Device (6) for an apparatus for forming fibre blankets, the fibres being formed from a material that can be attenuated by internal centrifugation and by attenuation by means of a gas stream, the device comprising a guiding duct (60) into which the fibres are intended to be channelled, of longitudinal axis (X), which duct has a first part (61) intended to form the inlet of the duct at the point where the fibres are intended to be introduced into said duct, a second or central part (62) and a third part (63) intended to form the outlet of said duct, characterized in that it includes articulation means (7) that are suitable for acting mechanically on the third part of the duct (63) so as to vary its dimensions and/or the position of at least one of its parts relative to said longitudinal axis (X).