Danny's original OB/Dipole sub design was a pair of 12" woofers in a W-Frame, the plans for which can be found on the GR Research site, in the specs for the woofer itself. He then designed an H-frame, which also uses two 12's in a single assembly, the plans for which are also in the 12" woofer listing. The single-woofer H-frame (six of them stacked atop one another) you have been looking at pictured here was designed only to make the subs more manageable, size and weight-wise. It wasn't done to make each woofer separate---the stack of six single-woofer H-frames could just as easily have been made as three dual-woofer H-frames, but each would have then been twice as big and heavy.