Greetings fellow speaker builders and web wanderers. This article chronicles my ongoing development as a speaker builder and covers my second high quality speaker project. The AC54W features an Aurum Cantus AC-!30MKII mid woofer and a custom tweeter waveguide. The waveguide enhances the overall quality and power handling by increasing the acoustic output of the tweeter over a very wide range. This waveguide operates from about 13Khz down to 800hz and produces a significant increase in output over most of that range. This allows a crossover slope that reduces the power applied to the driver allowing it to cross much lower at much higher power with much less distortion. More details as we proceed.
The build begins with an enclosure design. At the time I designed this I was limited to a table saw, router and drill press for enclosure construction so the enclosure has no fancy curves.
Baffle size ended up 11″Wide by 13″High by 12″Deep rather than the dimensions in the image, ignore the measurement markers on the image above. But the placement is accurate. it’s a simple design with placement dictated mostly by waveguide shape more than anything else. The waveguide dominates the baffle due to it’s enormous size.
Since I already had the woofers and I’m committed to using them in this project the next phase is to build the enclosures. I cut the boxes with my table saw and was surprised how well they fit. However, the first version I tried using 3/8″ rabbets on ALL joints and it was a nightmare. So much that I set the parts aside as scrap and redesigned the entire enclosure. First I used rabbets only on the box sides, taking my cue from the Parts Espress knock down boxes they sell. This means the baffle mounts flat against the enclosure front. Rather than simply gluing it to the front I decided to add cleats and make the baffle removable. Soon as the box was glued together I sprayed it with a couple coats of “Quiet Kote” spray on acoustic damping. It’s intended for automobile floors and fenders to reduce noise but seems to work well for speaker boxes too. I have no measurements but I can definitely hear the difference after spraying the Quiet Kote. The original box sounds “dead” when I rap on it and the inside of the box makes no audible sounds when it’s just sitting. After Quiet Kote however, the box sounds different just sitting still. I hear a high frequency echo effect inside the enclosure indicating the spray has significantly changed the acoustic characteristic of the enclosure. I haven’t measured the difference and I will do that some day but for the moment I’m convinced it reduces low frequency box losses and will help reduce coloration from the enclosure. It sounds like it converts the longer LF waves to higher frequency where simple damping material is more effective but without measurements it’s hard to tell.
With Quiet Kote sprayed and cleats glued near the corners I clamped the baffle in place temporarily and marked the baffle mounting screws locations for drilling. I then transferred the location to both baffles and drilled mounting screw holes in the baffle ONLY using my drill press. Later I lined up each baffle for with a box and hand-drilled the holes for the actual hurricane nuts. The result is that the baffles are specific to an enclosure so I have to keep them separate. This happened because of my old drill press. I can’t see where the bit will actually start to drill plus the arbor bearing is toast so it wobbles all around. Since then I’ve retired the drill press and purchased a new unit with laser sights and a build-in work light. I also purchased a CNC router so future projects will be much more accurate, and repeatable. I will be able to save the G-code for each enclosure and cut out a copy in very little time and they will be just like the original.
With the enclosures well on their way my next task was to route and mount the drivers and waveguide. First I drilled the woofer opening with a 5″ hole saw on the drill press. I use a 3/8″ bit to drill the hole saw arbor hole first then flip the actual arbor bit around so it’s a pin rather than a bit for the hole saw and use it to guide the hole saw rather than to drill the pilot hole. With a shaft instead of a drill bit the cut is more accurate and for some reason the hole saw cut cleanly through this time without any overheating, smoking or other issues I had before. Next I set up the router table with a standard 1/2″ round over bit and adjusted it for maximum round over without any overcut. The result is a smooth round over without any ridges that might cause diffraction.
Nice smooth round over. Next I decided to mount the woofer from the back side of the enclosure. The reason is twofold. 1. It moves the woofer back away from the listener compared to the tweeter and it precludes the need to countersink the square woofer frame.
With a rear mounted woofer the tweeter is only 8mm in front of the woofer voice coil in terms of time alignment. The means I won’t be using an asymmetric crossover, or it will be nearly symmetric. If you look closely you’ll notice I screwed the hangar bolts I used to mount the woofer a little too far into the baffle causing a slight bulge at about 10 O’clock position. Since I now have a cnc router I may opt to recut these baffles from scratch both to fix the mounting issues and so I have a baffle cutout drawing and Gcode for future use. Now comes the waveguide mounting. I purchased the waveguide from Pellegrene Acoustics. It came with an oval pattern that perfectly fits the outside shape of the waveguide. To mount it I first attached the pattern to the baffle with screws so it would not move while routing. I then mounted a 3/4″ round, 3/4″ high pattern cutting bit. It’s a regular 3/4″x3/4″ straight bit with a bearing on the collet (motor) side of the cutter. The bearing follows the pattern, cutting only where it extends beyond the pattern bottom.
