ACTIVE CONTROL OF LOW-FREQUENCY BROAD-BAND JET ENGINE EXHAUST NOISE

The low-frequency broadband exhaust noise generated by ground runup te sts of jet engines represents a significant source of environmental no ise. The low-frequency acoustic radiation from these operations is ext remely difficult to attenuate using passive noise control means and is capable of propagating over long distances, resulting in a noise and vibration problem in nearby communities, One type of engine test facil ity which has resulted in low-frequency noise complaints is known as a hush house, used on military air bases, Hn this work, the use of acti ve noise control (ANC) is experimentally investigated in order to redu ce low-frequency jet engine exhaust noise over large sectors in the ra diated far field, The tests were performed using a Pratt and Whitney J T15D turbofan jet engine situated in a test cell designed to approxima te the noise emissions of a hush house on a smaller scale, The control approach used was the feedforward filtered-X LMS algorithm with a ref erence signal taken from a microphone situated in the engine exhaust r adiated noise field, The active control sources were loudspeakers arra nged in various configurations in the far field and the error sensors were microphones, The results demonstrate attenuations of up to 15 dB at the error sensor locations in the one-third octave bands with cente r frequencies of 25 to 250 Hz. Using a multi-channel control system, t he overall sound level was attenuated by at least 5 dB over a sector w ith an area of at least 100 m(2). The results convincingly demonstrate the potential of ANC for reducing low-frequency jet engine exhaust no ise over a large area near hush houses. (C) 1996 Institute of Noise Co ntrol Engineering.