Evans Hydraulic Blue 12 Tom Batter Drumhead.With a fat & short sound, Hydraulics make starter kits sound big & tune easy. Hydraulics feature two plies that sandwich a thin layer of oil and give the fat, wet sound that was pioneered in the 70s The oil decreases sustain and enhances attack without affecting feel. Evans Hydraulic Blue 12 Tom Batter Drumhead With a fat & short sound, Hydraulics make starter kits sound big & tune easy. Hydraulics feature two plies that sandwich a thin layer of oil and give the fat, wet sound that was pioneered in the 70s The oil decreases sustain and enhances attack without affecting feel.