Copper, Bronze, and Brass are more dense than steel but not as stiff a metal. Sound travels through metal also and the velocity is highest in aluminum and steel (5,200-6000m/secc) and much slower through the denser Copper and Brass 4,300-4,700 m/sec. There must be some push/pull between mass/density and stiffness in regard to shell fundamental. The aluminum and steel higher and copper, brass, and bronze much lower. I've often wondered if you sprayed an rough texture or acrylic or lacquer coating on inside of a steel shell would it warm it up a bit? Like hammering produces alterations on shell surface that warms it up.