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CNC'ing New Subwoofers

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In 2015 I had a home theater mostly built out from Craigslist finds over the years, but my pride and joy was my Infinity SW-12 subwoofer. Then Interstellar came out on Blu-ray, and I saught to replicate that epic IMAX experience at home. I queued up the rocket launch scene and went for it. The engines roared to life and my body shook as if I was in the cockpit, riding this rocket into the darkness of space with my fellow astronauts. It was truly immersive — but then a sense I didn’t expect kicked in. The acrid smell of burning filled the room, but I didn’t have smell-o-vision. The shaking and rumbling stopped despite the rocket still racing through the atmosphere. My invincible Infinity SW-12 had reached its limit and fried itself.


DIYing The First Subwoofers

I started researching replacements and quickly stumbled upon the Martysub. The more I read, the more I wanted to try my hand at building the subs. I didn’t have quite the space necessary for the full Martysub design, so I went with the smaller Micromarty sub design, and then tweaked the plans to have a skinnier + taller cabinet width of 18” and 24” of height. I borrowed some tools from my family members, ordered the new drivers, and got to work.

Comparing the old burnt out 12” Infinity driver to the new 15” Daytons.

I spent the next couple weeks cutting out every piece, setting up glue jobs, and finshing the pair of subwoofers. This was my first major woodworking project and despite it being all straight cuts and square joints, it still came out a bit wonky.

The first cabinet with the final piece being glued in place. Note the sides don’t really line up. There’s also a distinct lack of bracing, with only one or two pieces in the middle spanning gaps.

I finished one cabinet before really diving into the next one, so I had some time to rethink things. For the next one I went with a window brace since I had some more material left over.

Not a great pic, but you can see the window brace, basically just little sections cut out of a solid piece of MDF.

Ultimately after a lot of sanding down they looked alright. I was just going to paint them black anyway.

I fired them up and put them through the same paces as the Infinity SW-12 the Interstellar rocket launch, and they blew me away. It easily reached 102 dB, and I believe this is with just a basic Behringer A500 amplifier giving 175 watts per channel — I later upgraded to a Crown XLS2500 which is rated for 775 watts per channel!

New House, New Theater

Towards the end of 2015 we received a letter adressed to the occupants of the home notifying us that the house was under foreclosure. Our landlord hadn’t been paying the mortgage. I’d been there for 3 years and I still wonder to this day what he did with my rent money. I set out to purchase a house for myself, and one of the key factors was a detached single family home with a good sized basement to start building out my dream theater.

A nice blank slate.

It took another few years to finally come to fruition, definitely longer than I expected, but it was absolutely worth it in the end. Many months of planning went into the theater, and many more to actually build it. I remember when HBO announced Game of Thrones Season 7 start date is when I really kicked into high gear with working on the theater to try and finish it in time. August 19th, 2017 I moved the couches into the room, and 20th had people over for a GoT viewing party. It sounded amazing and it was so incredible to be done. But this was only the beginning.

Hey look, Interstellar again! V1 of the theater re-used a cheap 92” screen from SilverTicket.

Eventually I stepped up to a 120” acoustically transparent Center Stage XD screen from Seymour AV, and was able to place the DIYSoundGroup HTM-12’s behind the screen. The subwoofers still hung out in front though.

Daniel Craig shows off the new extra wide screen.

Things were good. The theater was by far the best system I had ever put together, and the pair of subwoofers kicked hard. But… not as hard as I would have expected. The DIYSoundGroup speakers I picked for the main speakers and surround speakers had no practical limits, they could play cleanly to absolutely deafening levels. I tried a few times with various movies with hearing protection and it was incredible, but the subs couldn’t quite keep up. It was loud, sure, but the pair of subs would hit their limits within listening volumes… and that just wasn’t enough. Plus they were just… there. The point of going with an acoustically transparent screen was to have a totally clean view up front, just you and the movie. Right now it was you, the movie, and those two massive boxes up front.

As luck would have it, in September 2018 the basement flooded. I managed to stay on top of the water as it came in (staying up from 1:30 AM through 5 AM one night with a shop-vac…) so no equipment was damaged, but the walls were shot. It was a bit of a blessing in disguise though as I ended up redoing the front false wall to make space for the subwoofers, and it just so worked out that I could fit another two subwoofers up front too. I finished rebuilding around March 2019.

With the screen removed, everything’s exposed. The subwoofer stacks fit nicely between the main speakers, and the 18” width I had picked before for my last place happened to be perfect.

Look ma, no visible speakers!

Designing the New Subwoofers

Alright, so I knew I wanted two more subwoofers, but I also didn’t want to make these again by hand the same way I had before. While building the theater I had purchased a few flat-packs from DIYSoundGroup for the HTM-12 and Volt-10’s, and was amazed at the precision and how they came together so effortlessly.

The parts for the Volt-10’s. My cat did not appreciate more speakers entering the home.

The HTM-12 enclosure dry fitting together. The dado grooves cut into the enclosure meant every piece just snapped right in which made gluing up the cabinet so much easier.

I wanted to replicate the same precision with my next build, and a friend of mine had access to a CNC through work. He offered to help out if I could create the models and tool paths necessary and suggested using Fusion 360. I fired it up and dusted off my CAD skills I hadn’t touched in 10 years, then stumbled through creating a model from scratch.

Ultimately the design is still fundamentally the same as the subwoofers I built a few years prior, the external dimensions and port tuning are all identical. But the inside is where things differ the most, and it now features a super strong internal bracing structure.

