Abstract
Measurements of near- and far-field noise pressures are presented for rocket engines having about 5000-lb thrust, and 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 diam distance downstream of the nozzle near the transition from supersonic to subsonic flow. At a radial distance of two diameters the noise pressure levels varied from about 145 db just upstream of the nozzle to about 170 db downstream of the nozzle and no consistent trends were noted as a function of nozzle exit pressure. In general, the spectra at points downstream of the nozzle contained considerably more low-frequency noise than those measured upstream of the nozzle. It was noted that the maximum farfield noise was radiated in the downstream direction at angles of 30° to 45° from the jet axis. The noise energy radiated from all nozzles averaged about 0.5% of the mechanical energy of the exhaust stream, the least noise being radiated by the nozzle having an exit pressure less than atmospheric.
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