355 Footwell Subwoofer

Started this project and thought others might be interested. I'm building a fiberglass enclosure for a sub for my 355 spider. I know, the music's behind your head, but I like music and take a lot of long trips with the car, so I enjoy a good sound system. Last year I did my first fiberglass enclosure for my wife's Boxster and was really pleased with how it turned out, so now I'm doing my car. The car still has the stock Ferrari/Sony stereo and CD changer, but I added a 5-channel Sony amp in the nose and a switch to let me feed my iPod directly into the OEM head unit. For the moment, I've left the OEM door and dash speakers until the sub is finished and I can get an idea of the overall performance.

First step in a fiberglass enclosure is to protect the car, so I've put down multiple layers of tin foil & painters tape to protect carpets and leather. Wrapped everything else nearby in trash bags. Fiberglass is unbelievably messy to do, so you cannot do to much to protect your work surfaces.
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Next is to do a few layers of fiberglass to get an accurate shape of the car. These pictures show the initial glass. There's about 6-8 layers on the lower portions and sides. Probably only 2-3 layers on the upper sections since gravity wants to peel down the upside-down portions.

I've let the glass lay over into the gap between the dash and the center console. This is partly to make the box more secure when it's done (the shape will kind of lock in the box from sliding), and partly to add as much airspace as possible to the enclosure. There's no left side to build the glass against, so for the moment that edge of the box will be open and I'll close it in later. I also stuffed a few rags into key places under the tin foil to make the shapes a little more normal for laying in the fiberglass mat. It's difficult to get a nice smooth finish if you try to match every little crack and crevice.

I'm hoping to be able to use a 10" woofer, but I'll do an 8" if necessary. Matching the sub enclosure volume to the woofer's needs is possibly the most important criteria for getting good sound quality out of a box.
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Now the initial fiberglass has cured overnight and hardened nicely. The next step is to get the shell out of the car for further work. This was a lot harder in the 355 than in the Boxster for two reasons - first, I used more painter's tape than I probably should have, so the shell was securely anchored to the carpet. Second, the complex shape around the center console and the AC floor vent above it made it awkward.

They key here is to be slow and gentle so you don't crack your shell. It took about half an hour, but I was able to work the shell loose and pop it out. The pictures below show the rough shell with all the protective material that came out.

The last photo is after I peeled off all the excess tin foil and tape. There's still a lot of tape and foil embedded in the resin in the back, but that's fine - it can stay. Right now the backside of the box is extremely sticky due to the painter's tape. The glass resin essentially becomes plastic as it cures, so it's not sticky at all. In fact, to cure the painter's tape stickiness, I'll end up lightly basting the outside with a coat of the resin so that it becomes smooth and clean. Eventually I'll take a Dremel tool and smooth off all the nasty edges, too, but that's down the road some.

Now that the shell is out, I'll flip it upside down and layer up the upper half some more, working just on the inside since the outside is now as close to a perfect fit for the car as you can get. This is also where you can find the imperfections due to the cramped working space in the footwells. They'll be a few bubbles or pockets where the glass mat didn't seat tightly against the prior layer. You can cut those out with the Dremel and layer over them, or if they're not deep, just fill them with resin. You try to be sparing with the resin since it's dramatically heavier than the mat, but it's not like a pound or two will make much difference in a Ferrari. Right now the whole box probably weighs less than two pounds. By the time it's done, I'll bet it turns out to be 6-7 pounds without the woofer. Much lighter than a wood box, and you don't lose all the volume that you would to wood.

Next I'll get a rough volume measurement on the box to see whether it will support a 10" woofer or if I need to stay with 8". After that, I can start cleaning up the rough end for where the center console is going, and then cut out a woofer support for the front.
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Thanks! At this point, I don't know exactly how much I'll lose from the footwell. I glassed it out 7" from the deepest part of the floor, so that would be the maximum lost. However, once I know which sub I'm using, I may end up cutting that back if I need less volume, so it could be as little as 5". It's not really the depth of the sub that's the key so much as how much airspace it requires to sound right. There are lots of subs meant for small enclosures, so those are the ones I'm considering. An 8" would need about .3 cubic feet and a 10" would need around .67. I'll basically shift the box as tightly to the firewall as I can while still getting the volume I need.

In Susie's Boxster, I only lost an inch or so at the bottom, but since the box followed the slope of the footwell, it was about 8" deep at the top. This one has a Kicker square woofer in it, meant for low box volumes.

Also, as you can see with Susie's, it's really easy to pop out and I put her wire on a quick-disconnect plug, so if she needs the full footwell, the woofer can be pulled out in just a couple of seconds.
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I did not consider the side of the center console, mostly because there's just no way to get enough air space for a real sub. You could put some 6.5" woofers there maybe, but that really doesn't give you the full sound I'm looking for.

But it's OK - there's good news. Tonight I measured the approximate box volume. Since it's a very complex shape, it's tough to do just with measurements and math. So instead, I filled the enclosure with packing peanuts to my estimated 7" line. Next, I transferred the packing peanuts from the sub enclosure to a normal rectangular box where I could measure volume.

