When we finished last month, we had created the center seat -- a really radical modification to our Alpine Civic Si demo vehicle. The whole idea behind this process was to make the soundstage perfect, a concept not easily achieved in a normal car. With the seat in the middle and so far back, we had a great starting point for a truly awesome sounding system. The final step was to build a front speaker array that would take advantage of the center seating position, getting the best sound quality and quantity possible.
To do this, we decided to build a four-way front stage. This would consist of a 12" front sub, a 6" midbass, a 5-1/4" midrange, and a 1" tweeter. The four-way arrangement gives us the most output, power handling, and control, so it was perfect for this car. We also wanted to achieve a smooth visual progression from large to small speakers, so these driver sizes did that for us as well. If you've never heard a car with big subs in the front, you don't know what you're missing. Up-front bass adds a realism that's difficult to achieve any other way.
If you remember our 15" subwoofer enclosure construction a few months back, then you'll remember the basic procedure for building the front speaker enclosures. These are a little trickier, however, mainly due to the odd shapes of the firewall and the windshield area. We started by creating an MDF shell to fit the floor, cutting the pieces from cardboard templates. These pieces were super-glued together temporarily. With the floor shape taken care of, we created sides, tops, and bottoms out of MDF as well. These panels were also super-glued in place to form the basic enclosure shape for the 12" front subs. We went for about 1 cubic foot for these, for the best overall response.
The enclosures for the 5-1/4" mid and tweeter had to be built a little differently. Since the lower corner of the windshield forms an odd shape, it would have been difficult to build an enclosure shell out of MDF. Instead, we taped the area off and attached aluminum foil to the tape (3M Super 77 spray glue works great for this purpose, by the way). Now, we could apply some 3/4-ounce fiberglass mat and resin (about five layers) to the foil, forming a fiberglass shell that fit the area perfectly. This would become the base piece for our mid/tweeter enclosures.
With the fiberglass drying, we moved back to our 12" and 6" enclosures. The small box on top of the 12" piece is for the 6" midbass, built with MDF and super glue. Duraglass-reinforced body filler (the green stuff) was then applied to the corners of the enclosures to seal them up and add some strength. Notice that the edges of these are also rabbeted, or notched, to accept the stretchy carpet forming material. The boxes were also secured at this time, by bolting them to the firewall of the car. We used nut-serts, a type of rivet with a threaded inner lining, to hold the enclosures firmly in place.
All of the enclosure base pieces were now finished, so the next step was to make the rings that would hold the speakers. We used a perfect circle jig on our router to create these shapes, then used a rabbeting bit to make a notch in the edge of each one. This notch would accept our forming material in the next step. Each ring is held in place using super glue, and small MDF sticks were also used in some areas where the ring is further away from the enclosure surface. The key here was to get both sides symmetrical -- we used cardboard templates, tape measures, and good old fashioned eyeballing to get this done. Remember, the ring positions are where the speakers will be, so it's important to take your time and make sure they are exactly where you want them.
The rings were placed properly, so the next step was to remove all four enclosures from the car (the 12" and 6" enclosures were bonded together). Stretchy trunk-liner-type carpet was stretched over each enclosure, and stapled into the rabbeted notches in the edges of the pieces. We decided to use grille cloth instead of carpet for the smaller 5-1/4" and tweeter pods, to minimize the chance of warping these pieces. We used super glue to attach the grille cloth to the fiberglass shell, since staples won't go through fiberglass. All four enclosures were then coated with fiberglass resin to harden the forming materials.
After allowing the resin to dry, each enclosure was built up with six layers of 3/4-ounce fiberglass matting (and more resin) to provide extra strength, along with added resistance to resonance and vibration. Notice that the holes for the speakers were taped off with masking tape to avoid getting resin in the precisely fit holes. Any resin that would have gotten in there would have to be ground out later, wasting valuable construction time. The final step was to give the enclosures a layer of Duraglass, then grind them down, to finish off the shapes and also to add a little more strength. After the grinding, they were bolted back in the car to check their fit and position.
01: The front speaker enclosures were created using pieces of 3/4" MDF attached with super glue. | 
02: Additional pieces of MDF were glued in place to finish off the basic rear sections of the 12" sub enclosures. | 
03: Masking tape was laid down to cover the upper corners of the windshield, then spray glue was used to attach aluminum foil over the tape. |
04: Fiberglass mat was applied over the aluminum foil, creating a strong shell that fit the shape of the car. The enclosure for the 6" midbass can also be seen in this shot. | 
05: After the enclosures were attached with super glue, they were reinforced with Duraglass to create a strong joint. | 
06: Three-quarter-inch MDF rings were created on a router to hold each speaker and were then glued in place on the enclosures. We kept an eye on the rabbeted edges, ensuring space for the forming material in the next step. |
07: Stretchy speaker box carpet was then stapled to the box frames to start the final three-dimensional shape. | 
08: With the carpet stapled and trimmed, the shape of the enclosure can now be seen. | 
09: Fiberglass resin was then applied to the stretched box carpet, creating a hard surface. |
10: Six layers of fiberglass mat were applied over the enclosure for additional strength, creating over 1/2" of total thickness. | 
11: This is the completed enclosure with the fiberglass cured on the surface. | 
12: The upper enclosure for the 5-1/4" and tweeter was built using grille cloth as a forming material, then fiberglassed to create the hard surface. |
13: Duraglass was then applied over the fiberglass mat to finish off the enclosures. | | |
That's it for this month, but check out Competition Corner next month when we fabricate the custom fiberglass dash for our Civic. Go online to www.SteveBrownUniversity.com, and keep those questions coming to sbrown@alpine-usa.com.