Modifies the skin of the chest and upper back to function as a vibration-producing membrane capable of generating low-frequency acoustic signals — below normal hearing range but detectable by other Drum Skin users and by vibration-sensitive equipment. The signals can carry encoded information at ranges up to 50 meters through solid structures (floors, walls) and 20 meters through air. Users develop a haptic communication language — patterns of chest-vibration that convey words, emotions, and warnings. The modification was originally developed by Zheng-Dao Bioelectric for silent communication in secure environments. It has been adopted by the Shelf's courier networks, resistance cells, and any group that needs to communicate without being overheard. The vibrations are inaudible but can be felt by anyone in direct physical contact with the user — a hug becomes a conversation.