Near-field acoustical holography applied to high-performance jet aircraft noise

Abstract

Structural fatigue, hearing damage, and community disturbances may all result from jet noise, especially as jet aircraft become more powerful. Noise-reduction technologies require accurate characterization of the noise sources within jets. Array-based sound pressure measurements were made in the jet exhaust region of an F-22 raptor to allow for sound-field visualization using near-field acoustical holography (NAH). This is one of the largest-scale applications of NAH since its development in the 1980s, and the most detailed near-field measurements made of high-power jet noise to date. The measurement was made using a large, dense microphone array, which scanned sound pressures over several measurement surfaces near the jet, resulting in more than 6000 measurement points. Fixed reference microphones, measuring simultaneously with each scan, were used to perform partial field decomposition (PFD) of the measurement planes. Guidelines for multi-reference jet-noise measurements in current literature are qualitative at best. The PFD allows for an analysis of reference microphone requirements. A method for determining the adequacy of the reference array using near-field coherence measurements is examined. [Work supported by Air Force SBIR.]