Active structural acoustic control of aircraft interior flow noise via the use of active trim panels

Modem jet aircraft interior noise can be categorized into two main types: tonal noise caused by engine imbalance or blade passage, and mid frequency broadband noise resulting from turbulent flow. This project addresses aircraft interior flow noise caused by a flow separation over the crown of the aircraft. The noise control approach is to mount piezoelectric actuators to the aircraft interior cockpit crown trim panel and use them to actively control aircraft interior noise with feed-forward adaptive LMS control algorithms. The experiments were performed on a Cessna Citation III fuselage with the production crown interior trim panel in place. Flow noise was simulated by three speakers mounted above the crown of the aircraft producing random noise with a frequency range of 500-1000 Hz. Several piezoelectric (PZT) actuators were mounted on the interior crown trim panel as control outputs. Sixteen microphones served as error/global attenuation sensors. Microphones and PZTs were selected from off-line optimizations. The control reference signal was obtained from either an accelerometer mounted on the skin of the aircraft or from the signal generator itself. Control was executed on a personal computer digital signal processing. Single frequency control experiments were performed to judge the feasibility of control. The main broadband tests were performed with a variety of controller configurations. The results using the active panel are compared with more traditional acoustic control utilizing speakers as control actuators instead of PZT's. The project concludes that a 2 dB reduction over the frequency range is obtainable at the pilot's ears in a 2I20 configuration with an accelerometer reference signal. Acausality and slow controller speeds were found to be the main causes which limit further reduction.