Aquesta tesi presenta treball original i aplicat en l'àrea del control i la col·locació de sensors/actuadors (S/A) en sistemes de Control Actiu de Soroll (ANC). Primer, s'han aplicat tècniques de control i identificació robustes per a aconseguir ANC. La fase d'identificació està basada en una proposta d'identificació robusta orientada al control, considerant descripcions del sistema tant paramètriques com no-paramètriques, així com quantificant la incertesa. El disseny del controlador compara les estructures de control feedback (FB), feedforward (FF) i híbrida (FB/FF). El controlador feedback és sintetitzat i avaluat en el marc del control robust, i s'ha dissenyat utilitzant control òptim H∞ plantejat com un problema de sensibilitats mixtes. El controlador FF és un identificador adaptatiu, basat en l'algorisme σ robustament normalitzat. S'han desenvolupat dues propostes per a decidir quina de les estructures de control és més eficient, aplicades a un conducte de 4 metres amb soroll de banda ampla. A més a més, s'han mostrat de manera explícita els compromisos entre identificació i control, les limitacions inherents a un llaç de control feedback, així com qüestions relatives a la implementació de sistemes ANC. També s'han tractat altres qüestions com la relació entre acompliment, ordre del controlador, models paramètrics/no-paramètrics i implementació en processadors digitals de senyal (DSP), així com s'han comparat resultats teòrics i experimentals en el conducte. Les llacunes que encara resten entre teoria i pràctica en aquest tipus d'aplicacions també s'han resumit. D'altra banda, en aquest treball també es tracta el problema de com quantificar la col·locació de sensors i actuadors, amb la finalitat de controlar un sistema físic determinat. La mesura per a determinar la millor localització de S/A es basa en un criteri de llaç tancat orientat al control, el qual optimitza tant acompliment com qüestions pràctiques d'implementació. Aquesta mesura hauria de calcular-se abans del disseny, implementació i prova del controlador. La utilització d'aquesta mesura minimitza la prova combinatòria de controladors en totes les possibles combinacions de S/A. Per a aconseguir-ho, s'han definit diferents mesures que pesen l'acompliment potencial en llaç tancat, la robustesa, el número de condició de la planta (guanys relatius entrada/sortida (I/O)) així com altres qüestions d'implementació, com l'ordre del controlador. Aquestes poden calcular-se utilitzant software estàndard, tant per a models d'una-entrada-una-sortida (SISO) com per a models de múltiples-entrades-múltiples-sortides (MIMO) i poden aplicar-se a múltiples problemes d'enginyeria, ja siguin mecànics, acústics, aeroespacials, etc. En aquest treball, aquests resultats també s'han il·lustrat amb l'aplicació ANC presentada i validat amb dades experimentals. Com a resultat d'aplicar aquestes mesures, s'obté la localització de S/A que aconsegueix la millor atenuació del soroll en llaç tancat amb el menor ordre possible del controlador. / Esta tesis presenta trabajo original y aplicado en el área del control y la colocación de sensores/actuadores (S/A) en sistemas de Control Activo de Ruido (ANC). Primero, se han aplicado técnicas de control e identificación robustas para conseguir ANC. La fase de identificación está basada en una propuesta de identificación robusta orientada al control, considerando descripciones del sistema tanto paramétricas como no-paramétricas, así como cuantificando la incertidumbre. El diseño del controlador compara las estructuras de control feedback (FB), feedforward (FF) e híbrida (FB/FF). El controlador feedback es sintetizado y evaluado en el marco del control robusto, y se ha diseñado utilizando control óptimo H∞ planteado como un problema de sensibilidades mixtas. El controlador FF es un identificador adaptativo, basado en el algoritmo σ robustamente normalizado. Se han desarrollado dos propuestas para decidir cual de las estructuras de control es más eficiente, aplicadas a un conducto de 4 metros con ruido de banda ancha. Además, se han mostrado de manera explícita los compromisos entre identificación y control, las limitaciones inherentes a un lazo feedback, así como cuestiones relativas a la implementación de sistemas ANC. También se han tratado otras cuestiones como la relación entre desempeño, orden del controlador, modelos paramétricos/no-paramétricos e implementación en procesadores digitales de señal (DSP), así como se han comparado resultados teóricos y experimentales en el conducto. Las lagunas que aún quedan entre teoría y práctica en este tipo de aplicaciones también se han resumido. Por otra parte, en este trabajo se trata también el problema de como cuantificar la colocación de sensores y actuadores, con la finalidad de controlar un sistema físico determinado. La medida para determinar la mejor localización de S/A se basa en un criterio de lazo cerrado orientado al control, el cual optimiza tanto desempeño como cuestiones prácticas de implementación. Esta medida debería calcularse antes del diseño, implementación y prueba del controlador. La utilización de esta medida minimiza la prueba combinatoria de controladores en todas las posibles combinaciones de S/A. Para conseguirlo, se han definido distintas medidas que pesan el desempeño potencial en lazo cerrado, la robustez, el número de condición de la planta (ganancias relativas entrada/salida (I/O)) y otras cuestiones de implementación, como el orden del controlador. Éstas pueden calcularse utilizando software estándar, tanto para modelos de una-entrada-una-salida (SISO) como para modelos de múltiples-entradas-múltiples-salidas (MIMO) y pueden aplicarse a múltiples problemas ingenieriles, ya sean mecánicos, acústicos, aeroespaciales, etc. En este trabajo, estos resultados también son ilustrados con la aplicación ANC presentada y validados con datos experimentales. Como resultado de aplicar estas medidas, se obtiene la localización de S/A que consigue la mejor atenuación de ruido en lazo cerrado con el menor orden posible del controlador. / This thesis presents novel and applied work in the area of control and sensor/actuator (S/A) allocation in Active Noise Control (ANC) systems. First, robust identification and control techniques to perform ANC have been applied. The identification phase is based on a control-oriented robust identification approach that considers both parametric and nonparametric descriptions of the system, and quantifies the uncertainty. The controller design compares the feedback (FB), feedforward (FF) and hybrid (FB/FF) control structures. The feedback control is synthesized and evaluated in the robust control framework, and it is designed using H∞ optimal control as a mixed-sensitivity problem. The FF controller is an adaptive identifier, based on the robustly normalized σ-algorithm. Two approaches are developed to decide which control structure is more efficient on a 4-m duct example with broadband noise. In addition, the compromises between identification and control, the inherent limitations of feedback and implementation issues in ANC are explicitly pointed out. Relations between performance, controller order, parametric/nonparametric models and digital signal processor (DSP) implementation are discussed. Theoretical and experimental results on the duct are compared. The gaps that still remain between theory and practice in this type of applications, are also outlined. Furthermore, this work considers the problem of quantifying the location of sensors and actuators in order to control a certain physical system. The measure to determine the best S/A location is based on a closed loop control-oriented criteria, which optimizes overall performance and practical implementation issues. In addition, it should be computed before the actual controller is designed, implemented and tested. The use of this measure minimizes the combinatorial controller testing over all possible S/A combinations. To this end, several measures have been defined which weight the potential closed-loop performance, robustness, plant condition number (input/output (I/O) relative gains) and implementation issues, such as the controller order. These may be computed with standard software, either for Single Input Single Output (SISO) models or Multiple Input Multiple Output (MIMO) models, and may be applied to many engineering problems: mechanics, acoustics, aerospace, etc. Here, these results are also illustrated with the prior ANC example and validated against experimental data. The outcome of applying these measures is the selection of the S/A location which achieves the best closed loop noise attenuation with the lowest possible controller order.

