Bronze is an alloy of copper and tin, usually mixed with other metals (Zinc, in the case of the trumpet).
Bronze is different, in that its heavy, yet bright sounding. The tin supposedly gives the sound a "sparkle," which would give the impression to some of a brighter sound. If you ever flick your fingers on a bronze bell, then a brass bell, you notice the bronze bell has a *much* ringier sound when pinged.
HERES a link to check out the sounds three different trumpets being pinged: A Bach, Benge, Besson, and Blackburn (ambronze, which is the same thing, pretty much).
http://www.whc.net/rjones/jlynch/cryo/index.html
Van