Building Resonance Free Subwoofers

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.

@forrestc  Thank you Forrest. I should add that this is the fourth try at making a high performance subwoofer that can match up to an ESL. The other three were not so successful, but they were a learning experience. What I learned was that any amount of reasonable mass is not enough to control a subwoofer driver and it is extremely difficult to control the resonances of a box structure. The balance force concept is a major advance and I did not invent this myself. I first heard of it from KEF, a brilliant idea and boy does it work. The enclosure design otherwise is my own. The problem for commercial manufacturers is the complexity of the design increases the labor involved by at least a factor of 10 pushing the retail price up into the ozone. I am sure they could find some efficiencies, but still. I published the pictures so that DIYers with woodworking skills could make their own versions. The key design features are balanced force and cylindrical enclosure. My favorite thought experiment involves cutting 15" diameter aluminum pipe to length, welding on ends and feet then getting the whole affair anodized. I have no idea how to weld aluminum but the hired labor cost would be minimal. The problem with a perfectly round cylinder is it is rather bland from an aesthetic standpoint. The decagon with facetted ends is far more interesting.