Abstract:
A new concept is proposed in this paper which can reduce noise radiated from multiple moving sources in the open field by using directional detecting microphones and directional secondary sources. At first, basic experiments concerning on a simple 1-1-1 ANC system were performed in the open field using directional detecting microphones, speakers and error microphones, compared with using non-directional ones. And the directional system was found to have an advantage that it did not create increasing zones around the noise reducing zones. Then, the above directional ANC systems were virtually put side by side along a road and noise reducing effects were predicted by using above experimental data. In this case the ANC filters were fixed and were not affected each other. This prediction proved that the lined ANC systems could reduce noise propagated from moving sources in the open field.

Abstract:
Axial cooling fan noise is characterized by tonal harmonics superimposed on a broadband noise floor. The filtered-x LMS algorithm has previously been used for feed-forward control of the tonal noise of a small axial cooling fan. This system consists of four actuator-sensor pairs surrounding the small fan. The ideal placement of these elements has been previously determined by Gee and Sommerfeldt (Gee and Sommerfeldt, J. Acoust. Soc. Am. 115, 228-236). With success in reducing tonal noise to the same level as broadband noise, it now becomes desirable to control the broadband noise of the small axial cooling fan. This is accomplished with multi-channel active feedback control. Investigations of the controllability of broadband axial fan noise will be discussed. Performance issues and global attenuation achieved with the broadband control system will be reviewed.

Abstract:
A local active noise control system aims to create a zone of quiet at a desired location, such as an observer’s ear. The resulting zone of quiet is generally centred at the error sensor, and is often too small to extend to the observer’s ear. To overcome this problem, virtual sensing methods have been suggested, which allow the zone of quiet to be moved away from the physical error sensor to the desired location of maximum attenuation, i.e. the virtual location. These methods are based on estimating and minimising the error signal at the virtual location. The control performance thus depends on the estimation accuracy of the virtual sensing algorithm. In this paper, limits on the broadband control performance that theoretically can be achieved at the virtual location are derived for a virtual sensing algorithm called the remote microphone technique. These limits are compared to the broadband control performance that can be achieved for the case of using a physical error sensor at the virtual location.

Abstract:
This paper presents a study of decentralised velocity feedback control on a rectangular panel using five electrodynamic inertial actuators. The aim is to generate active damping in such a way as to control the low-frequency lightly damped resonances of the panel and thus reduce the sound radiation. It has been shown that collocated direct velocity feedback (DVFB) with ideal point force actuators mounted on a structure generates active damping and is unconditionally stable. However, when an inertial electrodynamic actuator is used to implement velocity feedback control, the fundamental resonance frequencies of the actuator determine stability and control bandwidth limits. In this paper the design of a small scale inertial actuator with a low mounting resonance frequency is first presented. The main design issues related to the stability of a velocity feedback control loop are discussed. The simultaneous implementation of five control units is then studied. The stability of the five control systems is assessed with reference to the locus of the eigenvalues of the 5x5 fully populated matrix with the open loop frequency response functions between the sensors and actuators. Finally the control performance both in terms of vibration reduction and radiated sound is presented.

Abstract:
This paper presents simulations and experimental results of active structural control of a vibrating plate using multiple velocity feedback loops with inertial actuators. A time domain model of a plate with multiple collocated inertial actuators and velocity sensors is presented. A fixed gain, velocity feedback controller is designed by minimising the total kinetic energy of the plate subject to a control effort limit and this optimal controller is compared with the more simple case in which each local loop has an identical gain. The stability of the control system is then discussed in terms of the open loop frequency response and it is demonstrated that the gain margin of the control system decreases with increasing numbers of control loops. The design and construction of an experimental rig with twenty-four sensor and actuator pairs is discussed. The results obtained from experimentation are compared with those from simulations. Very good attenuations in the ten modes up to 200Hz are predicted and observed.

Abstract:
This paper presents a theoretical study of active damping on a double panel vibroacoustic system. The system consists of two vibrating panels, which are structurally and acoustically coupled via elastic mounts and the air in the cavity between the panels. The “source” panel is excited by an acoustic plane wave, while the “radiating” panel radiates sound into free-field. An array of sixteen reactive actuators is located in the air cavity, in order to implement direct velocity feedback (DVFB) decentralized control loops. The aim is to generate active damping across the two panels so that the low frequency resonant sound transmission is reduced. It has been shown previously that collocated DVFB with idealized skyhook force actuators generates active damping on structures and is unconditionally stable. However, when reactive actuators are used, performance and stability of the system may be affected by a feed-through effect via the mounting system or the air in the cavity. A mathematical model for the problem has been developed, which considers the fully coupled configuration. Simulations with various feedback arrangements have been performed, and the results discussed, focusing particularly on the control system performance and stability.

Abstract:
An active silencer with and without side-branch resonator was studied in this paper. The control principle was analyzed by using the method of active impedance control, based on the structure with a side-branch resonator. The analytical results show that the necessary volume velocity of the secondary source is relative to that of the primary source and the two structural parameters of the active silencer as well, including the length of the side-branch resonator and the length from the primary source to the inlet of the side-branch resonator. The necessary volume velocity of the secondary source can be reduced at and around the resonance frequency of the side-branch resonator. When the secondary source is directly mounted on the duct wall, the necessary volume velocity will be stronger than that of the primary source. In order to verify the validity of the analytical results, the active noise control experiments with and without side-branch resonator were also made, respectively. Compared with analytical results, the practical measured results are basically alike.

Abstract:
TechnoFirst has developed a single-channel oscillating flap system [6]. The feasibility of Active Noise Control using this system has been demonstrated and results have been shown on real diesel engines with duct diameter up to 130 mm. TechnoFirst work as sub-contractor of DMT (Deutsch Montan Technologies) in the frame of the NORMA European project [7] to make the exhaust system working for a 220 mm duct diameter. This new dimension has pushed TechnoFirst to re-think entirely the design of the system. Indeed, when controlling the first mode inside a hot duct with a large diameter, the control system shows bandwidth limitation at high frequency due to the system own inertia. To avoid this problem, a solution was to split the control into two channels, by using two flaps instead of one. TechnoFirst has then developed a new prototype of twin oscillating flaps working simultaneously and measured performances of this device on a new experimental test bench. The first results show that a good control of noise inside the duct can be achieved with two small flaps, with less sensitivity to the system inertia [9],[10]. This paper describes the way to use an oscillating flap as a loudspeaker inside a duct. Single flap and double flap behaviors are described and experiments results are discussed. This description of double flap confirms that such a system is able to control a frequency bandwidth larger than a single channel unit.

Abstract:
This paper deals with the design and test of a hybrid acoustic treatment combining porous material properties and active control techniques. Such an acoustic system is studied to reduce evolutionary tones in flow duct applications. A digital adaptive feedback control algorithm has been specially developed in order to operate independently cell by cell, and to allow an easy subsequent increase of the liner surface. An adaptive bandpass filter is used to prevent the development of instabilities due to the coupling occurring between cells. Large active panels featuring 54 cells are tested with flow velocities up to Mach 0.3. Convergence is very rapid and stable for all cells. A detailed analysis of the experimental results is given, with comparisons to results obtained thanks to numerical simulations in the no flow case.

