A Brief on Subwoofer Enclosure Construction
for Third-Generation F-Bodies
by J. Skinner - [email protected]

Introduction -
    One of the hardest, and most expensive parts of installing a decent aftermarket sound system into your car is the bottom-end.  Good, clear, loud bass comes from large, fairly high-powered speakers called subwoofers.  The fact that they move such enormous amounts of air, usually requires that they be mounted in some type of enclosure.  The "enclosed" space gives the sub a springy cushion of support while it moves throughout it's excursion range.  The subs would need anywhere from half a cubic-foot to several cubic-feet in order to function as they were designed. Fortunately, we're blessed with a trunk area with an enormous number of options and some unique opportunities.
    Depending on your preferences and desires this can be a somewhat simple, or quite complicated task.  You would have to consider retaining easy access to your spare tire, willingness for extensive modification to the vehicle's panels, or would you be willing to drill holes?  Maybe you want something showy?  Or maybe you want something as sneaky as a thief in the night (and that might not attract one).
    Once you decide your approach, you're all set to make your game plan.  Let's say you're kinda green in this field.  Ever worked with wood before?  Do you have access to the tools and work area in order to do this?  Or are you willing to put out the money to have a professional or a friend do the work?
    I'll write this as if you already know how to work with the materials involved.  There are sites out there that can teach you enough to do it all on your own.  I'll point some of them out to you at the end.

Your options -
    You have four main routes to take.

    Pick one.  There's good and bad things about all of them, it's just a matter of what you want..  There's certainly much more to think about if you're building a competition system, but for an all around stereo upgrade, they'll all work.  And for some work, it will definitely cost a lot less than just taking it to a shop and saying, "Here, this is what I want."  Which is cool, but you can't take as much pride in it.  Okay, I lied:  it's a matter of what you want, and how much you want to spend.

How to get started -
    If your car came with RPO D27 (rear compartment security cover), you've got one step there already.  With any of the previously mentioned options, you'll probably end up making a board that fits the contours of the trunk.  Just take off the cover, and trace the outline onto a sheet of wood.  If you don't have the cargo area cover, then you'll have to measure your particular trunk and make the appropriate sized piece.  A good starting point is a 42" by 16" board made from 3/4" medium density fiberboard.  Size it to your trunk by cutting off a bit at a time and test fitting it.  You want it tight, but don't ruin your carpet doing it.  Round off two corners on one of the long edges at the same radius as the inner trunk walls (the part closest to you and to either side when you're looking into the trunk).  Then you'll have to cut out the area where the latch housing sticks out into the trunk.  This is about 15 inches wide at the widest, to 8 inches wide at the narrowest, and is about 2 3/4" deep*.  These are just guidelines for those who don't have the fortune of a template to trace; you'll have to exact it out by eye or by numbers.
(*For the cars that have the electric pull-down motors.  Those without are shallower.)
  If you're making the baffle board, you're just about done.  Cut the appropriate sized holes, carpet it, and mount the speakers.  How to cut the holes?  If you don't have access to a hole jig, like what's used in a car audio shop, then you'll have to play it by ear.  Find the center of where you want the hole.  Put a nail or tack there and attach a string (or I would recommend a metal file ruler, the kind with lotsa holes in it) and draw the circle of your sub's size (usually 5/8" smaller than the advertised subwoofer size, but always, refer to your speaker's spec sheet).  Try to make the measurements at tight at possible for the hole.  You want as much wood between the mounting holes for the speaker and the edge of the hole itself, to make a sturdy place for the speaker's mounting hardware to go through.
 

This little trick can help you save some time for shaping your main panel.

Trunk measurements -
    (Coming soon)  We all have to take into consideration the differences between individual thirgens for any measurements I post here.  These are not guaranteed to be completely correct, so be prepared to use your own judgment.

With electric pull-down motor - Bigger bulge at the back where the motor and rails are housed.

With normal hatch striker - Shallow bulge.

With rear-compartment cover option - Make allowances for the studs where the lid mounts to.

With T-Tops - Be alert for the storage provisions for the T-Tops

Convertibles - Have the same trunk dimensions, but you must be aware of the overhang of the lip of the trunk.  That, and the fact that it's harder to hear when the back seat is upright.

