Acoustic noise reduction for large aircraft

Noise reduction from large military aircraft is significant as airfields encroach on residential areas. Two methods of noise reduction are possible: active and passive. This proposal will outline a tunable, passive method that can stand alone or be usedin conjunction with active methods to reduce aircraft engine noise. This proposal will attempt to apply passive methodologies to this noise problem that have been successfully demonstrated in an active DARPA funded Phase 2 effort in Acoustic BandgapDevices and Materials. Acoustic Bandgap cowel-type enclosures, suitable for aircraft engine shapes and frequencies, have been modeled with predicted results of greater than 20 dB over a wide range of directional angle and frequency. These prototypedevices, currently in testing, are surpassing predicted attenuation levels. These device configurations are frequency tunable with changes in geometry (static or dynamic)have substantial potential to passively reduce noise from from both military andcommercial aircraft. These acoustic bandgap materials and devices are totally passive and can be tuned to attenuate certain ranges of frequencies. There is no power required and their are no moving parts. These design principles for which models havebeen devloped can be applied to aircraft engines and airframes in both commercial and military applications.