Abstract:
This paper presents an analytical investigation into the active control of sound transmission into a structural-acoustic coupled system. An enclosure with four acoustically rigid walls and two simply supported flexible plates is considered. One of the two plates through which a harmonic sound wave is transmitted into the enclosure, is called as incident plate here, and the other as the receiving plate. Based on a fully coupled vibro-acoustic model, six different control strategies, i.e. control arrangements, are compared and discussed. They are: (a) incident plate control using distributed piezoelectric actuators, (b) applying acoustic control source in the cavity (called as cavity control), (c) receiving plate control using distributed piezoelectric actuators, (d) hybrid control combining incident plate control and cavity control, (e) hybrid control combining receiving plate control and cavity control, (f) simultaneous use of both incident plate actuator and receiving plate actuator. The analytical results show that hybrid control combining incident plate control and cavity control and simultaneous use of both incident plate actuator and receiving plate actuator are desirable configurations for the active control of sound transmission into a structural-acoustic coupled system surrounded by two flexible plates in terms of the acoustic potential energy inside the enclosure.

Abstract:
This paper presents the activities developed by the authors within the research project named M.E.S.E.M.A. (Magnetoelastic Energy Systems for Even More Electric Aircraft) funded by the European Union. One of the main targets for the MESEMA Consortium consists in reducing the level of disturbance noise in turbofan aircraft. A noise & vibration control system using magnetostrictive actuators has been designed and will be developed with the goal of controlling noise & vibrations in a frequency range between 150 – 500 Hz. The environmental noise & vibration excitations will be representative of a small/medium turbofan aircraft case. A numerical model of the test article (a fuselage section) has been developed coupling the structural part with the interior acoustic volume. The experimental vibro-acoustic characterization of the test-article has been carried out and the numerical model has been correlated with experimental results. Genetic Algorithms have been employed in order to optimize the positioning of the actuators and reach the maximum theoretical interior noise level reduction; the algorithm prediction is based on frequency response functions derived numerically. The G.A. permitted to choose the best control strategy and to evaluate the influence of the number of actuators and of their spatial distribution on the interior noise reduction.

Abstract:
This paper presents an analytical optimization tool for an active automotive suspension. Genetic algorithms are used in combination with optimal control to optimize road noise attenuation inside an automobile by active structural acoustic control (ASAC). First, a laboratory test bench consisting of a wheel/suspension/lower A-arm assembly is characterized. Frequency response functions are measured and used for the validation of a quarter-car suspension model. A twelve degrees-of-freedom discrete suspension model is combined with experimentally measured dynamic stiffness of some suspension elements to build the analytical model of the suspension. This model is then used within a genetic algorithm to optimize the actuator configuration for the ASAC control of an active suspension. Orientation, position and control inputs are optimized for a single actuator configuration and for a two-actuator configuration. Using a vibroacoustic model for noise prediction inside the cabin, it is shown that the optimization tool allows to optimize the noise reduction efficiency of the active suspension.

Abstract:
Car manufacturers expend lots of resources in traditional acoustic passive design to improve sound quality in a vehicle cabin. A compromise is often made between desired and achievable sound field into the vehicle cabin at driver and passengers seat location. In the Work Area “Sound Quality of Interior Noise” from the European project InMAR (Intelligent Materials for Active Noise Reduction), leading by Ford, the objective is to design a Sound Profiling Active Noise Control System and realize the vehicle’s interior sound quality through the balancing of engine orders. The first available system is composed of two loudspeakers placed in the roof-liner, two error microphone sensors on the seat-headrest and a specific hardware electronic designed by TechnoFirst. In order to reach desired SPL levels on each engine order, additional electronic and algorithm was designed to handle accurately the angular position and load of the engine in real time. L.E. Rees and S.J. Elliot [1] specify the use of a C-FXLMS adaptive algorithm which is in charge of producing the desired sound field at driver and passengers’ seat locations. The feasibility has been made on a Ford Focus C-Max. SPL Target values were reached at +/- 2dB during an engine run-up from 2000 to 6000 RPM (70 to 200Hz). This article introduces the built-up of the Sound Profiling Active Noise Control System and first results obtained on the Ford Focus C-Max.

Abstract:
Low frequency cabin noise and vibration in propeller aircraft is dominated by the propeller blade-pass frequency and its low-order harmonics (< 500 Hz). Passive control at these frequencies is difficult, as it usually requires unacceptable weight penalties. Previous studies have shown that properly optimised propeller synchrophase angles can result in significant global cabin noise and vibration reductions (up to about 10 dB), not just the redistribution of this noise and vibration. However, most existing synchrophasers use fixed synchrophase angles, precluding the adaptation to changes in the cabin noise environment brought about by different flight conditions. This paper presents the preliminary results from an on-going investigation into the effects of different altitudes and airspeeds on the optimum synchrophase angles in a 4-engined military propeller aircraft, and the potential for active synchrophasing to compensate for these effects.

Abstract:
The focus of the work presented here is to explore the potential of a field programmable analog array (FPAA) hardware controller to reduce the low-frequency interior noise level inside a Holden Commodore station-wagon. The sound pressure levels measured inside the cabin when travelling at 80km/hr with a window open and with all windows closed were approximately 115dB and 90dB at 15Hz, respectively, which is a source of infrasound exposure to occupants. A simple feedback active noise controller was constructed to demonstrate the potential for a FPAA controller, not to optimise the noise reduction inside the vehicle. The predicted results matched the experimental results, which showed a small reduction in the interior sound pressure levels. The results indicate that active noise control has the potential to reduce the low frequency noise inside the cabin and that the FPAA controller is an extremely useful tool for the development of analog controllers.

Abstract:
This paper presents a numerical investigation of an active structural acoustic control strategy for coupled aircraft-style bays. While structural coupling can destabilize or limit the performance of some model-based decentralized control systems, fully coupled centralized control strategies are impractical for typical aircraft containing several hundred bays. An alternative is to use classical rate feedback with matched, collocated transducer pairs to achieve active damping. Unfortunately, due to the conservative nature of this strategy, stability is guaranteed at the expense of achievable noise reduction. Therefore, this paper describes the development of a combined control strategy using robust active damping in addition to a high-authority controller based on linear quadratic Gaussian (LQG) theory. The combined control system is evaluated on a tensioned, two-bay model using piezoceramic actuators and ideal point velocity sensors. Transducer placement on the two-bay structure is discussed, and the advantages of a combined control strategy are presented.

Abstract:
This paper presents theoretical and experimental results on the implementation of decentralised active wedges on a smart panel. The active wedges are made by a triangularly shaped piezoelectric patch actuator, with the base edge aligned along the border of the panel, and a point velocity sensor at the tip of the actuator. In this way the actuator generates bending moments along the lateral edges and a transverse point force at the tip. Thus, when a velocity feedback control loop is implemented, the collocated transverse force excitation and velocity detection produces an active damping effect on the panel. When the panel is equipped with a large set of these control units then good reductions of vibration and sound radiation can be obtained at low frequencies where the response of the panel is controlled by lightly damped resonances of low order modes. The paper is focused on the design and implementation of a single control unit. The stability of the control loop is analysed theoretically with a fully couple model of the sensor, plate and actuator elements of the system. In particular the effects produced by the bending moment and transverse point force actuation components are analysed independently to better understand their effects on the stability and controllability of the feedback control loop. Simulation results have been validated using experimental open loop and close loop measurements taken on a prototype smart panel.

Abstract:
The concept of using an active structural acoustic control system attached to an airframe structure to address the problem of excessive tonal interior noise levels inside vibration sensitive aerospace applications can on occasion produce unwanted simultaneous increases to the structure vibration levels. In this paper, the trade-off in global interior noise reduction performance required to accommodate the additional requirement of there being no net increase to the global structure vibration levels, under the control action of an active structural acoustic control system, is examined. The potential for controlling both interior noise and structure vibration levels simultaneously will be discussed across different frequency cases (representing scenarios of different coupling complexity) for the example of a stiffened cylinder with floor structure and enclosed cavity. Simulation results support the notion that simultaneous control of interior noise and prevention of structure vibration increase (under controlled conditions) is both plausible and reasonable with a relatively small trade-off in the interior noise reduction performance. This occurs when measures for both global interior noise levels and global structure vibration levels are considered integrally into the cost function that guides the selection of the optimum vibration actuator locations using a genetic algorithm search optimisation procedure.