(Soon you will be able to click on these to get a blueprint for a box or two for the application)

Wooden enclosure-
    Hopefully, I'll end up posting blueprints for two generic boxes.  One, if you can handle angled cuts, and another for those who are all right angles.
    If you're designing your own, here are some tips:

Under construction.  I wish this picture showed the internal structure better...

General information -
    Cut all your panels first,  then test fit them to each other.  Measure, measure, measure.  Use glue and wood-screws to attach everything, then seal it up with more glue (I like Liquid Nails).  Use a framing square for everything; this will keep your project from looking like a middle-school woodshop assignment.  A counter-sink is optional, but highly recommended, because it will keep the wood from stressing, and bring your screws flush with the surface.
Materials -
    MDF or birch is the best (and also the heaviest and most expensive).  Particle board will work, but it is weaker, doesn't hold carpet as well, makes a mess of dust, doesn't like getting bumped, and is extremely sensitive to water and humidity.  On the other hand, it's cheaper, easy to cut, and lighter than most wood of equal thickness.  You'll need a saw: circular saws are good, but table saws or radial arm saws are better because you'll pull straighter lines and can work with accurate angles.
Instructions -
    Your first measurement should be from the lowest point to the highest point.  This depends on where you want the sub.  You might want it flush with the "shelf" at the bottom, the flat part where your T-tops stow (if you have 'em), or flush with the lip of the hatch.  Stay aware of clearances for anything that moves, like the hatch supports, the underside of the deck lid, the storage box on the driver's side.  Take into consideration access to your jack and spare tire.  When you measure, do it loosely against the carpet.  Too loose, and it'll move around.  Too tight and you'll ruin the carpet getting it in.  Measure the rest of the three dimensions, and write them down with drawings.  Draw in any internal bracing you are planning, and calculate the volumetric displacement of each piece and subtract it from your total internal volume.  By thus keeping an eye on your final airspace measurement, you can end up with an accurate final product made especially for your equipment.
    Test fit each piece before assembly.  If you trust your measurements, put it together, but I recommend using only screws first and test fitting in case something is off somewhere.  When you measure, remember to account for the thickness of your material.  You can use anything from ” x 1” to 2” x 4” (pine) for internal bracing.

Fiberglass Enclosure -
General information -
    The fiberglassing process itself is easy to figure out.  Instructions are provided on the can.  A finished fiberglass product is a cured (dried) resin permeated in packed or weaved fibers.  The fiber is usually sold in strips in the form of mat, which is thick and loosely packed strands, and cloth, which is thinly woven strands.  The mat is good for thick uncomplicated applications, and the cloth is for smaller, thinner, more intricate places (like corners or features like tweeter domes if you were making a kickpod).  The resin comes in two parts: the resin and the hardener.  They have to be mixed in a certain proportion immediately before application (which is done with a brush).  Too much hardener makes it brittle.  Too little and it'll never cure (or take forever to) or end up flexible.  The mixed resin has a short pot-life and might eat through cheap plastic containers (like butter buckets) so a wide-mouthed glass container is recommended.  Don’t get any on your skin because a) it’s not fun to get off and b) it burns.
Materials -
    Fiberglassing materials can be found in hardware stores, auto parts stores that stock bodywork material, marine shops (for patching boats and jet skis), some surf shops (for board repair), and larger Wal-Mart stores.  A quart of resin sells for around $8, and a pack of fiberglass around $3.  A box of this size can call for about a gallon of resin, and 5 or 6 packs.  You will also need brushes of various sizes.  Get the cheap disposable ones, not the good ones you paint with because once it’s soaked in resin, it’s easier to toss it for a new one than to try to clean it.  Also once you dip the brush into it, the resin is already curing so about 8 minutes later, you’re trying to brush clumps onto your project and it’s not cool.  Buy masking tape, plastic drop cloths, a roll of aluminum foil, spray adhesive, and some acetone for cleanup.
Instructions -
   Using masking tape, line the entire trunk with aluminum foil (fiberglass doesn’t stick to non-porous substances like foil, smooth plastic, or glass).  Then use a spray adhesive, and put down another layer, overlapping as you go up (so that things dripping down the sides will run down like shingles).  Mask off everything you don't want to ruin (usually the whole damn thing, right?)  Pay attention to the bottom so excess doesn't find a way through and into the carpet down there.
    Now, if this box is permanent, this shaping method will work.  If you want to remove it, you'll have to find a way to bring the back edge forward enough so that when you pull up on the whole thing it will slide out and not catch on anything.

