Delf Sachau

Active noise control in light jet aircraft

Active systems for noise reduction are especially of interest when considering applications in which low‐frequency noise is a main source of disturbance and only limited amounts of installation space and payload are available. This makes the adaptation and implementation of such systems plausible in vehicles such as automobiles and aircraft where passive reduction methods are restricted. In order to achieve effective active control in these environments, aspects such as the control method and actuator‐ and sensor‐type as well as positioning must be considered. The noise characteristics are often known beforehand or are easily accessible by measurement. Using this data, an upper bound for possible noise reduction may be determined, e.g., by means of linear prediction methods. A current research project is aimed at developing an audiosystem for the cabin area of a light jet aircraft which, at the same time, should also function as an effective noise reduction system in order to enhance the cabin comfort as well as the audio quality. Using data from a measurement flight in a typical light jet aircraft, limitations of active control are determined. Furthermore, the testbed, an acoustic mockup, is presented, currently beholding a multichannel ANC‐ Audio system for tonal and broadband noise.

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.

Identification of noise sources by means of inverse finite element method using measured data

An inverse finite element method for noise source identification in an aircraft cabin is presented. If all sound sources are located on the boundary on the cabin, the equation system resulting from a matching FE model can be re- sorted in such a way that computation of the unknown boundary data is possible from measurement data taken in the cavity. The method is first validated using a simplified 3D COMSOL model. The numerically calculated data inside an inner sub-domain are impinged with a stochastic error and used as simulated measurement data to re-calculate the boundary data. In a next step, the sound field in the cavity of an aircraft mock-up excited by both interior and exterior noise sources is mapped with a custom- built microphone array. A matching COMSOL model is verified and compared to the mapping data. Consequently the inverse calculation is performed for this more realistic model.

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.

Noise source identification in a 2D cavity based on inverse finite element method

The identification of noise sources in enclosures proves to be particularly difficult in the low-frequency range, because the emerging standing wave field does not allow direct conclusions as to the location of sources. This paper presents an approach to reconstruct sound pressure and particle velocity on the boundary, based on an inverse finite element method (IFEM). This procedure requires sound pressure measurements in the interior first. In a second step, these data are associated to the nodes of an acoustic finite element model of the cavity. If all sources are located on the boundary, the equation system resulting from the numerical model can be re-sorted in such a way that the boundary values can be reconstructed. The IFEM is verified by a two-dimensional simulation. Including an energy-minimizing solution norm, performance on arbitrarily shaped boundaries is improved. Finally, the IFEM is applied in a two-dimensional laboratory experiment. By means of regularization techniques, a loudspeaker included in the boundary of a test facility can be identified.

Adaptive Support of an Aircraft Panel

The vibro-acoustic behavior of large lightweight structures such as aircraft fuselage has to be tested, especially to improve active and passive noise control means. Acoustic transmission laboratories with reverberant and anechoic rooms supply reliable test conditions for aircraft panels of a size up to several square meters. Shock mounts support these panels to realize free or pinned boundary conditions with minor damping, which is sufficient at high frequencies. At low frequencies, the vibration of the whole fuselage has to be considered. Therefore the support of the panel under test should provide the same dynamic impedance as the fuselage to which the panel would be connected in the aircraft. This frequency-dependent boundary condition can be realized by an adaptive support. The paper describes a numerical investigation of this supported panel which should behave like integrated into an aircraft fuselage.

Performance evaluation of an active headrest considering non-stationary broadband disturbances and head movement

An application of active noise control (ANC) is the active headrest, using actuators (loadspeakers), sensors (microphones), and a controller to create a local zone of quiet around the occupants head. In this paper, the attenuation performance of various ANC-algorithms for active headrests known from literature is evaluated considering non-stationary broadband disturbances and head movement. Numerical studies are performed to determine the optimal plant models and parameters for the ANC-algorithms. Based on the findings of the numerical studies, several real-time experiments are conducted with and without head tracking examining the distribution of the 10 dB zone of quiet and the attenuation at the occupants ear using either a head mounted microphone technique, the remote microphone technique, the virtual microphone technique, or the virtual microphone control method. It is found that none of the algorithms using a virtual sensing technique can produce a 10 dB zone of quiet for all considered non-stationary broadband disturbances and head movement. For the algorithm using a head mounted microphone, it is possible to form a 10 dB zone of quiet, but placing a microphone at the ear is not feasible in most situations.

Robustness of the extended delayed-X harmonics synthesizer algorithm applied to engine exhaust noise control

Exhaust noise of a combustion engine contains several harmonic components of the fundamental frequency that depends on the engine speed. Active noise cancellation (ANC) in the exhaust line of a diesel-electric drive faces serious challenges such as high temperature, high sound pressure levels, standing waves phenomena, a time-varying environment, and slightly varying disturbance frequencies. In the case of a non-acoustic reference sensor, also an offset or a mismatch in the estimation of the disturbance frequencies negatively affects the performance. The extended delayed-X harmonics synthesizer (DXHS) is known from several researches to be appropriate for control of periodic noise with multiple harmonic components. The algorithm is robust against observation noise and fluctuation of the plant under control. In this paper, the extended DXHS is implemented in an ANC-system for a diesel-electric drive and experimentally investigated in laboratory experiments. A modified update rule, a convergence factor regu...