Building Resonance Free Subwoofers

$250/hour! I got in the wrong businesses. Beautiful workshop. Nothing like enjoying the fruit of your labor.

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?