Active Noise Control in an Aircraft Cabin

In propeller driven aircraft the main source for internal noise are almost tonal disturbances caused by the propeller blades that are passing the fuselage. In a certain four propeller military transport aircraft the maximum sound level in the cabin can reach up to 110 dB(A), not taking into account any noise control treatments. Inside the semi closed loadmaster working station (LMWS) the sound level must be reduced down to 86 dB(A). It is proposed to reach this goal with an active noise control system. To get the minimum number and the optimum positions for the secondary sources and the error sensors, the method of stepwise reduction is applied on different finite element models and a full-scale test bed in order to validate the numerical results. The comparison of the numerical and experimental results shows a good conformity at low frequencies. Furthermore, the test bed shows a good system-performance. Therefore, it is possible to get an optimized set of actuator and sensor position with the method of stepwise reduction. INTRODUCTION Nowadays, noise is classified as a special form of pollution. Therefore different industrial safety rules have been introduced to reduce noise levels. Furthermore, it is an important selling factor to have a quiet aircraft interior. In propeller driven aircraft the main sources for internal noise are narrow banded – almost tonal – disturbances caused by the propeller blades that are passing the fuselage in a certain blade pass frequency. One large study on aircraft interior active noise control (ANC) has been the ASANCA project, where a Dornier 228 has been equipped with an ANC-system[1], [2]. In a certain four propeller military transport aircraft the maximum sound level in the cabin can reach up to 110 dB(A) without taking into account any noise control treatments. Inside the semi closed loadmaster working station (LMWS) the sound level must be reduced down to 86dB(A). Passive treatments are heavy at low frequencies and therefore an active system is proposed for reaching this aim. To get the minimum number and the optimum positions for the secondary sources and the error sensors, evaluations on different FEM-models are performed in the frequency domain using FEMLAB [3]. The results of this evaluation are used to build a full scale test bed in order to validate the numerical results and to improve the system performance [4]. NUMERICAL EVALUATION If you have to select K sensors, or secondary actuators, from L possible locations by solving this as a combinatorial problem you need C calculations [5] with: ( ) ! ! ! L C K L K = − . (1) Especially for a large number of possible locations it is very time consuming to solve such a problem because of computational borders. Therefore a new method of selecting actuator and sensor locations is established. In a first step the number of actuators is reduced to N by deselecting L-N actuators with lowest amplitude at the controlled case. After this first step the number of actuators (and sensors) is reduced by selecting the best solution out of the N calculated solutions. This procedure continues with selecting the best solution out of the recalculated remaining N-1 solutions until the desired number of K actuators is reached. It is possible to use this method because of the linear behaviour of the system. This reduces the number of necessary calculations D for the reduction from L to K actuators to