Reduction of Aft Fan Radiation Using Herschel-Quincke Resonators

Technology in Blacksburg and the Vibration and Acoustics Laboratories at Virginia Tech propose using hybrid Herschel-Quincke (HQ) resonators as a method to control aft fan noise radiated from turbofan engines. The noise control mechanism of the HQ tube system is as follows: as noise propagates through a duct past the entrance of a HQ resonator, some of the acoustic energy enters the HQ resonator while the remaining acoustic energy propagates down the duct. The sound waves in the resonator recombine with the duct acoustic waves at the tube exit. The fact that the waves in the HQ resonator travel a distance different than those in the duct results in a phase shift between the resonator and duct waves. Noise attenuation occurs at frequencies for which the recombining HQ resonator and duct waves are out of phase. Classical HQ resonators are passive devices with no moving parts whose noise attenuation characteristics are functions of the resonator¿s length and cross-sectional area. For fan noise control applications the geometry of the HQ resonators are optimized to attenuate frequency at the blade passing frequency. Hybrid HQ resonators could be developed with variable geometry and used in conjunction with adaptive control to optimize their effectiveness over a wide range of engine operating conditions.