Radial bridges. Drums are about the only instrument without a bridge, why? Who knows but it certainly makes a difference when they have them to carry the head tension and allow the shell to resonate freely without holes and hardware distorting the vibrations causing high frequency odd harmonics. Peavey radial pros have an external bridge, Whitney drums have a "internal bridge" which allows the shell to actually be larger than the head giving them a deeper sound than others of the same size. Pearl free floating snares have a "bridge" on one side which still allows independent tuning of the heads. Sleishman drums are completely free floating but without some sort of a "bridge" the batter and reso heads end up at the same tension and although limiting the player from tuning the heads differently it still allows them to be very resonant. American Percussion now have drums available with a type of "external bridge" they call a sound ring which reduces stress on the shell but does not eliminate it like a true bridge would but still works extremely well as there are no holes or hardware mounted to the shell itself.