Building Resonance Free Subwoofers

That's quite a project, thanks for posting.  I really envy some of those machine tools. 

@baylinor The $250 includes labor, tooling, supplies, electricity and rent if applicable. There is also a distinct danger element. Much of the work woodworkers use to do has been taken over by CNC machines with which we can not compete. For me it is the challenge of doing things that mark everything I do as handmade, things that a machine can not possibly do. 

@brunomarcs I think you envy the wrong tools. I can make anything with a band saw, a lathe and my hand tools. All those other machines just buy me time which is important but secondary. I encourage young aspiring woodworkers to start collecting hand tools, get a band saw and learn how to sharpen. The other stuff comes down the line. 

Wow, and I thought my design and build was extreme!

I originally was going to make the enclosure for the Rythmik F15HP DIY kit employing the design Danny Richie shows on his GR Research website: a double-wall box, with the space between the two walls filled with sand.

After considering the resulting weight, I instead built a dual-wall box, the inner layer MDF, the outer Baltic Birch plywood, with no space for sand. I then braced the Hell out of it: a 1.5" x 1.5" BB ply brace every 5" in every plane---front-to-back, side-to-side, and top-to-bottom. The bracing prevents the enclosure from "expanding" in reaction to the low frequencies contained in recordings, minimizing the resonance of the walls and raising the frequency of that resonance to way above the frequencies the sub reproduces.

By the way: Rythmik sells the factory-built F15HP with a 3 cu.ft enclosure, but for DIY buyers recommends a 4 cu.ft enclosure for greater minimally-greater output at very low frequencies. I did 4 cu.ft., which ended up measuring 24" H x 18" W x 24" D. Those dimensions may be manipulated in any way one chooses to create the 4 cu.ft. internal volume. I chose mine purely on aesthetic grounds.

I also built the Rythmik Audio/GR Research OB/Dipole Subwoofer, again with a double-wall MDF/Baltic Birch plywood construction. I built the OB "frame" in the "W/M" style, rather than the more common "H" style. Siegfried Linkwitz also chose the W style frame for his OB sub. I added a brace across the "open" side of the frame, ’cause why not?

@bdp24 Good for you! Keep it up.

Now, put on a bass heavy tune and turn it up. Put your hand on the sub. Any vibration you feel is audible. Ideally, you should feel none. I went through 3 other designs before developing this one including extreme mass (sand and in my case solid surface material, Corian) which did not perform at the level I was looking for. Two important concepts are the cylindrical enclosure which does not require bracing and balanced force topography, opposing drivers. You can get cylinders in all sorts of materials like Aluminum. You can get 15" aluminum pipe cut to length, figure out a way to finish the ends and plant two drivers in it, one at each end. I'm not a metal worker, but I think you could weld aluminum caps on the ends and cut  holes for the drivers then have the whole thing anodized. I thought about coopering cylinders out of hardwood and turning them round on the lathe but because my system doubles as a theater the subs had to be black and I wanted a more dramatic shape then a plain round cylinder. Size is always an issue for aesthetic reasons. I was thinking of using 15" drivers, but the enclosures would be almost double the size and since I am using 8 drivers 12" is more than enough. Because the drivers brake each other you want a driver with a BL product certainly above 20, the higher the better. This also allows you to keep the enclosure small. Rhythmic drivers are made by Dayton by the way. 

Another important concept for assessing bass performance is AB ing the system with headphones. This takes the room out of the equation and lets you know how much detail is in the recording and what the system is glossing over or adding in.