Left Channel (5′ full height)
This is a CBT36 line array speaker. It was built from a kit that contains unfinished speaker cabinets, drivers, and electronic components suppied by Parts Express. The design is by Don Keele of Audio Artistry and is unique in it’s use of five shaded line array banks that produce an impressively even stereo image throughout the listening room. “CBT” stands for Constant Beamwidth Transducer. The “36” refers to the number of 3.5″ full range drivers in the speaker pair. Each speaker contains 90 drivers (18 x 3.5″ full range drivers and 72 modular tweeters). The speakers are bi-amped, running off of a modular set of four 250 watt amplifiers and a programmable external speaker management system for cossovers between the tweeters, full ranges, and subwoofer.
Most of my custom work focused around custom cabinet finishing and hardware. All of the examples I found of finished CBT36 cabinets showed glossy coats of paint on the front panels, which didn’t appeal to me. I wanted more of an old school look with mahogany and brass bolts. I stained and buffed the wood veneer with fine grit sanding, steel and carnuba wax. The end result looks sort of like shiny brushed copper at a distance.
The unfinished cabinets are shipped with the front panels bolted on for safer shipping. The undersides of the front panels are extremely delicate where they have been precision machined to accept the tweeter arrays at very tight tolerances. In some places the MDF is no more than a millimeter thick, and can be easily damaged by pressing too hard into it. Luckily, these parts of the front panels get protected both by hardware and the fact that they are all internal structures.
Notice I received two identical right enclosures. Parts Express was extremely helpful with resolving this issue quickly so that I could start the project as soon as possible. Although the front panels are interchangeable (before mounting), the cabinet enclosures are not. Left and right cabinets have pre-drilled holes and hardware attached at specific locations so that the full range drivers line up inside the tweeter arrays. This becomes important for Phase correction when the external crossover is calibrated for left and right speaker arrays.
This angle shows the pre-drilled holes at the bottom of the cabinets which will mount the speaker cabinets to thier bases. Inside each cabinet is permanently fixed hardware that accepts the bolt threads for all the attached components (mounting brackets, cable routing, and panel bolts). Also visible are the internal walls that isolate the internal tweeter sections from the full range driver sections. This is needed so the air pressure generated by the full range drivers does not interfere with the operation of the tweeter modules. The final cabinets use additional sealing foam (provided in the kit) to completely seal off the airflow between the two sections after the front panels are re-installed with all the drivers attached.
The kit also suggests using speaker putty to seal the cable routing holes, but I substituted hot glue instead because I have had problems with speaker putty staying put and giving an airtight seal if there is no compression involved.
The front panels have to be completely finished before any of the speaker work starts. My veneering process involved a bit prep work before that could begin. I created a guide for outlining each of the 36 circular panel inserts for the full range driver openings. The work surface, materials, and tools for doing this are seen here. The adhesive mahogany veneer can be ordered through Rockler Woodworking and Hardware. I applied a heat source after mounting to cure the adhesive to the porous MDF. The bladed compass scored the veneer surface before cutting with the Exacto blade. The razor blade was used to trim the veneer after mounting it.
Front veneer cutting
The rest of the veneering was prepared by transferring the front panel hole positions to the veneer using an airbrush. I sprayed black ink carefully around all of the holes with the panel in a flipped position. To keep the panel firmly applied to the veneer while spraying I used a series of 10lb lead weights I could move and position as I sprayed without shifting the panel.
I cut slightly inside the transferred pattern on the veneer to keep some excess material for mounting. This allowed for small corrections in alignment during mounting and final trimming. Notice the six uncut tweeter holes in the bottom left of the image. These show the airbrush pattern that was transferred before cutting. I used the red hole punch to make the smaller cuts for the tweeter holes and bolt holes, and the Exacto blade to make the larger cuts for the full range driver holes. The Optivisor was handy for precision work. The blue towel protected my hand from the bolt at the top of the hole punch handle. Typically the hole punch is struck with a hammer, but I got better results through applying even downward pressure with my body.
This is a close up of the trimming process I used for the internal circular veneer sections on the front panels. I had to carefully follow cuts perpendicular to the grain of the wood so as not to cut into the MDF. As I approached the midpoint where the grain aligned with the cut I had to use extra caution not to overcut the veneer and damage the MDF. This is where I relied upon my Optivisor and a steady hand to methodically complete each cut. The second cut to the interior of the veneer was even trickier because there was no flat surface to brace the cutting tool against, so I had to make each of those cuts with the Exacto blade instead of the razor blade. A portion of a fully finished cut can bee seen in the upper right of the image. Each section took approximately 30 minutes to complete, so all 36 sections represent about 18 hours of work. I didn’t have to worry about rough edges at this point because the top veneer would cover the cut edges of the circular sections.
