July 2003, Sub Enclosures

When we left off last month, we had completed the mechanical conversion of our 2003 Civic Si demo car to center drive. We even threw in the stock seat, and took it for a spin to make sure all of our mechanical changes worked properly. The next step would be the construction of the real seat, but first, the enclosures for the 15" subwoofers would have to be completed. The reason for this was that the ports for the vented enclosures would eventually fire through the seat, so the enclosures had to be built first.

Whenever you see an Alpine demo car, the speaker screws will not be visible. This is because we always build an enclosure for each speaker first, then build a trim panel over the top of it to hide the screws and flush-mount the speaker. We do this because it looks better, and it also makes the enclosure more solid, as well as easier to get in and out of the car. There are many different ways to build a speaker enclosure, but we have found a way that is quick, solid, and easy, albeit a little unorthodox.

The first step in the case of our 15" drivers was to cut the stock side panels of the car to accept the depth of the subwoofers (01). Next, it was time to start the framework of the box. To do this, we first built a ring from 3/4" MDF and temporarily screwed it to the car (02). Then, we cut multiple pieces of MDF and superglued them to the inside of the frame piece to take advantage of the space inside the side panels (03). Superglue works great on many different materials, and MDF is no exception. An additional piece was glued to the bottom of the box to begin the building of the outer part of the enclosure. The shot also shows the mock-up 15" sub that we use for fitment, complete with a "magnet" of MDF.

With the addition of a few more pieces, it was time to create the mounting baffle for the subwoofer. An MDF ring was created on the router to hold the sub, and it was mounted in our desired position with blocks of MDF (04). Notice that the sub is at a considerable angle. This makes the sub more visible from the back of the car. At this time, the completed frames could be removed from the car so the final fiberglassing procedure could be performed (05).

Although the frame was fully formed, it needed to be strengthened before fiberglass could be applied. Superglue works well to hold the MDF together, but some additional strength is a good idea with larger enclosures like the ones for our 15's. We used Dura-glass, a reinforced body filler, to strengthen the corners of the enclosures on the insides. With the Dura-glass applied to all the inside corners, the boxes were ready for the next step.

That step involved fiberglass, along with something that most people would never guess we use in our high-end installs -- cheap, stretchy carpet. This is the same stuff that you would find on an inexpensive speaker box, but as it turns out it also works very well for fiberglassing. We stapled this stuff to the outside of our box, being sure to stretch it tightly between the edges (06). At this point, fiberglass resin could be applied directly to the carpet, forming a very strong first layer that is almost 1/4" thick. We only use this carpet on panels that are very structurally strong, otherwise "warpage" will most likely occur. Since our sub enclosures are very solid, we didn't have to worry about this problem.

The carpet alone would be quite strong once the resin had dried, but we decided to add six or seven layers of thin, 1/2" fiberglass mat to the surface to add additional strength, since the carpet tends to be somewhat brittle (07). Notice that the hole for the sub has been taped off to prevent resin from building up on the surface. This is important, since grinding the resin off of the wood surface will leave it lumpy, negatively affecting the tight seal of the woofer to the box.

After allowing the fiberglass to dry, we added a layer of Dura-glass to the outside of the enclosure to complete the structure. This filler material adds even more strength, while making the outside of the enclosure fairly smooth. We ground the Dura-Glass with an angle die-grinder, equipped with a coarse 24 grit disc to take the material down very quickly (08). The mounting brackets can also be seen clearly in this shot. They are built from 1/8" flat steel, and are secured to the box with bolts. The boxes are permanently attached to the car using nut-serts, a threaded insert that allows a machine bolt to be used in thin sheet steel. This technique holds the boxes securely in place, preventing them from coming loose and rattling.

With both enclosures mounted, the symmetry of both sides can be seen (09). This was achieved by carefully measuring the mounting position of each ring before gluing it in place. Also notice the square holes in the bottoms of each enclosure -- these are the beginnings of the ports that will mate up with the custom-built seat. Utilizing the expert advice of our resident SPL guru, Jim Dolan, we built square ports that measured 5" across, and we made them 18" long (10). This tuned the boxes to about 35Hz, perfect for the obnoxious amounts of bass we planned on making with the car. The ports were also slightly angled on each end to better mate them with the seat. The final step for the ports was to make a 1/2" wide mounting flange on the ends to allow them to seal with the seat area, which we'll discuss more fully next month.

For our last picture, we'll give you a little sneak preview of the center seat's construction (11). The metal frame you see here is the basis for the seat, and the wood squares on the ends of each port are the beginnings of the center seat. These squares will be fiberglassed into the seat so the mating flanges will line up perfectly. The wood squares were attached to the ABS ports with double-sided tape, making them solid yet easily removeable later.

That's all for this month, so check in with us next time when we'll show you how the seat was created from scratch. Keep those questions coming to sbrown@alpine-usa.com, and check out SteveBrownUniversity.com for some unique opportunities for you to learn hands-on about the techniques we use to build our demo cars.

01: The basic frame for the 15" enclosures was created using 3/4" MDF.	02: This frame was temporarily screwed in place until the inside portion was finished.	03: The inner section of the box, along with the top and bottom, were built with MDF and attached to the enclosure with superglue. This shot also shows the 15" woofer frame used for sizing the box.
04: A 3/4" ring was routed and superglued to the box in the desired position, creating the basic shape of the frame.	05: At this point, the completed frame was removed from the car so that it could be covered with stretchy speaker box carpet.	06: Carpet is now stretched over the enclosure frames, and Chris is applying resin to the carpet.
07: After the resin had cured on the speaker carpet, six layers of fiberglass mat were applied to the surface for extra strength.	08: Dura-glass was spread over the fiberglass, then sanded down with an angle die grinder to complete the enclosure. Notice the square hole in the bottom of the box for the port outlet.	09: The enclosures are both installed in the car now; notice the metal brackets holding them in place.
10: The ports were built using 1/4" ABS plastic, and they were bolted to the side of the boxes. Notice the angles on the ends to allow them to mate up to the seat.	11: The ports are now completed, and MDF rings were temporarily attached to them to form the beginning of the attachment point for the center seat.