The Meinl Sea Drum features little steel balls sealed between its two drum heads, one synthetic and the other natural goatskin. This means you can choose between two completely different timbres within the same drum. The Sea Drum"s sealable filling port allows you to add or subtract the amount of steel balls inside.16" or 20" Rubber wood frame Hand-selected goat skin head and synthetic head Sealable filling port allows you to control the amount of steel balls inside