Building Resonance Free Subwoofers

@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. 

How does the sub you made measure for distortion vs the standard sub box shaped enclosure.

@daledeee1 Good question. I know they are very low, but I do not have the equipment to measure it. I can tell you that all other things being equal, among the finest drivers, the primary source of distortion is the size of the driver. As the cone moves farther the suspension starts to get stiffer until the driver can go no further. That nonlinearity creates distortion. Larger drivers do not have to travel as far to create a given volume. My subs, with two 12" drivers, are equal to one 15" driver in distortion levels. The drivers I am using are Dayton's Reference Series "low distortion" drivers for smaller enclosure volumes. They have very strong motors, have very light and stiff aluminum cones and are extremely well vented. They are without question the finest drivers I could find for my design and money was not a factor at all. Having said all that the greatest source of "distortion" in subwoofers is the enclosure. If you have subs, turn up the volume with a bass heavy number and put your hand on the enclosure. Any vibration you feel is audible distortion. These subwoofers do not vibrate at all. Another interesting characteristic is you can hear each bass note clearly on records where it was previously difficult. You know there is bass there but you could not discern the note clearly. The large surfaces you have in a box enclosure are resonance prone. In order to get the resonance frequencies up above the subs operating range you have to use internal bracing. Even with bracing the forces generated by a single driver are going to cause even extremely heavy enclosures to vibrate a lesson I learned by building 200 lb Corian enclosures.