I dunno, but by the look of 'em both I'd say they were dreamed up by one of those "designers" off daytime TV home improvement shows and knocked up out of MDF and an old colander or two!
Why not just look it up on their website and read what they say?.....
"It is the first of the PMC range to feature the radically engineered RadialT bass unit. Its colossal exo-skeleton provides exceptional physical and thermal stability by ensuring the cone has a perfect rigid structure to produce enormous excursions and the coil is kept cool by heat sinking properties created by the exposed fins."