Exclusive to the Evans line, Hydraulic heads have a layer of oil between the 2 plies to give you the fat, wet sound that was pioneered in the 70's. The oil acts as a natural muffler to decrease sustain and increase attack without affecting the feel of the head, as internal and external mufflers often do.
Hydraulics have a solid attack and a controlled, quick sustain, and are best suited for rock. They are available in Glass, Black, and Blue, and can be used both live and in studio. A coated snare Hydraulic drumhead is also available for increased articulation and use with brushes.