Abstract:
A theoretical analysis of the active control of low frequency radiated pressure from submarine hulls is presented. Two typical models are examined in this paper. Each of the models is a water-loaded finite stiffened cylindrical shell with a hemispherical shell on one end and conical shell on the other end, which forms a simple model of a submarine hull. The conical end is excited by an axial force to simulate propeller excitations while the other end is free. The control action is implemented through a Tee-sectioned circumferential stiffener driven by pairs of PZT stack actuators. The actuators are located under the flange of the stiffener and are driven out of phase to produce a control moment. In general, it was found that the control system was capable of reducing 40% to 95% of the total radiated pressure from each of the submarine hulls for the first three axial modes.

Abstract:
This paper presents a free vibration analysis of a geometrically imperfect, smart FGM laminated plate that is composed of a shear deformable functionally graded layer and two surface-mounted piezoelectric actuator layers. The theoretical formulation is based on Reddy’s higher order shear deformation plate theory and includes the effect of a pre-vibration state induced by a temperature change and an applied control voltage. By adding an incremental dynamic state to the pre-vibration state, the governing differential equations are derived for the flexural vibration of imperfect smart FGM laminated plates. A semi-analytical method that makes use of one-dimensional differential quadrature and Galerkin technique is proposed to predict the vibration behavior of laminated rectangular plates with two opposite clamped edges and different initial imperfections. Numerical results are presented in both tabular and graphical forms, showing the influence of imperfection mode, control voltage, out-of-plane boundary support, temperature change, and the side-to-thickness ratio on the vibration characteristics of the plate.

Abstract:
A new type of distributed active vibration absorber using MR fluids embedded in a sandwich beam structure was designed to suppress unwanted structural vibrations. The paper introduces an analytical model for the MR sandwich beam structure based on the Kelvin-Voigt model and the Hamilton’s principle. The relationship between the magnetic field and the complex shear modulus of MR sandwich beam in the preyield regime is presented. The governing partial differential equations describing the dynamics of the MR sandwich beam are derived and a model analysis is performed. An active structural vibration controller based on the Lyapunov stability theory is designed. Simulation results show the stable response and improved transient performance provided by the MR fluid damper.

Abstract:
The paper summarises the results of experimental testing of a controllable friction magnetically responsive damper containing magnetorheological (MR) fluid. The damper was tested under various loads applied in the conditions of magnetostatic and fluctuating magnetic fields. Based on the acquired data, force responses and energy dissipated by the damper in terms of the input current and piston velocity were determined. It was revealed that the damper response time can be significantly reduced when using current driver. Finally, the linear model of the damper driven by a current driver is identified and verified.

Abstract:
In August 2005 Boeing Commercial Airplane Group successfully flight tested a morphing aerostructure driven by shape memory alloy (SMA) actuators. These devices, known as Variable Geometry Chevrons (VGC) are serrated aerodynamic surfaces mounted at the trailing edge of the jet engine’s primary and secondary exhaust nozzles. The presence of chevrons has been shown to greatly reduce jet noise by encouraging advantageous mixing of the free, fan, and primary streams. VGCs utilize compact, light weight, and robust shape memory alloy actuators to morph the chevron shape to optimize the noise reduction or meet acoustic test objectives. A thermally induced change in SMA microstructure results in a macroscopic actuator shape change which provides the actuation capability for the VGCs. The VGC system operated in both autonomous and powered modes. In the autonomous (unpowered) mode the system relies on changes in environmental and fan exhaust temperatures to heat and cool the actuators. This provided one chevron shape (tip immersion) at take-off in the heated or transitioned state and another during the cooler cruise state. During powered operation, the heaters were used to control the actuator temperature and hence tip immersion. The VGC program had a number of unique features. This was the first flight test of a shape memory alloy actuator used to morph an aerospace structure to reduce jet noise. The VGC system demonstrates a new capability for validating designs with flight tests. With variable geometry technology, a component can be tested in numerous configurations during one flight test. Finally variable geometry technology demonstrates new and as yet unexplored capability to optimize aircraft performance for multiple flight conditions. Designers can use this technology to optimize an aircraft component for different flight conditions rather than being limited to a compromise design point. This presentation will highlight these features of the program, discuss the system design, and conclude with VGC system flight test results.

Abstract:
Piezoelectric materials exhibit impedance-dependent stiffness and damping properties when an electrical network is connected across the material. This behavior has been exploited to implement a variety of passive and semi-active shunt-damping schemes for the purposes of increased energy dissipation on structures with adhered piezoelectric patches. More recent work has explored the use of negative impedance or negative capacitance shunts to provide even greater flexibility for adapting of stiffness and damping properties, including the possibility of "negative stiffness". This paper explores the use of negative impedance circuits for active vibration control on a cantilever beam. Shunted piezoelectrics may be interpreted in terms of classical fully active feedback control, and represents completely collocated sensing and actuation. Particular challenging aspects of the use of negative capacitance circuits includes the tuning of the impedance applied to the piezoelectric by the circuit to achieve desired ends. The recently proposed concept of wave-tuning is shown to be equivalent to reactive input power reduction. Results are presented for the experimental application of the concept for reduction of cantilever beam vibration.

Abstract:
We present here the results of the investigation on the acoustic feasibility of local feedforward control in call centers, the objective being to reduce the ambient noise at the ears of the operators thanks to their headset. The acoustic field of a call center combines multiple sources of broadband non-stationary noise with a non-anechoic room, and can then limit the performance of feedforward control because of loss of coherence or causality between the reference signal(s) and the error signal. In order to see the influence of those two acoustic constraints upon the control and to select the position and number of reference microphones leading to the best attenuation on the error microphone, frequency and time-domain simulations of optimal control were computed on experimental data. The data were recorded in an anechoic room, in a call center and in a work office, by using multiple reference microphones and an artificial head, as the error microphone. We show the results of single reference control computed with either remote or close reference signals, for anechoic and non-anechoic conditions and for various numbers and positions of white noise primary sources. We also show the results of multiple remote reference control compared to single close reference control of multiple non-stationary sources in the three types of rooms. The conclusion is that the control seemed to be more limited by loss of coherence than by loss of causality, which led to the selection of the "single close reference" configuration. The control simulated with this configuration performed up to 20 dB attenuation of the ambient noise of the call center in the 0-1500 Hz frequency range.

Abstract:
Multichannel active control is a most topical field in recent years, because it has become fundamental theory for solving a wide variety of practical problems. Compared with the signal-channel ANC system, the complexity of multichannel ANC with many inputs and outputs is significantly higher. The performance of ANC will be more limited because of the interaction between each channel and the causality constraint. In our study, by introducing an auxiliary adaptive filter, in which both reference signals and target disturbances are whitened by using multichannel Yule-Walker algorithm, we can reach the optimum solution very fast because both covariance matrix of reference signals and disturbances are normalized to unit matrix. When the auxiliary adaptive filter converges, we can transform the optimum solution to a general multichannel optimum solution with regard to plant model. In this paper, some fundamental matters of multichannel ANC system and multichannel Yule-Walker algorithm are described. Then a new structure and theoretical results for multichannel active control problem are given. As a practical example, some numerical simulation results for a 2 channel ANC problem are also included.

