Evans Rock Tom Pack (10 inch, 12 inch, 16 inch) glass (clear) Hydraulic two-ply drum heads feature a thin film of oil between the plies that suppresses unwanted overtones. A classic sound, short in sustain. Tension them tight for a real thump or loose for a more slappy sound. Evans Hydraulic series features two plies of thin film offering maximum durability, a fat, deep and short sound pioneered by early 70's rock and roll. Hydraulics are famously easy to tune, making them a perfect fix for troublesome drums and younger players.