Low disturbance wind tunnels, such as the Klebanoff–Saric Wind Tunnel (KSWT), offer an ideal environment to study boundary layer transition. In particular, the leading-edge receptivity of sound can be measured by creating acoustic disturbances that interact with the leading edge of a model to create Tollmien–Schlichting Waves. The magnitude and composition (sound, turbulence) of the background disturbances can affect these experiments, so the background disturbances should be minimized and documented thoroughly. The purpose of this thesis is to document the background acoustic signature of the KSWT and describe infrastructure upgrades for acoustic receptivity experiments. The measurements presented in this thesis will support future receptivity measurements in the KSWT. Microphone measurements revealed several important acoustic features in the tunnel. Cross correlations showed that two sources of low-frequency unsteadiness (the extended diffuser and corner two) create large pressure fluctuations that dominate the pressure spectrum. Directional separation of waves in the test section revealed that motor and blade passing noise travels primarily upstream into the test section. Finally, the acoustic treatments in the plenum are effective at removing sound from the tunnel. A wall of speakers was installed in the plenum to enable acoustic receptivity experiments. The speakers create both the primary downstream traveling waves and reflected upstream traveling waves in the test section. An adaptive closed loop control system was installed to reduce the amplitude of the reflected waves during acoustic forcing. Although the performance of the control system is frequency dependent, the technique was implemented successfully. The reduction in the diffuser reflection will increase the quality of future acoustic receptivity experiments in the KSWT.