Abstract:
A feedforward energy-based active noise reduction system has been applied to earth-moving equipment cabs, for which tracking of rapid changes to the reference input signal is required. In order to better understand the potential advantages or disadvantages of minimizing acoustic energy density (ED) versus squared acoustic pressure (SP), efforts have been made to quantify and compare the convergence rate and tracking speed of the two control approaches. For both filtered-x algorithms, the upper bound of the convergence parameter is established to provide a consistent means of comparing performance. Results will be presented to demonstrate initial convergence time for a stationary reference input signal and time-averaged noise level reduction for a non-stationary input signal.

Abstract:
Various structural measures against vibration and noise were taken in a training ship, Oshima Maru. However, an unpleasant sound persisted in the mess hall, where crews take their breaks. In order to reduce the noise, active controllers were investigated and one of them, the phase corrected error LMS algorithm, is corrected to satisfy causality on its update processes. Some of them were preconditioned using the inverse of the plant because their convergence rates are limited by the dynamics and coupling within the plant response. The algorithms were compared under the same conditions to investigate differences in their properties and also corrected to satisfy the causality of their update processes. Simulations for a control system were introduced using plant responses measured from a loudspeaker to a microphone in the mess hall inside Oshima Maru. After investigating the convergence speed in various gradient descent adaptation algorithms, the results were integrated with the actual plant response and applied to the active control of ship interior noise.

Abstract:
The development of feedback controller for flexible structures is usually undertaken by developing a modal model of the structural response. Because it is not possible to model the infinite number of modes which make up the total structural response, a truncated model is used where the influence of all modes resonant above a certain frequency is ignored. The effect of these unmodelled modes is usually taken into account using an approximation known in the model, this improving controller's spillover and stability characteristics. In some cases, it is desirable to control a structure over a finite bandwidth which excludes some low frequency modes. In this case, the optimal model excludes both low and high frequency modes. The work described in this paper is directed at developing approximation terms to include in the model the effect on the structural response of both low and high frequency unmodelled vibration modes. Procedures for calculating the optimal correction terms that include the lower and higher order mode contributions are outlined. The new model has a lower order compared to the standard model that usually includes all the lowest vibration modes up to the highest frequency of interest. The implication of this approach is that the optimal controller so obtained will have a lower order structure, making it more effective in controlling the vibration modes of interest.

Abstract:
Active ear defenders are increasingly being used in industry where low frequency noise can cause communication problems between workers. These devices typically make use of a dynamic loudspeaker fitted inside each earmuff to provide active noise cancellation while at the same time improve communication. In this paper an analytical model of a feedback control ear defender including porous absorption material and leakage is presented for low frequencies. In the first instance the effects of the loudspeaker and its properties on the sound transmission characteristics of the passive earmuff are studied. The results show that the presence of the diaphragm can adversely affect the performance of the system if wrongly selected. The effect of leakage is more complex and may introduce a new resonance into the system response while at the same time shifting the original resonance frequency to a higher value. In the second instance a model for the electroacoustic frequency response of the active ear defender is developed. It is shown that the frequency response depends on the speaker characteristics and the amount of porous absorption and leakage. As a result the performance of the feedback control ear defender also depends on its electro-mechano-acoustic properties. Simulation and experimental results are presented to gain an insight into the mechanisms of feedback active ear defenders and can be used as a guide in their design.

Abstract:
The presented semi-active friction device is composed of friction pads acting on a mobile component. Two piezoelectric stack actuators, generating the normal force on the pads, are controlled by a nonlinear feedback designed either by the Lyapunov method or the feedback linearization approach. An original experimental implementation of the controller is realized with a frequency subband approach which allows adding a phase-shift compensation in the nonlinear feedback. Experimental results were obtained with a clamped-free beam excited by a sinusoidal force. The results show the influence of the control parameters on the dynamics of the mobile component, the dissipated power by the pads. It appears that for a proper choice of the phase-shift compensation, the sticking period per cycle is reduced, and consequently the power dissipation is increased.

Abstract:
The response of a parametrically excited Duffing-van der Pol oscillator under linear-plus-nonlinear position feedback control with a time delay is investigated in this paper. By means of the averaging method and Taylor expansion, the equations governing amplitude and phase of its response are derived. Based on the Routh-Hurwitz criterion, the stability analysis of the trivial and nontrivial solutions is presented. It is found that the stable condition of the trivial solution is only associated with linear feedback, whereas the stable condition of nontrivial solutions is associated with both linear and non-linear feedback. The stability analysis shows that the trivial solution can be stabilized by linear feedback control. And the bifurcations of nontrivial solutions can be modified by linear-plus-nonlinear feedback control. It is shown that the numerical results obtained from the direct integration of the original equation are in good agreement with the theoretical analysis.

Abstract:
Nonresonant bifurcations of codimension two may appear in the controlled van der Pol-Duffing oscillator when the two critical time delays corresponding to two Hopf bifurcations have the same value. In the vicinity of nonresonant Hopf bifurcations, the presence of periodic excitations in the controlled oscillator can induce complicated behaviour of the system. In addition to nonresonant response, some types of resonances including primary, super- and sub-harmonic resonances, additive and difference resonances may appear in the forced response of the controlled system, when the frequency of the external excitation and the frequencies of the Hopf bifurcations satisfy a certain relationship. With the aid of normal form theory and the centre manifold theorem as well as a perturbation method, the nonresonant response of the controlled system at the nonresonant bifurcations of codimension two is investigated by studying the possible solutions and the stability of the four-dimensional ordinary differential equations on the centre manifold. It is shown that the nonresonant response of the oscillator may exhibit quasi-periodic motions on either 2-D or 3-D tori. Illustrative examples are given to show the quasi-periodic motions. The analytical predictions are found to be in good agreement with the results of numerical integration of the original delay differential equation.

Abstract:
Broadband active sound quality control (ASQC) algorithms based on frequency-domain and delayless subband adaptive filters are developed in this paper. When compared to a time-domain active noise equalizer, these algorithms provide a faster convergence rate and a reduced computational complexity. The shaping filter is designed based on the equal-loudness compensation and the graphic equalization methods. Computer simulations validate the usage of these algorithms in applications requiring high-order adaptive filters.

Abstract:
Wideband active noise control systems usually have hundreds of taps for control filters and the cancellation path models, which results in high computational complexity and low convergence speed. Several active noise control algorithms based on subband adaptive filtering have been developed to reduce the computational complexity and to increase the convergence speed. The subband structure is similar to the frequency domain structure but differs in the time domain processing of the subband signals. This paper is going to discuss several issues of implementing the delayless subband active noise control algorithms on a DSP platform, such as the modeling of the cancellation path in subbands and the partial update of different subbands.

Abstract:
This paper discusses some modifications made to a well known FXLMS adaptive filtering algorithm in order to improve its performance, namely rate of convergence, minimum mean squared error and computational load of the implemented algorithm. The presented modification – Filtered-Inverse LMS (FILMS) algorithm uses the inverse of the secondary acoustic path in order to improve system convergence. Stochastic analysis of the FILMS algorithm is performed without the use of independence theory (IT) assumptions, for the case of slow adaptation of filter weights. Simulations, performed in order to confirm the accuracy of our approach, are presented in the paper showing very good agreement between the developed theory and the algorithm performance. Some initial laboratory results using single and multichannel FILMS algorithm configurations are presented at the conclusion of the paper.

Abstract:
Sound field reproduction finds applications in music or audio reproduction and experimental acoustics. For audio applications, sound field reproduction can be used to artificially reproduce the spatial character of natural hearing. The general objective is then to reproduce a sound field in a real reproduction environment. Wave field synthesis (WFS) is a known open-loop technology which assumes that the reproduction environment is anechoic. For classical WFS, the room response thus reduces the quality of the physical sound field reproduction. In this paper, adaptive wave field synthesis (AWFS) is analytically investigated as an adaptive sound field reproduction system combining WFS and active control with a limited number of reproduction error sensors to compensate the response of the listening environment. The fundamental behavior of AWFS is illustrated by analytical considerations and simple free- field simulation results. As demonstrated, AWFS is fundamentally related to WFS and “Ambisonics”. The paper introduces independent adaptive control of sound field reproduction on the basis of radiation modes, via the singular value decomposition of the transfer impedance matrix. Possible practical independent control of radiation modes for AWFS is discussed.

