A jet propulsion system comprising a jet engine, a pylon and a noise reducing device, the jet engine comprising: a cylindrical casing; a cylindrical partition wall that is inserted in the casing while protruding partially from a trailing edge of the casing; and a compressor that is configured for compressing air that, in use, is taken into the cylindrical partition wall, with the inside of the cylindrical partition wall serving as a duct in which, in use, a core stream of high-speed air flows, and the space between the cylindrical partition wall and the casing serving as a duct in which, in use, a bypass stream of low-speed air flows, wherein the engine is connectable with a wing of an airplane by the pylon, the pylon comprising a projection portion that extends beyond the casing in the downstream direction with respect to the core stream and the bypass stream, the noise reducing device comprising at least one nozzle having a distal end, said at least one nozzle being arranged on the pylon downstream of the casing, said at least one nozzle being arranged on a downstream end portion of the projection portion of the pylon, characterized in that the at least one nozzle is configured for injecting a fluid toward a noise generation source that, when the jet engine is in use, is produced from a mutual approach of the core stream and an ambient air stream that is produced outside of the bypass stream and the core stream and in that the at least one nozzle is connected with at least one supply channel that is connected to the compressor so that, in use, the fluid that is injected from the at least one nozzle toward the noise generation source is air and a portion of the air that is discharged from the compressor is injected from the at least one nozzle. a cylindrical casing; a cylindrical partition wall that is inserted in the casing while protruding partially from a trailing edge of the casing; and a compressor that is configured for compressing air that, in use, is taken into the cylindrical partition wall, with the inside of the cylindrical partition wall serving as a duct in which, in use, a core stream of high-speed air flows, and the space between the cylindrical partition wall and the casing serving as a duct in which, in use, a bypass stream of low-speed air flows, wherein the engine is connectable with a wing of an airplane by the pylon, the pylon comprising a projection portion that extends beyond the casing in the downstream direction with respect to the core stream and the bypass stream, the noise reducing device comprising at least one nozzle having a distal end, said at least one nozzle being arranged on the pylon downstream of the casing, said at least one nozzle being arranged on a downstream end portion of the projection portion of the pylon, characterized in that the at least one nozzle is configured for injecting a fluid toward a noise generation source that, when the jet engine is in use, is produced from a mutual approach of the core stream and an ambient air stream that is produced outside of the bypass stream and the core stream and in that the at least one nozzle is connected with at least one supply channel that is connected to the compressor so that, in use, the fluid that is injected from the at least one nozzle toward the noise generation source is air and a portion of the air that is discharged from the compressor is injected from the at least one nozzle.