Grotta Sonora’s friction balls are designed to bring those high cosmic sounds from your gong. The set includes 4.5, 5 & 6.5 cm diameters. Built with a smooth rubber head and a clear PVC flexible stick which acts as a resonator: by adjusting hand placement along the stick, you can fine-tune the resonance for lower or higher notes. Includes set of 3 friction mallets - black