The Reduction of Choked Jet Noise

Some methods of reducing that noise peculiar to choked jets are investigated, the mode of operation being a reduction of one or more of the factors occurring in the gain criterion qηsηtηd gt-or-equal, slanted 1, in the notation of an earlier paper. The methods used consist of the notching of the exit edge of the jet nozzle, the introduction of cambered radial vanes and ventilation of the nozzle by means of a gauze extension. The magnitude of the noise of a jet emanating from a plain exit is shown to be susceptible to the state of the exit edge and the reflecting area presented by the nozzle body in the plane of the exit. It is suggested that the presence of initial turbulence or noise will also tend to reduce the magnitude of the sound, and this effect may be of importance in the practical case of the jets of propulsive mechanisms. The methods used influence other types of noise always present to some degree, a measured reduction in actual subsonic noise only being found in the case of the radial vanes.