respect to the insertion speed such that it corresponds to an average flow of the product on extraction equal to the average flow of the product on insertion for a striking frequency below a limit frequency it is possible to adjust the variable speed waveform 20 such that the angular velocity is eliminated which corresponds to a stoppage of the extraction rollers and therefore to a discontinuous feed when the striking frequency is above this limit frequency it is then no longer possible to wait for the elimination of the angular velocity u03c9 and the situation is then that of periodic slowing down 21 the lower part of the modulation waveform 20 21 corresponds to a needling zone 22 during which the needles 8 penetrate into the fibre fleece 2 the limit striking frequency between the discontinuous feed conditions and the slowing down conditions depends on the maximum acceleration which the motor reduction unit 124 can provide which itself depends on the inertia of all of the driven mechanical parts on the reduction ratio and on the maximum torque of the motor furthermore it is also possible to define a limit average feed speed beyond which for a given striking frequency it is no longer possible to obtain discontinuous feed conditions because of the flexibility of control offered by the use of an electronic variator and a variable speed motor it becomes possible to control with precision the phase shift between the striking cycle or needling cycle and the feed speed modulation cycle in effect it is possible to program the control and processing unit 11 in order to synchronise the two cycles and to predetermine either an advance fig2 b or a delay fig2 c of the modulation cycle with respect to the striking cycle in the first case.