Modification of directivity curves for a rocket noise model

The spatial extent and downstream origin of rocket noise sources can significantly impact the physical interpretation of directivity index measurements. Valuable updates to historical rocket noise directivity indices, based on recent measurements of Space Shuttle reusable solid rocket motor (RSRM) boosters have been published by Haynes and Kenny (AIAA paper 2009-3160). However, measurements at a radial distance of 80 nozzle diameters from the RSRM nozzle exit plane are insufficient to be called the far field at low frequencies and thus require modification to the apparent source origin prior to their use in the empirical sound pressure level prediction methodologies, such as described in NASA SP-8072 (1971). In this analysis, estimates of plume source sound power level as a function of distance along the plume axis are combined with frequency-dependent, far field directivity indices to predict sound pressure level as a function of angle and range. With geometric modifications in place, the predicted overa...