Building Resonance Free Subwoofers

@phusis No argument from me. Dirty bass messes up everything, no bass kills any elusion of a live performance. I might point out that you are doing exactly the same as me. You use a slower crossover but counter by lowering the crossover point to keep the sub out of the midrange. Exactly what slope sounds best I think would depend. on the main speakers you are using. Such steep filters are not good at higher frequencies. I cross from my bass transformer to the treble transformer at 500 Hz 2nd order. I stepped into the twilight zone by bi amping the Sound Lab's transformers using a digital electronic crossover. My initial experience included totally trashing a pair of JC 1s, right into the dumpster. 

I do not think I am adverse to large loudspeakers, but I only have so much room on a 16 foot wall which is mostly taken up by a 113" diagonal screen and two 36" wide loudspeakers. Two of the subwoofers had to go outside the panels to create the line source and I only had 15 " left for each sub. 12" drivers just made it!

The title is just a tad misleading. There is no such structure that’s "Resonance Free". If a structure has mass and stiffness associated with it, there are resonant frequencies. You may have pushed it out of the sub’s operational range or not (test or sim tool could let you know), but, there are some resonances around..

You can either dump your CAD and material properties for the wood (modulus, poisson’s ratio, etc) in some FEA tool and all resonant frequencies can be computed. You will need to know how to work with FEA tools in this case.

Or get a DAQ and a couple of accelerometers... Start sweeping the driver and determine if any resonances can be caught (coarse). You will need to know some basics of signal processing in this case..

@deep_333 technically you are correct, however if you can not hear or feel any resonance, they are from a human sensory perspective, resonance free. Any resonance frequency is well beyond the subs operational limit and extremely well damped. I am speaking about the enclosure only. The drivers do have their own resonance frequency which I think is in the 50 hz region. I do not have the equipment to measure it. Run with a flat sine sweep at 6 inches they are down 3 dB at 20 Hz with a very smooth curve. Using room control and digital EQ they are plus 15 dB at 20 Hz, remain flat to 60 Hx then drop smoothly to zero dB at 100 Hz. This is as measured at the listening position and by design. The crossover is at 100 Hz 10th order.  As long as the drivers are capable and there is ample power you can make a sub do anything you want with digital signal processing. 

Great job, Mike!  Your fine engineering, design, and workmanship inspires me to get out to my workshop start a new project.  But after seeing your workshop, I think that I need to do more than just a few updates first to it to say the the least.   Everything looks great, and I would sure love to hear it one day.