The harmful effects of aircraft noise, with respect to both comfort and occupational health, have long since been recognised, with many examples of sound control now implemented in commercial aircraft. However, the single engine light aircraft cabin is still an extremely noisy environment, which apparently has been side-lined by both cost and weight constraints, especially with respect to low frequency sound reduction. Consequently, pilots and passengers of these aircraft are still exposed to potentially damaging noise levels and hearing damage can only be avoided by the proper use of ear defenders. Minimisation of the noise around the occupants of the aircraft reduces the dependency of personal ear defenders and is conducive to a more comfortable, hygienic and less stressful environment. This thesis describes the basis of a theoretical and experimental project, directed at the design and evaluation of a practical active noise control (ANC) system suitable for a single engine light aircraft. Results from initial experiments conducted in a single engine aircraft demonstrated the viability of ANC for this application. However, the extreme noise, the highly damped cabin, the multiple tone excitation, the severe weight limitations and the requirement of air worthiness certification severely complicated the problem of achieving noise reduction throughout the entire aircraft cabin. Compromising the objective to only achieving local control around the occupants still presented difficulties because the region of attenuated noise around the error sensors was so small that a nearby observer experienced no sound level reduction whatsoever. The objective was therefore to move the control zone away from the error sensor and place a broad envelope of noise reduction immediately around the occupant's head, through the use of virtual sensors , thus creating the perception of global noise control. While virtual sensors are not new (Garcia-Bonito et al. (1996)), they are currently limited to acoustic pressure estimation (virtual microphones) via the initial measurement of an observer / sensor transfer function. In this research, new virtual sensor algorithms have been developed to: 1. minimise the sound level at the observer location, 2. broaden the control region, 3. adapt to any physical system changes and 4. produce a control zone that may ultimately follow an observer's head The performance of the virtual sensors were evaluated both analytically and experimentally in progressively more complex environments to identify their capabilities and limitations. It was found that the use of virtual sensors would, in general, attenuate the noise at the observer location more effectively than when using conventional remotely placed error sensors. Such a control strategy was considered to be ideal for a light single engine aircraft, because it would only require small light speakers (possibly fitted into a head-rest) to achieve a broad control zone that envelopes the region around the occupants heads. / Thesis (Ph.D.) -- University of Adelaide, Dept. of Mechanical Engineering), 2000.

Local active noise control systems generate a zone of quiet at the physical error sensor location. While significant attenuation is achieved at the error sensor, local noise control is not without its problems, chiefly that the zone of quiet is generally small and impractically sized. It may be inconvenient to place the error sensor at the desired location of attenuation, such as near an observer’s ear, preventing the small zone of quiet from being centered there. To overcome the problems encountered in local active noise control, virtual acoustic sensors have been developed to shift the zone of quiet away from the physical sensor position to a spatially fixed desired location. The general aim of the research presented in this thesis is to improve and extend the spatially fixed and moving virtual sensing algorithms developed for active noise control thus far and hence increase the scope and application of local active noise control systems. To achieve this research aim, a number of novel spatially fixed and moving virtual sensing algorithms are presented for local active noise control. In this thesis, a spatially fixed virtual sensing technique named the Stochastically Optimal Tonal Diffuse Field (SOTDF) virtual sensing method is developed specifically for use in pure tone diffuse sound fields. The SOTDF virtual sensing method is a fixed gain virtual sensing method that does not require a preliminary identification stage nor models of the complex transfer functions between the error sensors and the sources. SOTDF virtual microphones and virtual energy density sensors that use both pressure and pressure gradient sensors are developed using the SOTDF virtual sensing method. The performance of these SOTDF virtual sensors is investigated in numerical simulations and using experimental measurements made in a reverberation chamber. SOTDF virtual sensors are shown to accurately estimate the pressure and pressure gradient at a virtual location and to effectively shift the zone of quiet away from the physical sensors to the virtual location. In numerically simulated and post-processed experimental control, both virtual microphones and virtual energy density sensors achieve higher attenuation at the virtual location than conventional control strategies employing their physical counterpart. As it is likely that the desired location of attenuation is not spatially fixed, a number of moving virtual sensing algorithms are also developed in this thesis. These moving virtual sensing algorithms generate a virtual microphone that tracks the desired location of attenuation as it moves through a three-dimensional sound field. To determine the level of attenuation that can be expected at the ear of a seated observer in practice, the performance of the moving virtual sensing algorithms in generating a moving zone of quiet at the single ear of a rotating artificial head is investigated in real-time experiments conducted in a modally dense three dimensional cavity. Results of real-time experiments demonstrate that moving virtual sensors provide improved attenuation at the moving virtual location compared to either fixed virtual sensors or fixed physical sensors. As an acoustic energy density cost function spatially extends the zone of quiet generated at the sensor location, a fixed three-dimensional virtual acoustic energy density sensing method is also developed for use in a modally dense three-dimensional sound field. The size of the localised zone of quiet achieved by minimising either the acoustic energy density or the squared pressure at the virtual location with the active noise control system is compared in real-time experiments conduced in a modally dense three-dimensional cavity. Experimental results demonstrate that minimising the virtual acoustic energy density provides improved attenuation in the sound field and a larger 10 dB zone of quiet at the virtual location than virtual microphones. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1522526 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2010

