T24 pair (CNC cut)

[commander_dan]

This project began after I finished my T30/OT12 project back in 2015.. it was completed with the help of 3 separate people over the span of about 18 months, whenever we had a spare moment and or remembered to do work on it. With a bit of a push toward the end but without rushing, I finally managed to finish these on NYE 2017! A couple of friends had requested help to build their own BFM sub after hearing the T30's, so we sat down and worked out what would be best to build. The plan was to build a pair of the same sub which would suit both people, for ease of building. Originally T18 was considered, and this would probably have sufficed since one friend only listens to records and the other wanted something for his HT but also the odd party. It was for this reason that we settled on T24 (party friend has a tendency to turn up, so I wanted some headroom), loaded with 3010LF, at 18" wide. I realize this is overkill for what they will primarily be used for, but I'm an advocate of the phrase "It's better to have it and not need it, than to need it and not have it"

Once the decision was made, plans were bought for both people as well as a pair of 3010LFs from the US, and we promptly set about converting inches to mm for my friend who has access to a 5 axis CNC router (furniture company). We selected a lightweight structural 5 ply wood, and a few weeks later received our pre-cut flat packs! I checked each panel for accuracy and as expected, all panels were accurate to the mm. I oversized the braces deliberately, opting to sand them back if needed. We got the panel layouts marked up, and the baffles cut, drilled and blocked. From this point of the build, things progressed pretty slowly... a panel here, and a panel there, and each panel had to be straightened with jigs because the ply had warped significantly. It made me glad I chose marine ply for the T30's, that stuff hardly warped at all (but it's heavy).

[commander_dan]

Next came panels 1-4, note the extreme jigging and de-warping techniques used. This ply was terrible for warp, but not too bad once installed.

[commander_dan]

The time in between when panel 4 and panel 5 was installed was probably something like 6 months, life got in the way somewhat. Panel 5 was installed in August 2017. From here, the build progressed considerably faster and the goal was set to finish them by Christmas. Panel 5 was warped pretty badly even with the 5/6 brace in place but we were able to fix that before the second side went on.

[commander_dan]

Panels 5, 6 and 7 and braces installed. Note the cut in the brace in the 2nd pic. We slid that brace in too hard (not enough sanding) and it bowed P7 badly, but we only realized once we were installing P8 and the PL was cured. So we ran a handsaw through the brace and re-PL'd it which brought it back into spec.

[commander_dan]

Panels 8, 9, 10 and 11. So far the cabs had progressed without major issues, but when we came to install P11, we realized it wasn't cut to the right length, protruding by about 1/2". We must have gotten the specs wrong when we sent them to the CNC, and I was annoyed initially because I couldn't replicate the 50' cut easily. But it was easily solved, I ran them through the table saw on the 45' side, problem solved.

[commander_dan]

Final brace, jack plate and handles. Note the different colour plywood for the final brace. Yep, sent the wrong dimensions for those too! They were identical, but were miles off what they should have been. So I cut a couple of new braces from some scrap we had available, which happened to be marine play of some sort. After some deliberation about where to put the jack plate, it was decided to be put at the bottom of P9 in the dead zone. Mainly because we wanted it central, and being a single brace per panel cab, central meant there was a brace in the way. The small jack plates fit nicely along the bottom of the cabs. That was also where we decided to put the handle holds, the thinking being that to move the cab, one would tip it onto its top and lift using the handle and the final brace. As it turns out, these came out pretty light anyways and handles were probably not even needed. Speaker wire installed by now as well.

[commander_dan]

Access panel flanges, and a shot of the what the cabs would look like very soon!

[commander_dan]

At this point I whipped up a box for one of the T24's plate amps. Both were planned to be run off plate amps initially, and this one is a 300W amp. I cut some ventilation in, though it probably wasn't necessary with the heat sink on the top. The other box was done a few days out from Christmas.

[commander_dan]

Back to the T24's and it was time for the drivers to go in and the second sides to finally be installed! Note the temporary braces fitted up with "pulling" wires for after the second side had cured. The back panels were still warped/bowed even with the braces in, so we used this technique for both cabs.

[commander_dan]

The next day I routed the edges and sanded both cabs back. One was planned to have a duratex finish, the other a stain/varnish. I sanded the duratex cab to 80 grit and the varnish cab to 180. Then blew the dust off with compressed air and piled them into the car to take back to my place for testing. The fit nicely on the back seat of my car, unlike the T30s!

[commander_dan]

Both cabs passed their air leak tests with flying colours I fired up a 25hz tone and got a 1/4" excursion with the access panel off, and listened around the driver seal with a plastic tube "stethoscope". Heard no leaks, so ran the stethoscope along all the joints I could find, then put the access panels back on and did the same on the exterior. Once I was satisfied, each cab took turns to sit atop my unplugged T30's in the corner for a program test. I tested them both individually with their correct HP @ 45hz with some volume and they sounded great, and then I rolled the HP back to 30hz to see what their response was corner loaded. Pretty decent. Nothing on the T30's, as expected. In fact, their natural response was very close to the advertised graph (using my ears, I didn't measure with a mic). And then I had to hear both of them together, since they would be living separate lives.. this also doubled as a phase test. I did a quick setup on the DCX to tune them for the room, and they sounded very similar to the T30s, nice clean fundamental bass notes, as expected, just not quite as loud and not nearly low enough for my tastes.. I took a couple of pics to show the size difference between the T24 and T30.

[commander_dan]

So with the testing done, and about 3 days out from Christmas, it was time to get the duratex coat on. I had a small amount left over from previous builds but I was a little worried it wouldn't be enough so I primed the T24 with matt black paint in the hope that it would extend the duratex a little. I think this worked, because I had enough duratex for two full coats including mouth of the T24, and also the plate amp box. I now have a tiny bit left which will be for touch ups, some of the T30's are already needing some.

[commander_dan]

I was pretty happy with the finish I ended up with, considering it was a hot day. I've found you have to move fast with roll on duratex, get the coat thick and even with as few strokes with the roller as possible, and then role once for the finished texture. Any more than this and the texture starts to become rougher. I prefer a smoother textured finish. The corners were the last piece of the puzzle. I had left over Penn Elcom corner chevrons of the large variety. Unfortunately, where I had cut the handle holds in was just too close to the edge of the cab so the chevron partially covered the handle on the bottom side. I ended up screwing the chevron to some scrap wood, and used the same diameter hole saw bit that I used for the handles to cut the required semi circle in the two chevrons required. After sanding them smooth, they fit perfectly.

[commander_dan]

And that's about it! Final resting place of the varnish cab below. We need to isolate or float the record player as it's causing feedback from a certain volume with the sub in the corner and so close. Will work on solving that in the next few days. The black T24 is about to be setup in a HT at the other friends place, but it did its first party NYE, and was a great success.

[Grant Bunter]

Awesome!
Thanks Dan...