Abstract:
The article shows digital systems of active sound control in close space. The first group describes digital systems, enabling active sound control according to the set criterion. Theoretical fundamentals of these systems are based on digital filter theory, based on which digital graphic, phase, inverse and adaptive equalizers have been designed and programmed. The described systems have been implemented into two cards with signal processors TMS320C25 and TMS320C31 made by Texas Instruments. The article presents mathematical models of the above-mentioned equalizers and results of experimental tests, which verify their performance.The second part of article presents acoustic climate control system on the example of the City of Cracow. The system core are information layers made in the SoundPlan, MapInfo and GRASS software. In order to carry out acoustic climate control, we decided to use proven spatial information system technology (GIS). Acoustic climate control, and thereby sound control in large areas shall be performed using the program running based on genetic algorithms and graph theory. The system will be based on numerous constant and variable symptoms maintaining correct and even comfortable acoustic climate conditions on the maximally largest area of the town, urban agglomeration or commune.

Abstract:
Active noise control of the nonlinear noise processes uses nonlinear control structures such as Volterra filter or different kinds of neural networks. Filtered-s LMS (FSLMS) algorithm is a recently proposed algorithm, which uses functional link artificial neural network as its control structure. It is seen that filtered-s LMS algorithm outperforms the ANC algorithms like FXLMS and also the Volterra filtered-x LMS (VFXLMS) under higher order nonlinearity case. It is also seen that the nonlinear ANC algorithms like FSLMS or VFXLMS algorithms involves higher computational complexity than the linear ANC algorithm such as FXLMS algorithm. In this paper, Hartley transform is used to implement exactly the same FSLMS algorithm involving lesser number of computations.

Abstract:
In this paper a multi-channel feedforward adaptive control algorithm is described which has good convergence properties while having relatively small computational complexity. This complexity is similar to that of the filtered-error algorithm. In order to obtain these properties, the algorithm is based on a preprocessing step for the actuator signals using a stable and causal inverse of the transfer path between actuators and error sensors, the secondary path. The latter algorithm is known from the literature as postconditioned filtered-error algorithm, which improves convergence speed for the case that the minimum-phase part of the secondary path increases the eigenvalue spread. However, the convergence speed of this algorithm suffers from delays in the secondary path, because, in order to maintain stability, adaptation rates have to be lower for larger secondary path delays. By making a modification to the postconditioned filtered-error scheme, the adaptation rate can be set to a higher value. Consequently, the new scheme also provides good convergence if the system contains significant delays. Furthermore, an extension of the new scheme is given in which the inverse of the secondary path is regularized in such a way that the derivation of the modified filtered-error scheme remains valid.

Abstract:
High-technology lightweight applications will have to satisfy increasing structural-dynamic, damping and acoustic requirements in order to fulfill the rising comfort and environmental demands. Here, the specific use of fiber-reinforced multilayered composites with integrated actuators rise the possibility for new lightweight vibro-acoustic solutions. Therefor, material-adapted simulation concepts are necessary combining the actuator effect with the directional dependent material properties and the directional dependent sound radiation of the anisotropic composite structures. At the ILK advanced analytical and numerical simulation methods were developed considering “passive” composite-specific material parameters (e.g. fiber-angle) as well as actor-specific variables (e.g. actuator-size) within the design process. Especially, the analytical calculation procedures offer the possibility of effective parameter studies on the influence of practically relevant variables like actuator forces, locations, distribution and phase-shift. This modeling approach can be used to create a material-adapted feedback control of the vibration loads by means of an optimal distribution of the used actuators. Thus, the calculation engineer now can use an effective tool for the design of new actively damped lightweight structures for high-technology applications with optimized dynamic and acoustic properties.

Abstract:
In discrete frequency control, the design aim is to minimize the vibration at specific measured points on a structure and a number of well established design methods are available . The optimum solution, however, is often based solely on information local to the actuators and implementation can, in practice, result in increased levels of vibration at remote locations. Attainment of a globally optimal solution may therefore necessitate the implementation of a locally sub-optimal one. In this paper a number of new results are presented that define the freedom available to the designer for providing both local and remote reductions in vibration and a simple, yet powerful, geometric design methodology is introduced. The work was motivated by the authors’ previous research that has been aimed at reducing vibration in marine systems and where it is often not practically viable to locate either sensors or actuators at the points where vibration attenuation is required. The new design methodology is illustrated using a laboratory scale test rig that has been developed to replicate the problems associated with the propagation of rotor blade vibration through the thrust bearing.

Abstract:
The “harmonic control” or “convergent control” algorithm is a well-known method to suppress harmonic disturbances in the active control of vibration. The algorithm iteratively changes the amplitude and phase of the control signal, using either the adjoint or inverse of the control path model at the disturbance frequency, so that the sum of the control path and disturbance path is driven to zero. In theory the algorithm should be implemented using a steady-state approach, where the control signal is updated only after the output of the control path has achieved steady-state in response to the previous update. Recent results obtained by the authors have established conditions under which an instantaneous implementation of the algorithm, whereby the control signal is updated at each sampling point, is stable. In this paper these results are extended to include the case of the simultaneous control of multiple harmonics. In order to validate the theoretical results, the algorithm is implemented on a rotor test rig having a 3-kg rotor supported by journal bearings and with a critical speed of approximately 50 Hz. The objective was to control the radial response at the rotor mid-point by using an actuator located outside the bearing span. Control forces were generated with an electro-magnetic actuator and excellent reduction of both the fundamental and first harmonic vibration modes has been demonstrated.

Abstract:
This paper describes an active vibration control to effectively reduce the unbalanced vibration of an overhung rotor system. A flexible bearing pedestal supporting the overhung rotor is installed in four pairs of U-shaped electromagnets as a control device. Feedback gains of the vibration control system are decided by solving the optimal regulator theory with Kalman filter. The control currents calculated from the feedback gains are fed back to the electromagnets in the flexible bearing to give the control force, and the vibrations of the overhung rotor are suppressed indirectly. Influence of weighting coefficients in the cost function is investigated, and the optimal feedback gains to give the effective vibration suppression performance are provided. Frequency response curves are simulated numerically and the effective vibration suppression performance is shown. The numerical results are compared with experimental ones for the overhung rotor system with the vibration control device, and both results show the similar tendency in our study.

Abstract:
In this paper a vibration control problem for an aeronautical structure is solved via a model-based co-located feedback strategy. A gray-box dynamic MIMO model of the flexible structure is identified by resorting to a modified subspace identification technique. The control law is obtained by analytically solving an optimal H2 control problem, guaranteeing strong stabilization of the controlled system and the rejection of a broadband disturbance in the frequency range [100,500] Hz. Experiments are carried out on an aircraft skin panel and results are presented both for the model identification and the disturbance rejection. Piezoelectric patches are used as actuators and a rapid prototyping control system is adopted as control hardware.

Abstract:
The work is aimed to control the spatial vibration profile of a panel structure using multiple accelerometers that are distributed over the structure. The spatial vibration profile is estimated based on the measurements from the accelerometers and a spatially weighted vibration objective is utilised to emphasise controlling a particular panel region. Experiments on a rectangular panel structure are performed to test the effectiveness of the spatial control strategy with the use of the FX-LMS based adaptive control together with the filtering of the acceleration signals.

