Characterization of Rocket and Jet Noise using Near-Field Acoustic Holography Methods

As rockets and jets on military aircraft become more powerful, the noise they produce can lead to structural fatigue, hearing damage, and community disturbances. Noise-reduction technologies and sound radiation prediction require accurate characterization of the noise sources within rocket plumes and jets. Near-field acoustical holography techniques were used to visualize the sound field in the region of the jet exhaust on a high-performance military jet. Holography requires a coherent measurement of the sound field, but the size of the jet made a dense measurement over the entire source region impractical. Thus, a scan-based measurement was performed, after which a partial field decomposition (PFD) procedure was used to tie together incoherent scans. Then, the effective aperture of the measurement was extended utilizing the rigid ground reflection and a processing technique called analytic continuation. Finally, the three-dimensional sound field was reconstructed using statistically-optimized near-field acoustical holography (SONAH). This is the first time such a map has been obtained for a full-scale military aircraft. [Work supported by Air Force SBIR.