The mounting and trimming of the front panel veneer was done in the same fashion as the circular sections. Also notice that I folded the veneer around the edge of the front panel to follow the rounded cut on the edge of the front panel. A hot iron was run over all surfaces to cure the veneer adhesive, especially along the rounded edges to relax the bend and keep the veneer from splintering at the fold. All trimming was done with an Exacto blade because I could not rest the blade or the panel on a flat surface to make accurate cuts. All rough edges can be seen because I have not done sanding on any of the surfaces at this point. This trimming process also represents about 18 hours of work.
The smaller tweeter holes allowed for a faster edging process with a power drill and grinding bit. I used the grinding bit to remove the excess veneer on each of the holes, enlarging them to the point they were before the veneer was mounted. This grinding process also had the added benefit of pushing the veneer edge down into the holes to cover the interior MDF. Extreme caution needed to be used to avoid punching completely through and removing too much material, so I took my time on each speaker hole (about 5 minutes). Five minutes may not seem like a lot of time, but it added up to about 12 hours work for 144 holes, and was the most nerve racking part of the prep stage because it was very tempting to rush through this part. I had already invested about 50 hours and a considerable amount of money on the kit and materials at this point. One mistake on any tweeter hole would have ruined the entire project.
Here is a close-up of a panel section after a few passes in the sanding process. Most of the rough surfaces have been removed and I have begun to smooth over the seems to blend them as much as possible into one another by removing any material that visually shows any edge overlap. There was not a lot of risk at this stage because no power tools were used. A simple sanding block took care of the flat surfaces and the rest was done with hand sanding.
The final finishing stage involved a light staining of the mahogany to bring out the texture in the woodgrain. That was followed by a pass of clear poly coat that I took all the way back down to bare wood with 600 grit and 1200 grit sand paper. The poly coat still filled the pores in the woodgrain so that final sanding could achieve as smooth and uniform a natural surface as possible without a loss of grain texture, which I didn’t want. Final buffing of the wood surface was done with carnuba wax. This is typically used to protect pool cues, which need to be clean and smooth for best results. It makes any wood surface slippery to the touch, and protects it from dirt and dust. It also gives the wood a very pleasant camphor smell to it.
At this stage the front of the panels are completely finished and flipped over. I have transferred them to another workstation with a felt surface that protects them from damage or abrasion as I work on mounting and wiring all the driver arrays. The kit comes with a manual and wiring diagram, seen in the center, so I’ll skip over most of the wiring and speaker installation process since it can be read in the manual. The local electronics store also gave some great advice for quick release connects that may come in handy if I ever have to do any maintenance work on these beasts.
Hot glue substitute
The CBT36 Line array manual instructed me to use hot glue to route wires and mount resistors on the sides of driver magnets and circuit boards, but I read in some forums that a better and more secure way to do this was with zip ties and and mounting brackets. The whole idea behind this type of mounting is to eliminate any type of vibration in any of the components that might interfere with playback. I took this a step further and mounted adhesive speaker foam in between any hard surfaces that could transfer vibration and loosen components.
The black bolts that came in the original kit I noticed had a very narrow flange and bit into the MDF they were holding down. I didn’t want to damage the work that I had done on the front panels by installing them, so I found replacements at Home Depot that had a much wider flange and the added bonus of the brassy finish I was looking for. Additionally, I added a rubber gasket under the flange to prevent loosening, air leakage, vibration, and biting into the wood. The gasket compresses, filling the area between the hole and flange with a protective air tight seal. The painful part of this hardware substitution was that all 40 bolts had to be ground down to the proper length, so they didn’t bottom out inside the cabinet. That involved about 12 minutes of work on 40 bolts for about 8 more hours of work. With everything added up, it probably took me about 200 hours of work to complete the project including speaker testing.
Here is a final image of the left CBT36 Line array being tested with a 150 watt test amplifier and my iPad running a frequency modulation and testing app. Not all of the front panel bolts are installed because I was still making final checks in wiring and enclosure testing before sealing up the cabinets permanently.