Introduction: High Performance Audio Monitors
I wanted a pair of relatively small, but unique and high performance audio monitors for my stereo system. I've built plenty of rectangular, boxy speakers, but I wanted something different. So my criteria were: as small as possible but still large enough to easily fill an average sized room with music, efficient enough to match my 21" subwoofer, and cool looking. So for the lower frequencies I chose an 8" die cast frame, carbon fiber woofer with rubber surround (info on parts below). And for the high end I chose a rectangular faced AMT tweeter (air motion transformer). This worked out well for a couple of reasons. First, the sensitivity of these two speakers was a good match, avoiding any further components to adjust, and B, the rectangular shape allowed a trim and pleasing triangular face shape, minimizing the frontal area and thereby reducing unwanted reflections. These type of tweeters are fast and clear, although they are somewhat directional, so for the best sound you'll want to sit on axis in the sweet spot.
I decided on a simple 12db per octave crossover with a 4.5Khz crossover point. My goal was to minimize any components in the musical path that aren't really necessary. There are a bunch of calculators on-line so you can determine the best components and crossover design for whatever drivers you use. I simply hot glued the components to a small plywood board which I later screwed into the base board (you don't want anything coming loose and rattling around in there).
Step 1: Basic Design
Once I decided on the drivers, I was able to design the face plate. Wanting to minimize the frontal area, I drew a circle about an inch larger in radius than the woofer on 3/4 mdf, tapering down at the bottom to give a substantial base, and up at the top to accommodate the tweeter. Then I decided on the depth which would give the required volume and room for the drivers, and cut a rectangular piece for the base, and temporarily screwed these two pieces together ( I don't use CAD, or even a drawing board anymore, just build stuff). Tapering and curving the sides makes good acoustical design sense because it reduces the possibilities of internal standing waves inherent in rectangular boxes. Taking it a step further, I thought it would look cool to curve the top also. To do this, I laminated three pieces of 3/4" Sycamore to cover the shape I wanted. I then cut that line on a band saw, and sanded smooth. You will need to bevel the sides of the top piece to fit the stainless side pieces. I attached this piece to the face also temporarily. Now with three sides in place, I was able to define the shape required for the sides. I used a piece of template board and drew out the shape, cutting and adjusting until it matched right. When I was sure I had the side shape correct, I copied the shape four times onto a sheet of 16ga polished stainless steel, then cut them out with my plasma cutter. As a metal fabricator, I know most folks don't have access to the tools and equipment that I do, but these services are available at most local welding shops, or even body shops for a reasonable fee. Once these are cut, you will need to curve the rear upper corner to fit the top and rear plate. I did this with my English wheel, but it can be done with a round shaping hammer and shot bag, or hollowed out tree stump. It's only a slight curve. The back plate will be a bit shorter than the front, as determined by where the back end of the top ends up, and should be parallel to the front. Ideally it should not be parallel, as explained above, but that would make construction considerably more difficult.
Step 2: Puting It All Together
Once you have the sides ground to fit the rest of the enclosure, layout and drill holes evenly spaced about 1 1/2" all around at 3/8" from the edges to allow for #6 X 3/4" round head wood screws positioned right in the center of the 3/4 MDF and hard wood. The bottom piece will be rectangular and beveled to fit the bottom edge of the sides, so these screws will have to angle up slightly. If you bought stainless sheet with a protective cover, leave this on until finished. You will have to disassemble some parts, so I suggest using only ever other screw hole for now. We just want to make sure everything fits correctly for now. When you are satisfied with the fit, you can disassemble all pieces and layout he holes for the drivers and their mounting screws, the hole for the wire terminals, and the port on the back plate if desired. Cut these holes with a jig saw, then smooth the cuts with sandpaper. MDF will finish well if you first coat with a sandable primer/filler, available at hardware and paint stores or home improvement stores. I ended up doing 4 - 5 coats to ensure a smooth paintable surface. Then I chose gloss black for the front, back and bottom. This took about 8 - 10 coats, wet sanding between coats to look good. I used a red shoe polish to stain the sycamore tops, then coated with about 10 coats of clear satin Deft for a nice finish.
Step 3: Wiring
Mount the drivers, then put all pieces together except the sides. This allows you access to the driver terminals, the crossover connections, and the input terminals. There are several design calculators on-line to help you with your crossover. Just be aware of the driver impedances, the slope you want, and the crossover frequency for your drivers, and design accordingly. Mount your crossover board on the base plate, and your input terminals on the back plate, and wire and solder all appropriate connections. A word of caution here... be clear on your design, and the correct wiring connections. You don't want to have to take everything apart later because something got wired wrong. When this is done, you'll need to acoustically dampen everything inside. Sound deadening material is available from on-line sources, and I also stuffed in some long fiber wool for extra damping effect. At this point it might be wise to connect to your stereo just to make sure all is right, but don't over drive them, easy does it while it's open.
Now you can do the final assembly. Caulk all joints you can reach with a silicone or similar caulk, and add a bead of caulk on all edges that seal against the sides. Replace the sides and drive screws into all holes. remember, you are driving into the edge or 3/4 MDF, so don't over tighten! Wipe off any excess caulk.
Step 4: The Fun Part
When you are satisfied everything is working correctly, place your speakers about ear level, about 2-3 feet from your back wall, and about 40-60 degrees apart from your position. Turn on you favorite record, and go sit in the sweet spot and enjoy!