The invention relates to a device and a method by means of which the air flow and the fibre flow on the roller (6, 8) can be matched to operating conditions using a controlled positioning of an air guide element (22). The above is achieved by means of said device, whereby said air guide element (22), comprising at least one reference point, lying on a line with a separation from the surface of the roller (6, 8), running essentially along the surface, may be displaced and by means of a corresponding method.