The harmful effects of aircraft noise, with respect to both comfort and occupational health, have long since been recognised, with many examples of sound control now implemented in commercial aircraft. However, the single engine light aircraft cabin is still an extremely noisy environment, which apparently has been side-lined by both cost and weight constraints, especially with respect to low frequency sound reduction. Consequently, pilots and passengers of these aircraft are still exposed to potentially damaging noise levels and hearing damage can only be avoided by the proper use of ear defenders. Minimisation of the noise around the occupants of the aircraft reduces the dependency of personal ear defenders and is conducive to a more comfortable, hygienic and less stressful environment. This thesis describes the basis of a theoretical and experimental project, directed at the design and evaluation of a practical active noise control (ANC) system suitable for a single engine light aircraft. Results from initial experiments conducted in a single engine aircraft demonstrated the viability of ANC for this application. However, the extreme noise, the highly damped cabin, the multiple tone excitation, the severe weight limitations and the requirement of air worthiness certification severely complicated the problem of achieving noise reduction throughout the entire aircraft cabin. Compromising the objective to only achieving local control around the occupants still presented difficulties because the region of attenuated noise around the error sensors was so small that a nearby observer experienced no sound level reduction whatsoever. The objective was therefore to move the control zone away from the error sensor and place a broad envelope of noise reduction immediately around the occupant's head, through the use of virtual sensors , thus creating the perception of global noise control. While virtual sensors are not new (Garcia-Bonito et al. (1996)), they are currently limited to acoustic pressure estimation (virtual microphones) via the initial measurement of an observer / sensor transfer function. In this research, new virtual sensor algorithms have been developed to: 1. minimise the sound level at the observer location, 2. broaden the control region, 3. adapt to any physical system changes and 4. produce a control zone that may ultimately follow an observer's head The performance of the virtual sensors were evaluated both analytically and experimentally in progressively more complex environments to identify their capabilities and limitations. It was found that the use of virtual sensors would, in general, attenuate the noise at the observer location more effectively than when using conventional remotely placed error sensors. Such a control strategy was considered to be ideal for a light single engine aircraft, because it would only require small light speakers (possibly fitted into a head-rest) to achieve a broad control zone that envelopes the region around the occupants heads. / Thesis (Ph.D.) -- University of Adelaide, Dept. of Mechanical Engineering), 2000.

Previous work has shown that active noise control is a feasible solution to attenuate tonal noise radiated by small axial cooling fans, such as those found in desktop computers. One such control system reduced noise levels of a baffled 80-mm fan in the free field with four small loudspeakers surrounding the fan. Due to industry specified spatial constraints, a smaller fan and speaker configuration was desirable. The smaller configuration maintains similar control performance, further facilitating practical implementation of the control system. The smaller control system employs a smaller fan running at a higher speed. Different loudspeaker configurations for control exist and have been tested. A configuration consisting of four control sources spaced symmetrically around and coplanar to the fan exhibits global control of the tonal component of the fan noise. A configuration with three symmetrically spaced sources is shown to perform similarly, agreeing with theoretical prediction. An analysis of the control system in a non-ideal reflective environment is also discussed.

