Thomas Kletschkowski

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.

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.

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.

Active Noise Control in a Semi-closed Interior

As a first example this chapter presents the application of the general design methodology for ANC-systems (proposed in Chap. 8) to a particular interior noise problem in an aircraft cabin. To show the practical use of the proposed (matrix) design approach, considering the mechatronic background of feed-forward ANC presented in part I as well as the ANC-system design tools discussed in Chap. 7, this chapter reports on five successive design steps that were performed to support the development of a robust active noise system (ANS) for a special working area (WA) of a certain military aircraft (MA). The chapter starts with a motivation and a description of the requirements. Afterwards, the output of all design steps, beginning with the feasibility study and ending with the robustness study, will be summarized. The project was intended to support the activities of Airbus Germany. It was carried out at the professorship for Mechatronics of Helmut-Schmidt-University/University of the Federal Armed Forces Hamburg between 2002 and 2009. This support has been continued in 2010 and in 2011.

Comments on Signals and Systems

To prepare the mechatronic foundation of active noise control, this chapter contains some elements of system theory. Since we will be talking about active noise control systems and signals, it is necessary to define these terms. Furthermore, it is necessary to introduce values and functions that can be used to characterize signals and systems in time domain but also in frequency domain. However, it is not intended to present a compact summary of system theory that is in great detail presented in textbooks, such as in (Cadzov and van Landingham in Signals, systems, and transforms, Prentice Hall, New Jersey, 1985), (Fliege in Systemtheorie, Teubner, Stuttgart, 1991), (Girod et al. in Einfuhrung in die Systemtheorie, Teubner, Stuttgart, 2005), (Johnson in Digitale Signalverarbeitung, Hanser, Munchen in Cooperation with Prentice Hall International, London, 1991), (Oppenheim and Willsky in Signale und Systeme—Lehrbuch, VCH Verlagsgesellschaft, Weinheim, 1989), (Sundararajan in A practical approach to signals and systems, Wiley Eastern, Singapore, 2008), (Ziemer et al. in Signals and systems, continuous and discrete, Macmillan, New York, 1983). A description of stochastic signals and random vibrations of both linear and non-linear mechanical systems is to be found in (Lajos in Zufallsschwingungen und ihre Behandlung, Springer, Berlin, 1973), whereas digital audio signal processing is discussed in (Zolzer in Digital audio signal processing, Wiley, Chichester, 2008).

Actuators for Active Noise Control

To establish an active control system, it is necessary to integrate actuators. Therefore, this chapter gives a short introduction to acoustical actuation for active noise control that is limited to electro-dynamical actuation by loudspeakers and electro-dynamical exciters. The first actuator type is widely used as secondary sound source. The second one is of special interest, if an elastic structure is to be used as an acoustical source. The equation of motions will be derived from simplified electro-mechanical actuator models and the particular actuator behavior is discussed in frequency domain. A general overview on loudspeakers is given e.g. in (Baranek in J. Acoust. Soc. Am. 26(5):618–629, 1954b) and (Havelock et al. in Handbook of signal processing in acoustics, vol 1, Springer, New York, 2008a). Comments on loudspeaker design and performance evaluation can also be found in (Havelock et al. in Handbook of signal processing in acoustics, vol 1, Springer, New York, 2008a). A detailed description of audio amplifiers is given in (Shea in Amplifier handbook, McGraw-Hill, New York, 1966) and the transfer behavior of electro-dynamical loudspeakers is analyzed e.g. in (Zwicker and Zollner in Elektroakustik, Springer, Berlin, 1984) and (Moser in Technische Akustik, Springer, Berlin, 2005). Sound radiation from vibrating structures has been studied extensively in (Cremer et al. in Structure-borne sound, Springer, Berlin, 1995) and (Fahy and Gardonio in Sound and structural vibration, Elsevier, Amsterdam, 2007).

A Sound Intensity Probe with Active Free Field

To avoid the time and cost consuming process of realizing artificial free field conditions by introducing passive damping to the interior (which in addition changes the global characteristic of the investigated enclosure) or the need of sophisticated numerical models required to apply inverse noise source localization techniques such as the IFEM, compare Sect. 7.2 and Chap. 13, a novel sound intensity probe with an active free field (SIAF) was proposed in (Sachau et al. in Schallintensitatsdetektor sowie Verfahren zum Messen der Schallintensitat. Patent. DE102004009644A1 22.09.2005, 2005a). This chapter reports on the application of the design methodology for ANC-systems proposed in Chap. 8 to the SIAF design process in which a first specification of the novel sound intensity probe was compiled in two successive design steps. The chapter also includes comments on the functional testing of the first SIAF realization. These experiments were especially focused on free field calibration considering the sound intensity measurements errors discussed in Sect. 4.2.2.

Dynamics of Basic System

The present work is focused on adaptive feed-forward control to low-frequency interior noise. The basic system in which the noise is present is the air filled interior. To establish a control concept, it is important to know what field variables can be used to describe the transfer behavior of this basic system or to receive information about the systems state. For this reason, basic acoustic field variables such as the acoustic pressure, the acoustic velocity, and the change in density are introduced in this chapter. It will be shown that interior noise fields are described by the wave equation or, in case of harmonic excitation, by the Helmholtz equation considering the associated boundary conditions. These partial differential equations will be derived from basic field equations that are used to express the balance of linear momentum, the conservation of mass as well as the thermodynamic state. To give an introduction into the nature of standing waves, this chapter also contains analytical solutions for one-dimensional waveguides. However, this chapter is far away from being a comprehensive summary of (engineering) acoustics that is in great detail presented e.g. in (Baranek in Acoustics, McGraw-Hill, New York, 1954a), (Heckl and Muller in Taschenbuch der Technischen Akustik, Springer, Berlin, 1995), (Henn et al. in Ingenieurakustik, Vieweg, Wiesbaden, 1984), (Kuttfurff in Akustik: Eine Einfuhrung, Hirzel, Stuttgart, 2004), (Morese and Ingard in Flugge (ed.) Encyclopedia of physics XI/1, acoustics 1, Springer, Berlin, 1961), (Morse and Ingard in Theoretical acoustics, McGraw-Hill, New York, 1968), (Moser in Technische Akustik, Springer, Berlin, 2005), (Skudrzyk in The foundations of acoustics, basic mathematics and basic acoustics, Springer, Wien, 1971), and in (Fahy in Foundations of engineering acoustics, Academic Press, Amsterdam, 2003). A compact summary of acoustics is given in (DEGA in Akustische Wellen und Felder; DEGA Deutsche Gesellschaft fur Akustik e.V. http://www.dega-akustik.de/publikationen/online-publikationen. Cited 05 May 2010, 2006).