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In the photos you can see that the internal width and depth are 20.5”. The internal height not shown is 21.5”. With these dimensions minus the amp compartment and bracing, the internal volume is about 4.5 ft^3. Technically this makes the system an infinite baffle system. I believe Small defines infinite baffle to be where the ratio Vas and Vb is less than 3. In this case Vb is about the same as Vas or slightly larger. Given that the Qt of the Bully driver is about .5+, it is ideal for this IB alignment.
The box is first built using 1” MDF using dado joints as shown. The internal braces are made with ¾” MDF and dado jointed into the box walls. Note that the rear brace also serves as the back of the amplifier compartment. The spacing of the amplifier compartment side walls are 12” on the outside. This needs to match your plate amp so check it’s mounting size.
The rectangular hole for the amplifier and the mounting hole for the woofer were both cut after most of these photos were taken. Both are recessed.
The photos reveal some mistakes made. I somehow cut the internal braces too short. As you can see in some photos, there is nothing there to slide into the dado cut on the bottom piece (the box is upside down in these photos) and then in other photos you can see where another half inch has been added by gluing a strip on the edge of the braces. I had not cut the dados in the bottom for the side pieces of the amplifier compartment so I just left those and let them butt joint to the bottom.
This is basically a very simple box and it’s not critical that you build it exactly as I have. There are probably better methods of bracing etc that you may come up with. The critical thing is that the internal volume be correct, that the box be very rigid, and that it has no leaks.
Once the 1” MDF box is assembled, the ¾” MDF with veneer is then added along with the ¾” Baltic Birch bottom. First I cut and glue on the Baltic bottom piece. It is simply cut to match the 22.5 x 22.5 in size of the box. Then I cut the ¾” veneered MDF front and rear pieces and glue them on the box. Next I cut the ¾” side pieces about 1.5” smaller than they should be to cover the entire depth of the box. The height of the pieces however does cover the entire height of the box. I then cut a dado along the front and rear edges of the side pieces and a matching dado in some solid wood trim, in this case, Cherry. These strips are glued onto the side pieces extending the width so that it fits on the side of the box and covers from front to rear. The idea being to provide a solid wood edge that can be rounded over with a router. Once glued in place the edges of the front and rear are thus covered with this rounded over solid wood piece. The top is made so that it is about 1.5” too small all the way around and solid wood edges are added to all four sides. These are rounded over and when placed on the top of the box, cover the top edges of the sides, front, and rear pieces. You now have a box with rounded edges and no raw MDF showing. The bottom which is Baltic Birch is just painted black. I installed casters on mine because the thing probably weighs a good 160 lbs or more.
According to Bully, the woofer will also work well in as little as 2.5 ft^3 but I prefer larger as opposed to smaller when it comes to sealed box volume. You do not increase power handling by putting a woofer in a smaller box. All you do is roll it off at a higher frequency. To produce a certain output level at a certain frequency, you must move a certain amount of air. There is no magic in a smaller box that enables it to have more low frequency output without bottoming the cone. If you move your woofer to a smaller box and find it no longer bottoms out on you then this is because it also does not play as low as it did in the larger box and because it's less efficient, not because you have increased it’s power handling. ( A larger box has a more gradual roll off on the low end) Yes, you can now play a particular track louder than before without bottoming so perhaps you have increased, “apparent power handling” but the real fact of the matter is that your smaller box is acting as a high pass filter not to mention that it has reduced efficiency. What’s worse is that you have also increased the total Q or “Qtc” of the system. It’s a much better thing to leave a woofer in a larger box and address the bottoming issue with more appropriate methods. The larger box will have better efficiency, and better damping. When you increase the restoring force by making the box smaller you just make the amplifiers job of controlling the cone motion all that much harder. The same is true for drivers with a very small Vas. While the Thiele Small parameters suggest proper damping and performance in a small box, the real performance is not the same. These compact systems don’t take up much space but they have a lot of difficulty resolving detail in bass. It’s what I call the, “one note fits all” sound. Seems like no matter what note is played, you get a lot of content of that one note that the tight restoring force likes to play.