick-panels a polyurethane foam is applied and carved to get a desired form. from there a layer of fleech is uses. t-shirts can be used. basically it is a layer that can add support for the extra 4-6 layers without folding.

Chupa 15th August 2004 05:17 AM

my plan is to make the bottom and the back out of the fiberglass as that is where the most curves are. for the top ill just use MDF. Would it be groovy just using liquid nails to join the 2 pices to eachother? making it air tight of course...

ive been looking at this guide for a while now:

SupraGuy 15th August 2004 06:31 AM

It's relatively easy to make fibreglass acoustically dead. As mentioned, part of the "trick" is to make as little of the flat parts out of it as possible.

How I generally make my fibreglass enclosures is to build all the flat parts from MDF first, and assemble them, with the exception of the "face" plate, typically where the woofer will sit. No flat surface will be made up of fibreglass.

If you use standard automotive or marine resins, you can use multiple layers, but you cannot allow full curing time between them. Allow just enough time for the resin to not be tacky to the touch.

Tips: Buy lots of inexpensive paint brushes. Bristle brushes, not foam. The cheapest ones you can find will be fine, as you will not be able to re-use them. Buy lots, you'll end up using one for every batch of resin that you make. While you're at it, buy a box of disposable latex gloves.

If it's a hot day, make your batches small, or you'll be throwing out a lot of hardened resin in the shape of the disposable aluminum pie plate that you'll be using to mix the resin in.

Cut smallish strips of fibreglass matt (or cloth) before you start, because you won't have time to cut it once you've mixed the resin. I find that a size about 3" by 6" is about perfect (And fits into your disposable pie plate very nicely.)

Use lots of layers of aluminum foil and/or duct tape to protect the interior of your car and the backing area where you are going to be laying your fibreglass. If there's a hole in there that the resin gets into, you'll tear something apart trying to get your box out to fit the woofer. Cover an area that is larger than the intended area.

Overlap the fibreglass onto the ends of the MDF about 3" or more. This will make sure that they never separate. You can also "paint" the inside of the MDF with fibreglass resin. This will add strength to the MDF as well as seal it to make it airtight.

Cover anything and everything in the interior of the car that even remotely might get fibreglass resin dripped onto it.

If you will be working inside the vehicle with a polyester based resin (Which a vast majority of fibreglass resins are) then wear a respirator! I cannot stress this enough. It's bad enough what you're going to do to your hearing once you've installed a killer subwoofer, don't kill off the brain cells, too.


rockthisway 28th August 2004 03:49 AM

you can use the same brush over if you have a cup of acetone and keep it in it until you need it again just watch it because it will evaporate fairly fast

zavier662 17th September 2004 10:16 PM

Great info guys, but I still have one question.

I built my first box recently and, let's just say it wasn't perfect. The fit is pretty good, but I do have some small gaps.

Would fiberglassing the exterior of the box be worth trying?

Or will liquid nails and some silicone be enough?

getafix 17th September 2004 11:25 PM

if the gaps are not that big you could use body filler to fill those gaps.

Stocker 18th September 2004 02:16 AM

Body filler is not so good for *exposed*, large gaps. If it's in a place where the corner of whatever you are hauling in the truck could possibly zap it, you will have a gouge out of your nice new box. You can glass over the outside. If you use fiberglass cloth and not too much resin for the exterior layer it makes a texture that you may or may not like. You can always try liquid nails and silicon first, if you want and if you feel more comfortable with it... if it is good enough for you, then it is good enough, period.

Wear a mask when sanding body filler. Wear a mask and preferably goggles when you sand glass. You do not want to breathe little bits of glass.

If you can smell fresh resin through your respirator, exit and re-fit the respirator

Flat panels of glass can be made MUCH more rigid if you include a bit of skinny rope in between layers, with enough resin to soak it. It makes the flat, curvy. I built the flats on my box this way, and knocking on the side is like knocking on a solid chunk of wood. Dead.

I used clear packing tape to mask in my car. It didn't leave adhesive residue and didn't pull up much carpet. make sure it lays down smooth though or it will be one with the your box!

zavier662 18th September 2004 03:10 AM

Sounds great. The gaps are small and what you are saying seems to be what I want. Thanks

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