Abstract: A continuous web which is drawn off a roll is perforated on its way toward the wrapping station of a machine for the making of plain or filter tipped smokers' products or filter rod sections. The perforating device is adjustable by a control system which receives signals from a photoelectronic testing unit serving to monitor the permeability of successive increments of the web between the perforating device and the wrapping station. The combined cross-sectional area of holes per unit area of the web is increased or reduced, depending upon whether the monitored permeability is less than or exceeds a preselected optimum permeability.