Never seen that type of design either, curious were you saw that? Found it here: http://www.diysubwoofers.org/prt/dual_chamber.htm "The primary advantage of the dual-chamber system over a simple ported system using the same driver is a further reduction in driver excursion, caused by the addition of a second port resonance within the passband of the system. This reduction in excursion results in increased power handling within the system's passband, making this type of system particularly suitable for use for drivers with low Xmax. However, because of the additional chamber and venting requirements, this system is considerably more difficult to design and build when compared to a simple vented system."