This is the trunk lined with aluminum foil.  The board is cut in half and beveled so that each side can face outward a little.

    Once you've protected the car, spray some adhesive (I use 3M spray adhesive not trim adhesive) onto the foil and put down some fiberglass strips (helps to precut them to shapes).  Mix some resin and slap it on there 'til all of the fiberglass is soaked.  Either hold your breath or wear a respirator when you're leaning over the trunk.  Once that first layer has cured (usually about 2 or 3 hours, faster if it's hot out), you can pull the fiberglass shell out of the car and work on it outside so you don't have to lean over the car, and there's less chance of spilling something on the car.  Pile on the layers, letting each one cure before the next, until you have about a half-inch to three-quarter inch thick box.  Put your board on top of it, trim the excess, and glass over that 'til it's all one piece.  Use Bondo to smooth rough surfaces and strengthen/deaden the box.
 

Protect your investment!  Note the back wall.  It's a board cut to the shape of the trunk, but angled so that the box can be slid out when it's hardened.

I was short some airspace so I cut the board into thirds and angled them back some to contour to the trunk.  Who would've thought all that sine, cosine crap would come in handy?  This pic also has a layer of fiberglass down.

Measure and cut a divider.  You will have to offset it some from center because the nature of the trunk has a less than symmetrical design.  I can't stress enough how important the measuring is.  My box came to just a bit less than a tenth of a cubic foot from the speaker spec, but wouldn't have if I hadn't measured again halfway through and made that last minute change.

Measure your airspace -
    For a wooden box, figure it out mathematically using volume formulas.  If your fiberglass box is fairly straightforward, do the same for it.  If you're too lazy, just fill it with water and measure how much water it takes to fill it, then convert it to cubic-feet or liters (depending on what measurement your speaker's spec sheet uses).

The great thing about fiberglass is its shaping ability.  I fit in a divot that let the factory trunk light shine though an acrylic cover, and a valley that lets me into the small storage compartment on the driver's side even with the box in place.

 

What to do if your box has too much airspace -
    Start filling it with some dead space.  Two-by-fours are good.  Just measure the volume of the pieces and stick 'em in and subtract from your total (you should be accurate to about 2% this way).  Use this opportunity to add to the bracing of the box.  The stronger, the better, because you would lose power to any flexing the box does.  But if you're way off, 2x4's will be too heavy, so use styrofoam blocks.  It's lighter but less accurate as it has quite a bit of air within it's own density.  Just measure the volume of the styrofoam blocks, and use that figure plus maybe 5% to come up with your displacement.

What to do if your box has too little airspace -
    There's not much you can really do to add airspace, but this little trick can help if you're only off by about 15%.  Stuff the whole box with fiberglass or polyester filling (Poly-fil can be bought at Wal-Mart at about $2 for a large bag).  Rule of thumb: about 1 lb. per cubic-foot.  This works by the fibers slowing down the sound waves and fooling the speaker into thinking it's in a bigger box.  There is lots of debate about whether this is true or not, but I do know it makes the box sound better.  If you're off much more than that, then you're SOL and need to either start over or get different speakers.

Covering your enclosure -
    3M's general trim adhesive (about $8 a can) works well enough.  I used a contact adhesive in a professional spray-gun, because that other stuff is too expensive and not strong enough in my opinion.  Pick some good automotive carpet, preferably with the hard rubber backing for further sound deadening, and that matches your interior.  Or if your taste suits it, vinyl is good (as you can see above, that was the route I took). If you only need to cover one face, spray some glue on the box and stretch that covering over and around the sides.  Then staple-gun it down (on the sides or where you can't see it) for added strength.
    If you're carpeting all the sides, you'll have to "roll" the box over the carpet, gluing as you go until you have four sides covered, then cut out pieces for the sides.  If you want a more professional look for the sides, fold the edges over and recess the side panel (just look at some of the pre-made boxes out there, you'll see what I mean).