Abstract:
When metal cutting is performed, the machining processes introduce productivity degrading vibration. By applying an active control scheme, theses vibrations can be reduced significantly with the result of improved surface finish of the work piece and increased tool life. Due to the large difference of boundary conditions during the machining process, a controller fast enough to follow these changes is needed, for example from no cutting to cutting - the actual engagement of the cutting process. If the controller does not success to follow fast enough, the tool tip might break, as is the case without any active damping. Different approaches based on feedback control are investigated; all implemented using an active boring bar. The first approach is based on a digital adaptive feedback controller; the feedback filtered-X LMS algorithm. The two other controllers are analog; one is a lead controller and the other is lead-lag controller, both with gain and phase orthogonally adjustable. This paper focuses on robutness, stability and convergence of the digital and the analog feedback controllers. The analysis is based on open loop frequency response function estimates during different operating conditions.

Abstract:
We investigate the problem of actively damping the transverse modes of a rigidly-terminated vibrating guitar string. After introducing a physical model that emulates the behavior of the system, we calibrate the model using a series of measurements. Next, we use the model to compare the controllability aspects of the closed-loop system for various sensor/actuator configurations. In particular, we consider the difference between damping only the vertical transverse modes, versus damping both the vertical and horizontal transverse modes. We choose to apply collocated control using a multi-axis piezoelectric sensor and electromagnetic actuators, which exert a localized force on the string without coming in physical contact with it. Finally, we compare the results of applying two control algorithms, which allow for large system delays, to an actual vibrating string.

Abstract:
Active magnetic bearings (AMBs) represent a smart solution for supporting rotors, that has a lot of advantages. Their application is limited to special fields where the advantages justify higher effort in development and hence higher costs. AMBs can generate a great benefit particularly in the field of high speed applications. An unbalance in the case of a overcritical operating rotor effects forces in the bearings and a deformation of the rotor especially in the frequency ranges of its flexible modes. To minimize these negative effects, the rotor must be balanced, which is extremely time-consuming. Several stops are necessary for adding weights to identify the system behavior. Active balancing devices (ABDs) are fitted to the rotor to reduce the effort. They work without contact and allow adjustment of the balancing state of the rotor during operation. To detect the effects of unbalance the AMB embedded tool unbalance compensation can be used. The motivation of this work is to emphasize the advantages, which are reached by the combination of AMBs and ABDs.

Abstract:
In this paper the application of distributed vibration control for a flexible structure is studied both analytically and experimentally. The purpose is to investigate the effectiveness of distributed vibration control strategies and compare them with centralized and decentralized methods. A simply supported beam is chosen as the illustrative flexible structure. Distributed control architectures are designed based on a system identification model and are used to minimize vibration due to broadband disturbances. Experimental results are presented for the control of vibrational modes below 600 Hz. It is demonstrated that the distributed control architecture presented here approaches the performance of a traditional centralized controller employing the same control effort. In addition, in comparison to centralized control, the distributed controller has the advantages of scalability for application in large systems and that it will continue to perform (although probably with diminished capability ) even if some processors fail.

Abstract:
The objective of this work is to demonstrate a fault-tolerant vibration control system applicable in higher order systems. System failures are detected and isolated by Beard-Jones (BJ) filters. When such a failure is detected, the Fault Detection and Isolation (FDI) filter output is supplied to a hybrid automaton, which switches the system to a new controller that is specifically designed for the faulty system condition. The closed loop system, therefore, maintains optimal performance and stability under failure conditions. The two most significant contributions of this work are: 1) the demonstration of a fault adaptive control system applicable to higher order systems, and 2) a new methodology for designing BJ filters for high order systems, and systems with feed-through dynamics (i.e. output is a function of input). The capabilities of such a system are demonstrated through simulations based on analytical and experimentally obtained system models. The results provide a benchmark for the design of detection filters for use in fault-tolerant active vibration control.

Abstract:
Active vibration isolation of large load structures typically requires significant actuator energy. With a zero stiffness support, which can be realised with a permanent magnet system, reactive forces may be applied to the structure with comparatively little effort. This paper presents simulation results for such a magnetic spring arrangement which is stabilised by a non-linear controller. Transmissibility less than unity is demonstrated over the entire frequency spectrum.

Abstract:
The engine is the heavy and rigid component in the automobile. As the engine prevents the deformation of the frontal structure to absorb the collision energy during automobile collision, it has been commercially applied that the engine is separated from the main frame in order to keep the deformation space. In contrast with this concept, we took advantage of the engine to reduce the deceleration of the car during the collision. The present engine mount not only mounts the engine in the vertical direction to isolate the vibration, but also controls the movement of the engine in the horizontal direction to reduce the deceleration by varying the damping force between the engine and the engine mount filled with MR fluid, whose rheological characteristics can be varied by an external magnetic field. In this study, the MR engine mount was designed and the characteristics of the mount were verified.

Abstract:
A submarine hull is excited by vibration transmission through its propeller-shafting system which originates at the propeller due to the fluctuating pressure field. Excitation of the submarine from the propeller-shafting system is a significant source of radiated noise. The vibration transmission can be reduced by using a resonance changer. A resonance changer introduces virtual elastic, damping and inertial influences by hydraulic means, thereby acting as a hydraulic dynamic vibration absorber. In this paper, active tuning of the resonance changer stiffness to minimise the low frequency vibration transmission to the submarine hull is examined. Results are compared with those obtained using a passive resonance changer with optimal parameters obtained by minimising the vibration transmission using a genetic algorithm.

Abstract:
The paper presents the analytical model, numerical simulation and experimental results for the implementation of various semi-active control systems with magnetorheological fluid dampers (MRFD), by using programmable logical controllers (PLCs). A new method of real time control and the on-line monitoring of the trial stand of Magnetorheological Damper Control Systems are presented. This allows the generating of the force-movement work behaviour and of the force-velocity hydraulic behaviour for circular movements done with a rod-handle mechanism. The automation system ensures the adjusting of the circular movement’s amplitude and frequency so that it would cover the entire range of applications necessary to the systems’ attempts of semi-active control. A frequency converter with vector control and electromagnetic flux control allows the adjusting of the entrapment revolution with a wide range and high precision for a prescribed velocity. Quasi-simultaneously the damping force and the movement are measured through the two multiplexing signals of the PLC central unit. The experimental and predicted results are in good agreement, showing the possibility of practical implementation of semi-active control strategies with MRFD, by using PLCs. The developed method is also useful for validation of the analytical models developed to portray the dynamic behaviour of MRFDs.

Abstract:
Adaptive Helmholtz resonators are used to reduce tonal noise propagating as plane waves in ducts. Optimal tuning of the resonator has previously been achieved by using a pressure sensor located in the duct downstream of the resonator. The work described here is concerned with the development of a cost function that could be used by a controller to optimally tune the Helmholtz resonator without any in-duct pressure sensor. The cost function that was developed is based on the phase difference between the pressure at the top of the closed end of the cavity of the Helmholtz resonator and the pressure at the neck wall, close to the neck duct interface. Damping in the system is taken into account using a correction factor applied to the cost function.

Abstract:
The presented work deals with hybrid reduction of acoustic transmission through a concept panel. A piezoelectric actuator is added to a classical sound package made up of two elastic plates and a poroelastic core. A complete description of the sandwich dynamical behavior is obtained using a finite element implementation in COMSOL environment. The poroelastic material is modeled using a pressure-displacement formulation. The piezoelectric effects are also included. Interactions between all layers are then fully considered. The transmission loss of such a panel is numerically predicted. It is shown, that the active patches improve performance in low frequencies, whereas passive layers are known to be efficient at high frequencies. Predicted results for these two operating modes are compared to experimental data. Discrepancies are discussed in terms of coupling and boundary conditions mainly. Numerical updating of the sandwich finite element model is also considered.

