Manufacturer page.

The main materials of circular sound attenuators are galvanized steel (Z275) and mineral wool. The SLU sound attenuators are made of strong outer spiral seemed tube and an inner tube made of sheet steel with small openings to be able to withstand mechanical cleaning and at the same time does not interfere with the insertion loss. The space between the inner and outer ducts is filled with mineral wool and the inner duct is covered with a nonwoven cloth to prevent fibres from getting into the duct system. Further information can be found at www.lindab.com/Ventilation Systems/Sound Reduction