The Jo-Ral 4C Bass Trombone Mute is crafted oa aluminum and copper to deliver an exciting tonal quality that results in near-perfect intonation. They are made of spun aluminum, and have been tested to perform evenly in all registers. Many models are also available with brass bottomsoffering a slightly warmer, moderately dark soundor with copper bottoms which produce an even warmer, darker, rich sound.