Pearl Masters BCX924XSP/C357 4-Piece Drum Shell Pack, Golden Bronze Glitter 6PLY 7.5MM Birch Shell Bass Drums feature Pearl's SP30 Spurs with retractable spike tip for slip resistance Renown for its "Natural EQ" sound in studio and live applications, Birch's boosted low and high tones power through amplification and make the drummer's voice heard with barking clarity.