The sub enclosure's volume turned out to be roughly 1.145 cubic feet. This is much larger than I needed for the subs I was considering - .3 cu ft for an 8" or .67 cu ft for a 10". In fact, that's enough for a 12" woofer if I wanted it. But the point here is good sound quality and minimum interior intrusion, so I think I'm going to do a 10" woofer and just shrink the enclosure back towards the firewall a bit to make more room for feet.
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Glad your car is lighter, Vince. But I can solve the stereo problem with the 'Off' button. Senseless judgmentalism is yours for life - like Herpes.

Now I've taken a Dremel tool and cut the left side (which will be over the center tunnel) clean. Make sure you wear safety gear when doing this - fiberglass dust can be very unpleasant.

As the closer picture shows, this cut revealed a couple of problems. Towards the back is a little hump where the two sheets of fiberglass cloth didn't sit tightly against each other. Further up is a hole where I didn't get any cloth in that area. It's tough working with such messy stuff upside down while crammed in a car footwell, but it's OK. Later on we'll trim out where necessary and just build those areas up again with new fiberglass, resin, or fairing compound.
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To close up the open side over the center console, I've run a bead of hot glue around the open side and then gently stretched a light, stretchy jersey-type fabric over the opening. Next I'll paint that fabric with the same resin that you use with the fiberglass cloth. That will make it stiff enough to maintain the shape for us, and then we can start layering fiberglass on the inside to build up strength.

In the following picture, since we knew the enclosure was a lot bigger than we needed, I re-measured enough packing peanuts for the box volume I actually needed. The woofer wants .67 cu feet, so I decided on measuring for .72 cu ft since the extra layers of fiberglass will eat up some of the interior volume as we progress. Having poured .72 cu ft worth of peanuts into the enclosure, I traced around the fill line with a Sharpie, so now we know about how big the final enclosure will be. Everything forward of that fill line will get cut off later. So it's actually going to take up quite a bit less of the footwell than the original lay-in suggested.

And of course, the box will be easily removable in case I need the foot space or if I'm racing Vince and trying to make up for his 70 lb weight advantage...
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Thanks, DonJuan. The square Kicker is the one I used in my wife's car (the Boxster photos further up). The piston appears flat, but the woofer structure itself isn't any more shallow than average - it still has a big basket and magnet structure behind it. I actually have a Kicker Solobaric S5 10" that I originally was going to use in the 355, but I think I'll keep that for something else. The advantage to the square woofers is not really depth (although they are built for low-volume boxes), but more that they have a larger cone area than an equivalent round speaker. Since the volume of air that a subwoofer can move is a function of the woofer cone area and the overall excursion of the cone, a square speaker in theory will move more air than the same diameter round speaker.

HotRod is right about allowing for the woofer volume within the box - I have already picked the sub now, which I should've mentioned. I'm using the Polk MM1040. They recommend .67 cu ft for that woofer, which already takes into account the volume that the sub itself will occupy.

The reason I chose the Polk over the Kicker that I also had was two specifications - one is the Fs, or free-air resonance of the sub. This spec strongly contributes to the lowest frequencies the sub can play efficiently, and the Polk's is significantly below the Kicker. The other spec was efficiency, or basically how much SPL a speaker can create for a given power input. The Polk is rated at 91dB, whereas the Kicker is around 86. Since the scale is logarithmic, 5dB is a very significant difference.

Since my car is a spider and the amp I chose is not ridiculously large, an efficient speaker will give me more volume/watt. Good points, guys!

 

OK. Back from vacation and back to work.

You can't lay fiberglass cloth into sharp right angle corners or it splinters and breaks and then has no strength left. Since we have sharp right angle corners where the box ends over the center console, we need to round them off to keep layering up the inside. To do that you mix fairing compound with the fiberglass resin. It turns it into a thick goo which can be put in the seams. When it dries, it can be sanded to a smooth corner.

This is my first time using it and I mixed the first batch a little thin. You're supposed to keep adding the compound until you get a very thick peanut-butter like texture so it will stay where you put it. Since mine felt thick but could've been thicker, I decided to do it in 1/3rds on this box. I did the first third and then put the box at a tilt so that it will stay in the corner as it hardens. Once it does, I'll flip the box over and do the same thing for the top edge and the back. Then I can go back to layering in fiberglass cloth on the inside.

It's hard to see from the photos, but I put enough in that it is forming a nice rounded edge by itself. I guess that's one advantage to having it a little thinner - probably won't need much, if any sanding to get that nice rounded edge.

Also, for those who have asked, I've ordered a book on making molds and so I may try to do that from this box. If it works, I will be able to reproduce the box for others who want one.
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After spending the last couple of days layering up the fiberglass to be thicker, today I finally rechecked the volume and cut the excess off the box. Now we can start to see the real size of the enclosure. The side view shows that we're only losing about 4-5 inches at the base.
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