Jet noise suppression by porous plug nozzles

Jet noise suppression data presented earlier by Maestrello for porous plug nozzles have been supplemented by the testing of a family of nozzles having an equivalent throat diameter of 11.77 cm. Two circular reference nozzles and eight plug nozzles having radius ratios of either 0.53 or 0.80 were tested at total pressure ratios of 1.60 to 4.00. Data were taken both with and without a forward motion of coannular flow jet, and some tests were made with a heated jet. Jet thrust was measured. The data were analyzed to show the effects of suppressor geometry on nozzle propulsive efficiency and jet noise. Jet noise reductions in overall sound pressure level of up to 15 dB were obtained with a 3.5 percent reduction in thrust, but reductions of 10 dB were obtained at thrust reductions on the order of 0.3 percent. The noise reductions for subsonic pressure ratios were much less than those for supersonic pressure ratios.