Abstract: A device for regulating the airflow in an air damper including a twist sleeve of flexible material, the cross section of which substantially conforms to, an outer tube in which the flow takes place, one end of the twist sleeve being disposed on the inner side of the outer tube and the other end of the twist sleeve being disposed in a tube sleeve which can be rotated under an axial displacement controlled in relation to the rotation. The tube sleeve may be provided with a helical slot. A guide pin is arranged on an inner side of the outer tube, which guide pin goes down into the helical slot. The operating apparatus comprises two cords, which cords are provided with fastenings against the tube sleeve and run along opposite circumferential directions of the tube sleeve between the outer tube and the tube sleeve.