The invention relates to a device to prevent the formation of solid matter due to projections on an air outlet, for example that of an armored vehicle, located near to such projections.It comprises a shutter 1 incorporating at least two vibrating fins 3 fixed onto a support 2, said fins being of two different types mounted alternatively and vibrating differently. One vibrating fin 3a is connected to the support by a number N of fastenings and the other vibrating fin 3b by a number N+/-1 fastenings. The support 2 for the vibrating fins is constituted by an assembly of bars 2a, 2b that are substantially perpendicular to the fins 3. Abutments 5 are placed on the fins 3 or on the support 2.