the corrections carried out will result in an effective improvement the processing of these values is also chosen as far as possible to reflect all the configurations or dimensions of the installations equipped with these regulating systems a preferred method of regrouping measurements of the mass of fibers per unit surface area is indicated in fig6 in this method for example the width of the felt l is divided into four sectors which partially overlap the regrouped evaluated measurements in these four sectors ensure that excessive importance is not given to measurements corresponding to the sides of the felt compared with the center part other methods of processing could of course be employed tests in each case show the significance of the method studied for resolving the problems encountered in practice by way of example tests have been carried out on a pilot installation for the formation of felt from glass wool this installation contained only one fiber forming device these examples are not intended to limit the invention the fiber forming device and the arrangement of guide duct and driving system are of the type represented in fig2 in this installation the felt has a width of 2 40 m it has a mass per unit surface area of 1 kg m 2 since only a single fiber forming device is used the speed of the receiving conveyor is relatively low being 5 25 m min at the exit from the stove the felt passes through an x ray absorption.