Venting your enclosure -
    Only if your sub is made for a ported enclosure.  Use Theille-Small parameters to find out what you need to do about your port diameter, length, or inner surface area.  There are some programs out there that can help you do this.  Blaubox and PerfectBox are two of the best (free ones) I've seen.  You can either build the port into the box in sort of a rectangular shape (again calculating it's inner surface area), or use PVC pipes or cardboard tubes (thick ones, not paper towel rolls).  If you go with either of those, round off the edges to reduce port turbulance (look at your home stereo's ports, see how smooth and rounded they are?).  You can buy ports, too, from some audio shops or Crutchfield.

Finishing touches -
    Drill a couple holes for the wiring (or mount a terminal plate(s)), and be sure to seal everything against air leaks.  When you drill the hole, consider where the wires are coming out, and position the hole to point the wires somewhere concealed.  If you are running dual subs (and most of you are, I'm sure), keep the wires to the amp at an equal length.  The difference in lengths can cause subtle problems (usually only noticeable at higher wattages and volumes).  If you like the look, or the additional protection, add grilles or a grillecloth (make sure it's "acoustically transparent" or in other words: too thin to hurt the sound).  Add feet or handles if needed/wanted.

Electrical -
    Crimp-on female spade terminals are good for the wires that attach to the speakers themselves.  Soldering directly to the subs terminals is also a good idea, but is a little more hassle if you ever remove the subs from the box.  However you connect the wires, make sure they are not loose.  This can result in bad connections, and in worse cases a scratchy or crackly sound, or can restrict the power flow.  I usually compromise on crimping the connector on the wire, and putting a drop of solder to seal the deal.  The speaker wire can go from the speaker directly to the amp through a hole in the box (that's been somehow sealed), or soldered to a terminal mounted to the box.
    If your amp or deck doesn't have crossovers built in, then adding one can make your setup sound a whole lot better.  You'd basically block out all the frequencies except those your speaker is best at playing.  For our application this is anything below 180 hz, but can be a bit higher, or much lower depending on the setup.  Unless you're setting up a truly competition setup, it doesn't matter a whole lot.  Just make sure your other speakers can pick up where your subs leave off (in other words, everything above the crossover frequency).  Hey, some people prefer it, but personally, I like to hear the music, not just the bassline.

Theftproofing -
    Mount the speakers with Torx screws for the best protection.  Use two screws of a different type on each speaker (such as an Allen, Phillips or flathead) across from each other to slow down or thwart potential thieves.  Or build your setup so that the screws are covered or inaccessible altogether.
    To keep the box itself safe, you can run a bolt through the bottom of the box and the floor of the well.  Use a Teflon or rubber washer underneath a large metal washer inside the box to maintain a seal, a stack of washers or a spacer block between the box and the floor to keep the bolt from pulling the floor against the box, and a large washer underneath the car to keep from damaging the floor sheetmetal.  Be sure to run the bolt upward into the box, instead of the other way so you don’t have a bolt sticking out underneath the car.
    Remember: the main enemies of thieves are noise, time, lighting and lack of knowledge.  Get an alarm.  Make it hard or time-consuming to remove components.  Don't show off your theftproofing (or the system itself if you can help it).  Don't blast it anywhere near places where you park regularly and for extended amounts of time (work, school, home).

Sound damping -
    I coat all the inner surfaces of the box with rubberized undercoating, and apply Dynamat to the larger panels to help deaden the sound.  It really does help to do the same to your trunk area.  You'll probably have to hunt down rattles and pad the trim, too.  The fold-down license plates are big offenders.  A little hot-glue and a rubber spacer was all mine needed.  If you don't mind the extra cost, or weight, you should dampen the panels where the 6x9's reside, the floor-pan up front, and maybe the doors.  The unreinforced panels of the roof skin are a good idea, too.  But this stuff gets expensive (and heavy! just go to a shop and pick up three or four rolls of Dynamat or Roadkill and you'll see what I mean!).
 

If you want to skimp on the sound deadening, these marked areas are the thirdgens' weakest points inside the trunk, so only covering these parts would be enough.  On the outside panels, it's the rear quarter-panels and the side pillars (where the 6x9's are) that are most prone to vibration.

The mounts that hold on the bumper are a challenge to make "rattle-free".

Some other ideas -
    Remove  your panels, and make your own, with the subs built in.  If you do it right, you could probably even keep your cargo space intact.  (There's a lot of free space behind those panels!)

Links - Alright, my little brief should help you out for making a box for your F-Body, but here are a few links if you need more research material for your project.