Feedback active noise control (ANC) has been applied as a means of attenuating broadband noise from a small axial cooling fan. Such fans are used to maintain thermal stability inside of computers, projectors, and other office equipment and home appliances. The type of low-level noise radiated from axial cooling fans has been classified as harmful to productivity and human well being. Previous research has successfully implemented feed-forward ANC, targeting specific narrow-band fan noise content related to the blade passage frequency (BPF) of the fan. The reference signal required for a feed-forward algorithm limits its ability to attenuate much of the noise content; however, it is also desirable to reduce broadband fan noise. Feedback control is a logical alternative in the absence of a valid reference signal. The fan used for this research was mounted in one of the six aluminum panels that constituted a mock computer case. The fan was surrounded by four miniature loudspeakers as control sources and four small electret microphones as error sensors. A feasibility study was conducted with a single channel of analog feedback control. However, for the majority of this research, the ANC algorithm was executed on a digital signal processor. Several electronic modules provided the necessary signal conditioning and conversion for the process. A method is proposed and validated for predicting the overall attenuation that can be obtained for a specific fan, based on its autocorrelation measurement. Studies were performed in order to determine the difference in performance between static and adaptive controllers. Comparisons are made between decentralized and centralized controllers, the results of which are presented in this thesis. Feedback ANC is demonstrated as a good alternative to feed-forward ANC for the reduction of BPF related tonal fan noise content. Some low-frequency broadband attenuation is achieved. The delay time associated with current DSP technology is shown to be too long to effectively attenuate flow noise (the main component of broadband fan noise). Adaptive control proved to be necessary for stability and performance in the feedback controller. Decentralized control is shown to outperform centralized control for this specific application.

No description available.

Techniques are available today to attenuate the output sound of the trumpet. All of these techniques involve using passive mutes. Due to the limitations in the sound one can obtain with passive mutes, another solution, using active noise control, is proposed to predictably attenuate the output sound of the trumpet. With the new system, it is theorized any desired output sound can be obtained. Within this thesis a model of the trumpet physics is derived and an investigation of the implementation of two analog feedback controllers and two digital LMS controllers is performed. The model of the trumpet mechanics is studied to understand the trumpet system before applying the control systems. Analysis is performed on the type and the location of the acoustic control actuator and the error sensor to be used. With the chosen actuator and sensor, the two types of controllers are designed and realized. The farfield spectrum of the trumpet's response to a single note is analyzed for each controller and the resulting attenuations compared. The model of the trumpet system is then used to demonstrate the coupling of the trumpet and the player and to show the effects of the controllers on the behavior of the player's embouchure. With the inclusion of the controllers in the trumpet system, the farfield spectrum was successfully attenuated at two harmonics of the tone passed through the trumpet. Testing was not performed with an actual trumpet player due to the high sound pressure levels (160 dB SPL) required from the control actuator. From a derived model of the control actuator, specifications for an acoustic driver capable of delivering the high sound pressure level were calculated. Design and fabrication of the proposed actuator will be completed during future work. / Master of Science

Methods for analyzing the structural-acoustic power efficiency of active structures are developed. For this work we define the power efficiency as the ratio of the sound power radiated by a structure to the maximum possible radiated sound power. An active structure is defined as one that has electromechanical actuators distributed over its surface for the purpose of structural-acoustic excitation. The power efficiency of planar, baffled structures with arbitrary boundary conditions is examined using a combination of methods based on numerical integration, variational principles, and finite element analysis. The fundamental result of this work is that computing the power efficiency of an active structure reduces to the solution of two eigenvalue problems. The maximum possible sound power radiated by a planar, baffled structure is shown to be equivalent to the largest eigenvalue of the acoustic power transfer matrix. The structural-acoustic power efficiency is the solution of a separate generalized eigenvalue problem whose parameters include the location of the electromechanical actuators and the type of electromechanical actuation. The advantage of this metric over other measures of radiation efficiency is that 0 and 1 bound the structural-acoustic power efficiency. Furthermore, solving for the power efficiency as a function of frequency yields a measure of the bandwidth of the structural-acoustic actuator. Power efficiency is analyzed for point force actuation and distributed moment actuation. Numerical simulations demonstrate that maximizing the power efficiency requires that the magnitude and phase of the structural modal velocity vector be matched to that of the eigenvector that corresponds to the maximum eigenvalue of the acoustic power transfer matrix. Matching the modal velocity to the maximizing eigenvector produces a vibration shape that maximizes the sound power radiation of the structure. Individual actuators are not able to achieve high efficiency over a broad frequency range for both types of electromechanical actuation. Multiple-actuator arrays are able to achieve higher average efficiency at the expense of increased number of actuators. An optimization problem is then posed to maximize the structural-acoustic power efficiency by varying the location and size of distributed moment actuators. We demonstrate that an average efficiency on the order of 0.85 is possible over a large bandwidth through optimal placement and sizing of a set of four distributed moment actuators. Experimental results on a baffled plate demonstrate that correct phasing of the actuators results in velocity distributions that correlate well with predicted results. / Ph. D.