Measurements of near- and far-field noise pressures are presented for a 1,500-pound-thrust engine and for several 5,000-pound-thrust engines for which the nozzle exit pressure was changed systematically in order to study its effects on the noise, level and spectra. Near field surveys indicated that the highest noise pressure occurred at about 20 exit diameters downstream of the nozzle near the transition from supersonic to subsonic flow. The acoustical power radiated from all engines averaged about 0.5 percent of the mechanical power of the exhaust stream, the least noise being radiated by the nozzle having an exit pressure less than atmospheric. The rocket engines of these, tests radiate more power per cycle at the lower frequencies than are reported for subsonic jets in other, related studies.
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