air entrance of Channel acoustically IMPROVED

nacelle structure (10) surrounding a fan comprising an air inlet duct (100) having an inner surface (100A) defining an air supply channel; and at least one ring (22) disposed in said sleeve and movable air inlet selectively between an extended position, wherein this ring is moved radially inwardly from the inner surface to provide an impediment to shock waves from of the blower, and a retracted position, wherein the ring is moved radially outwardly from the extended position to be flush with the inner surface and thus replenish the air supply channel.