Abstract
The National Aeronautics and Space Administration (NASA) has espoused the vision of developing technology capable of keeping all objectionable aircraft noise within airport boundaries. In order to achieve such an aggressive goal, new aircraft will have to be designed employing quiet propulsion, quiet lift, and quiet drag. Research activities are continuing on all these fronts. Dominant sources of propulsion noise to date on jet‐powered transports have been jet noise and fan noise. As bypass ratios of aircraft turbofan engines have increased to improve fuel efficiency, jet velocities (and therefore jet noise) have decreased, making fan noise the larger contributor in many instances. In NASAs Advanced Subsonic Technology (AST) Program, which ran from 1994 until 2001, several active control strategies for tonal fan noise were investigated. These included rings of wall‐mounted actuators, active impedance control, stator‐mounted actuators, and hybrid active/passive concepts. The most promising approach seems t...
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