A view without the braces in place, you can see each the dado grooves carved into each side panel and the base.

A view with the braces, 5 interlocking pieces that provide a ton of strength to help the cabinet from resonating.

A view with everything in place. The nice wood texture is just for show since it’ll all be painted black.

I was also able to use Fusion 360’s animation system to virtually assemble the pieces. This was crucial in understanding how the internal bracing structure would come together since it was a bit of a puzzle,

I rendered the assembly animation as a video to be able to reference it easily later on my phone.

Eventually I had to layout all my pieces to prepare them for machining. I watched a few YouTube videos to try and figure out how to do this, and I was a little surprised at how involved this actually was. In the past I’ve used a program that took in parameters about your final pieces and it could programatically figure out the best placement for cuts on a piece of stock material, but I didn’t find such a feature in Fusion. (It’s possible I missed it though.)

I ended up duplicating the models and then rearranging all the panels on two virtual 4’x8’ sheets of 3/4” material, manually trying to place them around to make the most of the material. Since I was planning on building two sets of subwoofers, I duplicated some of the pieces on one sheet and also included any double sided pieces on that piece.

The underside of the pieces shows some of the dado grooves on a few double sided pieces.

Getting CNC Certified and VCarving

Towards the end of 2019 when I was finally ready to start manufacturing this project I went to a local makerspace and took the CNC certification course. It was about 5 hours or so and covered creating a project in VCarve Pro, defining toolpaths, and then operating the CNC machine from start to finish. The makerspace had a pair a Shopbots for use, so everything was based on generating G-code for that system.

Conveniently VCarve Pro offers makerspace licenses for home use so makers can work on their project toolpaths at home, then pop into a makerspace to export the final G-code for their machine. I downloaded it and got to work exporting my project from Fusion 360 over.

My first challenge was simply exporting from Fusion to VCarve. I tried almost every format combination possible, eventually landing on DXF I believe. Other formats would either fail to import or just generate incomprehensible nonsense.

Eventually I was able to import correctly and start creating the toolpaths neccessary. I chose a 1/4” bit to start with, and after I created all the toolpaths I ran the estimate machining time. It came back with an astonishing 6 hour estimate… I was expecting something like 30 minutes! I then swapped the bit out for a 1/2” bit, re-ran calcuations and it came back with a much more sane 40 minutes.

VCarve at had a nice 3D preview mode that made it clear what I was creating. I ended up working with someone at the makerspace to tweak my file on the day I went in and massively reduced the number of tabs. (Unfortunately I do not have that version of the file handy.)

Firing up the CNC

Finally, it was time to turn turn my virtual drawings into something real! I took a day off work and reserved the CNC machine at the makerspace, then headed in. I purchased my material there, though unfortunately they didn’t have MDF available (which was my first choice.) Instead I ended up with baltic birch plywood. Since I had planned for MDF though, I had purchased my own router bit — a 1/2” Whiteside upcut bit. This created some problems.

I worked with the shop hand to double check my final VCarve settings, and we ended up tweaking a bunch of feed rate settings and things based on his experience with the machines. We loaded up the material and fired it up.

It was immensely satisfying watching this robot dance around and see my project come to life after nearly a year of planning.

Working on the second layout, the double-sided one. Note the alignment pegs on the edges so I could flip the piece and align it back up correctly. These didn’t quite line up as well as I had hoped which created some problems later during assembly. I’m not sure if wooden dowel was the best choice here and would revisit this method again.

Unfortunately since I had purchased an upcut bit intended for something softer like MDF, I ended up with a lot of chip out. I don’t think the makerspace had a downcut 1/2” bit either, so I was a bit stuck. The chip out also got worse as the day went on so I think the bit may have been dulling.

After 4 hours of CNCing, I had every piece cut and loaded on a cart ready to bring to my car.

Assembly time!

Once I got everything home, I started eagerly assembling the pieces. But I hit a few snags. The chip out necessitated sanding everything down to remove the burrs, but that was doable. The biggest issue was one was some of the pieces fit just fine into some dados, but didn’t fit into other dados. It was really puzzling, but I suspect one of the Shopbot axis calibrations was off, leading to a minor scale down effect. It was as if any dado that was cut on one axis came out fine, but on the other axis it was 99% the correct size.

I ended up having to go over a lot of the dado grooves with a hand held portable router and widen them enough to accept the pieces, which was a bit unfortunate and led to a lot more work that I didn’t expect.

The inner support structure comes together. I ended up having to sand off a lot of the hard edges too to make a bit of a wedge shape so things could slide into place.

I unfortunately ran into alignment issues as well. These were some of the double sided pieces that required flipping the stock in the CNC bed, then re-aligning the stock. Since that wasn’t perfect, the grooves ended up misaligned and needed some more reworking with the handheld router.

Several days later, one assembled subwoofer!

Two assembled subwoofers! I also took a roundover bit to most of the edges, and sanded down any mismatched surfaces. The top subwoofer needed a lot more sanding, thus the exposed plywood layer.

The finished subwoofer cabinets felt super strong and knocking the cabinets with your hand it feels like you’re practically just hitting a rock. Once installed in my theater the total 4 subwoofers have provided all the bass I could ever want.

I set them all up, rewired my Crown XLS2500 amps to power each pair of subs in bridged mode, and re-ran Dirac Live room calibrations. The subwoofers after correction are able to provide a relatively flat response from 80 hz all the way down to 15 hz, which is just lovely.