... Non-insulated Air Flexible Duct & Silent Insulated Duct)Made of Aluminum foil + pet + steel wire OR flexible duct+insulated sponge ... +aluminum foil.(Silent insulated duct Punched many small hole, can absorb most of sound from airstream) ... as school, supermarket, office, stadium, theater, factory...) high requietment of the enviroment area, such as hospital, laboratory, howff, bus, ...