Aluminum material is less expensive than copper and much lighter; making them much easier to lift and transport.  The bases are a post/clutch pedal design with a rod link that allows the bowl to be removed from the base.  Needless to say, as a percussionist, sometimes, the less weight you have to carry around, the better.  They are easy to move, and still have the quality sound you would expect from metal bowl drums.  I have also fitted them with the original version Planet Percussion tuning gauges.  23", 25", 28", & 30".