Harald Breitbach

Active Noise Control in a Semi-Closed Aircraft Cabin

An active noise control system is developed for the loadmaster area of a propeller driven transport aircraft. The loadmaster area is a small semi-enclosed volume connected to the large cargo hold. This coupling of a small room with a large room yields to new questions when an active noise control system is designed: Which effects does the coupling have on the noise field inside the small loadmaster area? Which influence do these effects have on the active noise control system? How complex is the uncertainty induced by the coupling to the large cargo? The analysis of this coupling was done in two ways. The first one, an energy based method, is used to analyze the acoustical energy flow between the loadmaster area and the cargo hold. The second one, a substructure technique, is used to study the influence on the eigenfrequencies and eigenmodes of such a coupled system. Afterwards, the theoretic results were confirmed with an experimental forced vibration analysis. The most important result is that the shape of the noise field inside the loadmaster area is less sensitive in spite of the large cargo hold. Moreover, an active noise controller was developed to handle the controller tasks. First experimental control results are given in the last section. These results have shown successful noise reduction up to 23 dB without any controller optimization.

Robust active noise control in the loadmaster area of a military transport aircraft

The active noise control (ANC) method is based on the superposition of a disturbance noise field with a second anti-noise field using loudspeakers and error microphones. This method can be used to reduce the noise level inside the cabin of a propeller aircraft. However, during the design process of the ANC system, extensive measurements of transfer functions are necessary to optimize the loudspeaker and microphone positions. Sometimes, the transducer positions have to be tailored according to the optimization results to achieve a sufficient noise reduction. The purpose of this paper is to introduce a controller design method for such narrow band ANC systems. The method can be seen as an extension of common transducer placement optimization procedures. In the presented method, individual weighting parameters for the loudspeakers and microphones are used. With this procedure, the tailoring of the transducer positions is replaced by adjustment of controller parameters. Moreover, the ANC system will be robust because of the fact that the uncertainties are considered during the optimization of the controller parameters. The paper describes the necessary theoretic background for the method and demonstrates the efficiency in an acoustical mock-up of a military transport aircraft.

Optimized Active Noise Control of Semiclosed Aircraft Interiors

A new method for optimizing the actuator and sensor positions applied to active noise control of semiclosed aircraft interiors is presented. This approach starts with the measurement of transfer functions between all possible actuator and sensor positions. Based on these evidences the minimum number and optimal position of the actuators and sensors are computed by optimization. The expected noise reduction and the necessary electrical inputs are calculated for the optimal configuration. These numerical results are then compared with experimental data for the optimal setup implemented into a mock-up of the semiclosed aircraft interior.

Optimization of actuator and sensor positions for an active noise reduction system

Different systems and strategies have been invented in order to reduce the noise level inside the fuselage of aircrafts. First of all passive methods like adding materials with high damping or vibration absorbing qualities were used. Due to mass reduction as a major aspect in aircraft design a lot of research is focused on active noise reduction (ANR). The level of attenuation gained by an ANR - system is depending on several attributes of the system like hardware and software in use. Another important parameter, which has a great impact on the performance, is the positioning of the actuators and sensors. Because of the high number of possible arrangements of actuators and sensors in three dimensional spaces, it is almost impossible to determine the optimal positions by experimental work. Therefore numerical optimization is applied. In this paper a hybrid evolutionary algorithm is introduced for the calculation of appropriate configurations for a fixed number of actuator and sensors out of a high number of possible positions for an ANR - system within a military aircraft. The presented COSA - algorithm (cooperative simulated annealing) connects qualities of two well known optimization algorithms, the simulated annealing (SA) and genetic algorithm (GA). A general description of the algorithm and the acoustical basics will be provided together with an overview of the results.

Active Noise Control inside the Loadmaster Area of a Turboprop Transport Aircraft

The paper describes the design and test of a prototype active noise controller for a semienclosed cabin inside a turbo-prop aircraft. Here, the eight active noise control loudspeakers and sixteen microphones are mounted at the cabin walls and ceiling. For the development process, an acoustical mock-up is built to reproduce the acoustic situation inside the aircraft on the ground. The acoustic requirements on such a mock-up are described and wellfounded by a coupling analysis. Moreover, the controller is based on a frequency domain implementation of the steepest descent algorithm. To achieve the optimal performance and robustness requirements, a method to adjust the microphone and loudspeaker weights is presented. The new method also considers the uncertainty of the system in the controller parameter design process. Experimental results show the efficiency of the prototype system for a typical load case.

Aircraft interior ANC with flat panel speakers

In propeller driven aircraft the main source for internal noise are 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, because passive solutions are to heavy at low frequencies. Optimal positions of the loudspeakers are found by finite element calculations. These positions have been realized in a full-scale test bed. A reduction of the sound pressure level of more than 30dB within a specified volume was achieved at a frequency of 100 Hz. HiFi speakers are used as secondary actuators in this test bed. These speakers are heavy and have unsuitable geometric dimensions for an aircraft. Therefore, other actuators, e.g. flat panel speakers, will be investigated with respect to the application in a mock-up of the LMWS.

Aircraft interior ANC with light‐weight actuators

In propeller‐driven aircraft the main source for internal noise is 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 semiclosed 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, because passive solutions are too heavy at low frequencies. The optimal positions of the loudspeakers are found by finite‐element calculations. These positions have been realized in a full‐scale test bed. In the test bed a reduction of the sound‐pressure level of more than 30 dB within a specified volume was achieved at a frequency of 100 Hz. In this test bed hi‐fi speakers are used as secondary actuators. These speakers are heavy and of unsuitable geometric dimensions for an aircraft. Therefore, other actuators, e....