Internal ring dampeners are made of a plastic sponge like material. The rings have adhesive on one surface, and stick to the bottom side of the playing heads. The drum heads need to be taken off to apply the dampening rings. The internal ring system is suitable for all the drums, including the bass drum.
If the front bass drum head has a sound hole in it, then the dampening rings can be applied to the drum without removing the heads.
Internal dampening ring systems are available in various size and thicknesses to suit the drummers requirements.