This left me with a 1/8″ deep oval in the baffle that fits the waveguide perfectly. Now to remove the excess material inside the opening. The tweeter and waveguide are quite large so the opening for the waveguide must be only 1/4″ or so smaller than the actual waveguide outside edge. I initially used a jig saw but later realized that was unnecessary so I chucked up a regular straight panel trimming bit in the router but changed the bearing on the top for a bearing 1/4″ larger in diameter. I got the bearing from a Whiteside rabbetting kit that comes with 7 different bearings and a 1 1/2″ rabbeting bit. Then I simply drilled a hole for the router to enter and start, set the bit depth so the bearing would run against the 1/8″ deep pattern cut and came at it from the back side of the baffle through the hole I drilled. Now the straight bit cuts a straight edge 1/4″ smaller than the pattern opening and I had a perfect waveguide mounting flange.
Now that I had my baffle cut out I got in a hurry and painted it black before really thinking it through. I now have black paint all over my baffle mounting surface. I got some partial silicone sealant to use to mount the waveguide so it could be removed if need be later and proceeded to mount the tweeter itself behind the waveguide. The waveguide was easily glued in place, picture below. Note the sanded but not yet painted again baffle. I may scrap these baffles and recut them on my CNC router when I get it set up. For now though they allow me to design a crossover and even listen to to the finished product for a while.
The tweeter mount (picture below) is easier than I thought it would be. Although the waveguide is strong, even strong enough to hold the tweeter itself, the soft glue I used to mount it combined with the weight of the tweeter over any period of time is more than enough to cause it to sag loose or even break. The maker of the waveguide recommends mounting the tweeter itself rather than relying solely on the waveguide for support so I took a hint from his mounting pictures and made some hardwood tweeter mounts. The tweeter plate is bolted to the waveguide using a set of inner mounting holes on the tweeter. The outer mounting holes are then used to screw the tweeter to the hardwood mounts which are in turn screwed to the baffle itself. I used paper shims to draw the waveguide tight up against the mounting blocks then screwed the tweeter through both the tweeter flange and the hardwood mounting blocks to the baffle beneath. This gives me 1/4″ bite into the underlying baffle to prevent the hardwood mount from moving but there is no chance it will come through the front. It’s also held in place by pressure from the paper shims so the tweeter is solidly mounted to the baffle and the waveguide is solidly mounted between them. This will prevent any undesirable tweeter or waveguide movement should the speaker be handled roughly or even dropped. Note also the hangar bold used to mount the woofer (protruding from the woofer mount flange). The hangar bolt is 1 3/4″ total length leaving a full 3/4″ of wood thread to hold it in the baffle. I was feeling in the wrong place when I screwed one of them and it went a little bit too far causing the end to protrude on the outside of the baffle a little bit. Hopefully I can dab a little glue in it, remove the hangar bolt and clamp it tight to prevent it from looking bad on the speaker side. My next task is to mount the baffle, test for leaks and check for a tight fit and begin box prep for veneer.
TESTING and XO… Unfinished.
Veneer is new to me. I’ve never used it before. I have used contact cement and I’m familiar with the general process so I’ll do my best. Box prep begins with sanding the box as smooth as possible. While I doubt any small cracks or seams will show through, high spots definitely will so I mount a piece of 60 grit to the sander and sand till it’s smooth. With the box absolutely smooth with no seams, dips, glue drops, gouges or other issues. Note the veneer is being pressed flat under the MDF scrap so it’s easier to work with.
I’m finally ready to apply contact cement and then the veneer. I cut the veneer without thinking about grain. MDF has no grain so it’s another NOOB mistake. I only messed up 1 piece (a side) however so I’ll have a seam down one of the sides, I can’t see having to recut an new piece of veneer. Grain will run front to back on all sides. Since I now have one side piece with grain running perpendicular I’ll rotate it, glue it in place then apply a strip to cover the 1″ or so area that’s not covered. Since the seam runs with the grain it will mostly likely not even be detectable without careful scrutiny but I will know. To match up the seam I’ll clamp the short piece of veneer to the box overlapping the existing veneer slightly and run a knife with a straight edge as a guide. This will produce a cut in both pieces at the exact same place. Then I should be able to remove the little strip of waste veneer and glue it’s counterpart in place on top of it. My other option is to pre-cut the extra veneer piece by clamping both in place prior to contact cement then just apply it as two separate pieces, aligning the seam.
Veneer went on without too much trouble except I didn’t keep the grain aligned the way I wanted to. But again, a NOOB mistake that I won’t make again. Got both speakers finished in two days. Now I wait 72 hours before finishing them. I may stain them or I may not, only time will tell.
But now that both are finished I’m ready to tape off the drivers and put a final coat on the baffles. Not sure yet how I’ll finish them. I may cut a hardboard grille and use grill ball and sockets or I might just leave them black without any finish. I’m not one that likes grille’s, they don’t offer any additional protection and the color the sound. Back in the days when drivers were ugly they might have served a real purpose but nowadays I can’t see the point.