The Helios FSD 125 is a sturdy flexible duct made from aluminium casing. Its inner surface is perforated to securely hold a mineral wool lining approximately 58.5 mm thick, providing effective sound insulation. The duct features spigots on both ends for easy connection to other ducts or attachment to fans using a pipe clamp connector (BM). This duct has a pressure drop roughly four times higher than equivalent rigid ducts.