A study of active control techniques for noise reduction in an aircraft fuselage model

A simplified cylindrical model is used to investigate the elementary mechanisms of control of sound transmission into aircraft cabins by two active control techniques: propeller synchrophasing and active vibration control. Propeller synchrophasing involves controlling the relative rotational phase of the engines to achieve maximum cabin noise reduction. Active vibration control involves structurally controlling the vibrational response of the cabin wall to reduce the important modes which transmit their energy into the cabin. Noise reductions for harmonic excitation at acoustic cavity resonance are shown to be in excess of 20 dB throughout most of the cavity whether synchrophasing or active vibration control is used. Off-resonance reductions are substantially less due to increased modal density requiring a larger number of actuators for effective control of the complex sound field. Additional studies were performed using synchrophasing in conjunction with active vibration control to study their joint capabilities in controlling complex sound fields. The dual control system displayed improved control performance with noise reductions on the order of 25-35 dB and a more uniform sound field. Also, the complementary control characteristics of the system clearly demonstrated effective control of orthogonal acoustic modes of the cavity. However, the improved effectiveness of the control system was dependent upon judiciously positioning the actuators for optimal control of the sound field. An independent study was performed to identify the effects of a complex geometry on sound transmission into an aircraft fuselage model interior. For this study, a geometrically scaled cabin floor was installed in the unstiffened test cylinder to investigate the structural and acoustic influence of the simulated cabin floor. Results indicated that the stiffening of the cylindrical model associated with insertion of the floor strongly influenced the structural response of the cylinder but generally had little effect on the coupled pressure response. Conversely, the modification of the interior acoustic cavity tended to have little influence on the cylinder response but substantially reduced the coupled pressure response. Thus, this investigation identified the fundamental mechanisms of control of sound transmission into simplified models of aircraft fuselages by active control techniques.