Abstract:
The design, fabrication and testing is presented of a micro velocity sensor to be used in combination with a piezoelectric patch actuator to form a closely located sensor–actuator pair for the implementation of direct velocity feedback control loops. The aim of this study is to describe the operation principles and the experimental results of this new sensor built using MEMS (Micro Electro Mechanical System) techniques. By means of an internal velocity feedback control loop, this sensor directly provides an output signal proportional to the velocity at its centre and also provides a low pass filtering effect. These response characteristics make more stable the control system since it improves the collocation and duality properties of the sensor–piezoelectric actuator pair and also attenuate the unwanted higher frequency excitation effects produced by the piezoelectric actuator itself. In this way a simple control system can be built where the micro fabricated sensor is integrated together with a miniaturized amplifier to the piezoelectric patch actuator. Arrays of these control units can then be bonded on thin structures in order to get large damping effects that reduce the overall vibration of the structure at low frequencies.

Abstract:
In order to yield high sound amplitudes at low frequencies in active noise control applications a electro pneumatic transducers as alternatives to commonly used electro dynamic loudspeakers are suggested in literature. Such a sound source consisting of a modulated air flow was investigated and control algorithms for this kind of nonlinear secondary sources were developed. Two models of this nonlinear secondary path were investigated: firstly, a black box model consisting of a time delay neural network (TDNN) and, secondly, a white box, respectively grey box, model derived directly from the underlying differential equations and a time discrete version of it, with partly estimated parameters. The details of modelling of the analytical model is shown and some general remarks on the modelling of the TDNN secondary path model are given.

Abstract:
The parametric array uses two ultrasonic sources to generate an audible sound wave by non-linear interaction. The audible sound wave frequency is equal to the difference between the two ultrasonic carrier frequencies. This scattered wave is far more directional than would be generated by a conventional transducer operating at the same frequency. This paper is reports a theoretical study into the use of the parametric array as a secondary source for active noise control. It is shown that a highly directional zone of quiet can be produced. A study of the required source strengths and carrier frequencies with respect to secondary source authority and frequency bandwidth is reported. The effect of primary and secondary source separation is also investigated. The high levels of ultrasound required to create useful secondary field levels are noted and the safety of such fields discussed.

Abstract:
This paper presents a new method for sensing the radiation mode amplitudes of vibrating beams with different boundary conditions by means of piezoelectric fibers. Two straight and one curved fiber bonded on the surface of the beam are used as sensors. The sensor output is defined as the output current of the curved fiber minus the output currents of the other two straight fibers. By designing the appropriate shape of the curved piezoelectric fiber, the sensor output can directly lead to the corresponding radiation mode amplitude of the beam. The equations which define the curved sensor shapes are independent of the type and position of the external excitation. Finally, a numerical example is given and the effects of shape error and location error of this type of sensor are discussed.

Abstract:
This paper delves into a new research area of noise masking for active noise control (ANC) systems. Methods to enhance the performance of existing ANC techniques for a more consumer-centric environment are explored. Snore ANC systems are implemented with an added noise masking using soothing audio to further increase the end user’s comfort level. Soothing audio is used to mask the residual snore noise which is still annoying to the human ear in a very quiet environment. To reduce the required audio volume, the masker is passed through a psychoacoustic processor that takes advantage of the limits in the perception of sound by human ear. As a further step towards improving the system performance, off-line and on-line modeling of secondary path using soothing audio have been implemented and evaluated by computer simulations.

Abstract:
This paper presents a first step towards the study of noise reduction inside a helicopter cabin. In order to completely evaluate the acoustic processing of a noise reduction system, psychoacoustic experiments with human subjects are needed. However, as it is undesirable to fly passengers in a prototype aircraft, for reasons of safety and cost, a mock-up can be advantageously used for such tests. Our goal was to reproduce, with high psychoacoustic fidelity, helicopter cabin noise recorded during a test flight. Moreover, as it is uncomfortable to expose subjects to intense noise environments for a long time, the noise must be reproduced at a lower level, while maintaining the same psychoacoustic frequency balance (loudness curves) as in the real situation. A solution was developed in which a sound reproduction system, based on a frequency domain active noise control algorithm, was used inside a helicopter cabin mock-up. By adding loudness equalisation, the noise could be reproduced at lower levels, while maintaining the same relative sensations at each frequency. Our mock-up has provided very good test results, and is now ready for acoustic control tests with human subjects.

Abstract:
Acoustic Reflectometry (AR) is a promising method of determining the geometry of unfamiliar passages, which can be described as an acoustic waveguide at low frequencies compared to the duct cross-sectional area. In its basic form, this technique measures the acoustic pressure fluctuations generated by cross-sectional area changes in the waveguide. Knowing the time difference between the initial excitation of the waveguide and the magnitude of the reflected acoustic pressure pulse will give an estimate of the distance from the acoustic source, the area change at this distance, and potentially other geometric features which will be discussed further in this paper. For this research, an analytical model of bifurcating ducts has been developed and techniques have been developed for estimating duct parameters from duct impulse response. Experimental verification of the technique has been performed on a small and mid-scale acoustic waveguide. Results indicate that estimation of geometry of the initial duct and the first bifurcation are possible, however the rapid increase in the impulse density due to multiple reflections results in an indeterminate geometry past the first bifurcation.

Abstract:
Most of the active noise control (ANC) schemes propose the attenuate of noise by spatially locating sensor(s)/actuator(s) strategically. The overall control effect may be a locally targeted sound attenuation at the expense of sound amplification at other parts of the spatial system. Unlike the conventional speakers used in many ANC problems, the parametric array loudspeaker (PAL) is unique because it produces a highly directional insonified column of audible sound by exploiting the nonlinear interaction of finite-amplitude ultrasounds in the atmospheric air. There is a dearth in the literature concerning the use of PAL as a controller for ANC purpose. However, it is also known that the PAL performance is hindered by inherent poor low-frequency response and temporal demodulated sound. This paper discusses an original study which utilizes a simple control algorithm to demonstrate the prospect of using a commercial PAL for ANC. Preliminary studies investigate the control at higher audible frequencies since the PAL has a more reliable tonal characteristic in that range. The result reveals that single-frequency control, of which the primary disturbance signals are controlled by the secondary PAL source, at a virtual localised point is promising if the distinctive characteristics of the PAL is taken into account.

Abstract:
Active control of wall reflections could be used to lower the cut-off frequency of large anechoic rooms. This paper outlines numerical simulations, using an image source method, of reflection control in a room equipped with wall-embedded secondary sources. According to Green’s integral representation of the scattered acoustic pressure, adequate linear filtering of total acoustic wall pressure provides error signals which account for wall scattering. Usual algorithms such as the Filtered-Error Least Mean Square algorithm (FELMS) can then be used to minimise the error signals. In the present paper, frequency-domain simulations are given for a 10m×7m×6m anechoic room with an 80Hz cut-off frequency. The equivalent finite wall impedance used in the simulations is taken from acoustic measurements in a large Kundt’s tube terminated by a set of absorbing elements taken from the Laboratoire de Mécanique et d’Acoustique (LMA) anechoic room. Control simulations with 68 monopoles in the walls show that a significant reduction in acoustic reflections can be achieved, over the frequency range from 10Hz to 80Hz in all parts of the central measurement area of the room.

Abstract:
Low frequency soundfields, such as sonic booms, can be accurately reproduced inside a room with a large number of loudspeakers. Among various soundfield reproduction strategies, the Boundary Pressure Control (BPC), whose underlying idea is that a given soundfield inside a volume can be obtained by control of the acoustic pressure at the boundary surface, is suitable for applications of this kind because it can be implemented using adaptive filters, which is useful to compensate for the reflexions in a room. In order to evaluate what performances can be expected for such a soundfield reproduction system, two studies have been carried out. On the one hand, advantages and defects of the BPC method have been studied in an anechoic environment via a 30 x 30 multichannel active noise control experiment. On the other hand, the behaviour of a reproduction cabin using BPC has been numerically simulated with modelled and measured secondary paths. The results of both studies show that BPC can be used to accurately reproduce low-frequency soundfields over an area large enough to enclose a listener.

Abstract:
A network of mobile microphone arrays (nodes) has been created to identify and track noise sources in an outdoor environment. Collaborative signal processing is exploited: individual nodes with limited processing power and access to the local environment relay information to a central-level composite tracking system (fusion center). The nodes are able to detect noise sources, calculate the relative direction, and perform gross classification. The fusion center is responsible for associating detections to form noise events, associating events to objects, and then tracking those objects. Association is accomplished by a series of classification algorithms and decision logic. Information is stored on the objects’ location, speed, heading, track, and noise character: impulsive, tonal, or broadband. Results from experiments run in an outdoor suburban environment demonstrate performance and the complexity of the problem. Challenges and directions for future research are discussed.

Abstract:
In a number of applications of active noise control, localized control of the sound field provides a satisfactory solution that is often easier to implement in practice. However, there are also applications where global (or at least extended) attenuation of the sound field is required. For such applications, one must attenuate the overall energy in the field in order to achieve this objective. This paper will look at two application areas where global control is desired and will outline possible techniques for accomplishing this task. The first application targets free-field radiation, and specifically focuses on sound radiated from small axial fans, such as are used in office equipment. To achieve global attenuation requires that the total radiated power be minimized. However, this represents a quantity that is difficult to measure and incorporate into a control system in real time. For this application, a method has been developed that uses the concept of minimizing radiated power, but implements practical measurement techniques. An analytical method is used to map the condition of minimized power into the corresponding pressure field response, from which the optimal location of the error sensors can be determined. A conventional active control system can then be used to minimize the field, with the outcome closely corresponding to the desired objective of minimized radiated power. The second application targets enclosed sound fields at low modal density and specifically focuses on attenuating the sound field in the cab of earth-moving equipment. For this application, the movement of the operator’s head dictates that localized control will generally be insufficient. Prior research has found that minimizing acoustic energy density can often achieve greater global attenuation in low modal density fields. Thus, this approach has been applied in the cab of earth-moving equipment, and the desired objective of global control has been largely realized. Another advantage of this approach is that the control system performance is less sensitive to the location of the energy density error sensor. As a result, the error sensor can be placed in a convenient practical location without sacrificing the control that is achieved. Both of these energy-based techniques will be developed, and results will be presented indicating the control that can be achieved and the global characteristics of the resulting sound field.

Abstract:
During the past fifteen years there has been a lot of research work on active control of sound radiation by smart structures with both embedded structural actuators and sensors. The main principles of this new approach have been investigated and are now well established. Both feed–forward and feedback control schemes have been studied. Feed–forward control is particularly suited to the control of tonal disturbances for which a reference signal is available. In this case the structural actuators are driven in such a way as to rearrange the vibration of the panel so that the total sound radiation detected via the structural actuators is minimised. Normally single channel systems are used with distributed sensor–actuator pairs designed to control the most efficient radiating mode of the structure. This approach is known as Active Acoustic Structural Control (ASAC). Feedback control is instead advantageous for the control of steady state broad band disturbances. Its implementation relies heavily on the collocation and duality of the sensor–actuator transducers in which case the feedback control system is bound to be unconditionally stable. In principle, the most advantageous approach is to implement single channel feedback ASAC systems, although some fundamental problems related to stability have been highlighted when distributed sensor–actuator pairs are used. However recent work has shown that excellent control results can also be obtained with arrays of decentralised feedback control systems using point actuators and sensors. In this case the control system is driven to implement Active Vibration Control (AVC) only which however can be set to minimise both the vibration and sound radiation by the structure. Although the main principles of both ASAC and AVC approaches for the control of sound radiation by smart structures have been pinned down, there is still a lot of research work in progress on the sensor–actuator transducers. In this paper the main features of both strain and inertial sensor and actuator transducers are discussed with reference to both ASAC and AVC control systems. Particular emphasis is given to the physics of control and to the stability issues involved in both adaptive feed–forward and feedback control systems. Finally some new directions for research are proposed where the control transducers are also made adaptive in order to provide better collocation and duality properties.

Abstract:
In this paper, the active vibration and motion control of ocean surface vehicles is discussed. Since the problem is associated with the nonlinear and coupled system dynamics, and random environmental disturbance, nonlinear control techniques will be used for the solution. In the meantime, the understanding of the mechanisms of coupling and its effect on the performance in vibration and motion control is also emphasized so that physical insight can be gained through the control practice. This paper starts with a brief introduction of the ship kinematics and kinetics, disturbance and control inputs. Then nonlinear feedback control techniques are applied to solve various problems of vibration and motion control separately. Finally, the coupling of vibration and motion, their effect on control performance and simultaneous control of vibration and motion are discussed.

Abstract:
This paper presents a process of developing active noise control (ANC) systems with a focus on digital signal processing (DSP) algorithms and implementations. We will use a snore ANC system as an example for demonstrating the complete process of developing commercial products for real-world applications. An ANC application evolves from system specification, acoustic and feasibility analysis, experimental setup and measurement, algorithm development and computer modeling, simulation and fine tuning, finite wordlength analysis, real-time experiment, hardware and software development, and system integration. This paper briefly reviews adaptive algorithms for ANC, their performance related to physical limitations, and introduces two important components for further improving system performance: virtual sensor and residual noise masking using psychoacoustic principles. We will discuss hardware and software of ANC development systems and the custom products. Hardware issues include processor, data converters, amplifiers, and analog input/out channels, and their relationship to DSP principles. Software involves development tools, programming methods, and high-level intelligence.

Abstract:
This paper addresses the issue of complexity and how it relates to active control, the design of acoustic arrays and virtual acoustic prototyping. For the purposes of this paper, the complexity of a system is defined as the number of degrees of freedom that would be required to accurately describe the (likely) behavior of the system at a given frequency. Mathematically this is related to the relative size of the singular values of a matrix describing the inputs to outputs of the system in question (and can also be related to the modal overlap factor for resonant systems). For active control purposes, complexity determines the minimum number of control channels required to achieve good performance. For other applications such as inverse problems or the design of microphone arrays the formulation is similar and the relative size of the singular values (i.e. complexity) determines the accuracy and hence performance of the system. For example the performance and complexity associated with a small array of microphones detecting sources in a free field is identical to the complexity (and control) of sound radiation from a structure into a free field. If a disturbance propagates through a number of sub-systems that are coupled together, then the complexity varies depending on the location between the source and receiver. Windows between the sub-systems can act as bottlenecks. Clearly it is advantageous to place control at the bottlenecks in the system where the complexity is at a minimum. The efficiencies of radiation modes for example, are a description of the complexity looking into a system through a specific window and determine the number of control channels required to stop a disturbance propagating through that window. It is noted that complexity in different sub-systems can change at different rates with frequency making the location of the bottlenecks, and hence control strategy, vary with the frequency range of interest. One final example that will be discussed is the recreation of 3-D acoustic fields around a user’s head using equivalent sources and the number of these sources that are required to achieve an accurate 3-D simulation. It is shown that the number of equivalent sources is related to both the complexity and size of the volume surrounding the head.