Active structural acoustic control (ASAC) in conjunction with the adaptive feedforward control has been proved to be an efficient practical approach to reduce structure-borne sound. ASAC works on the principles of reducing the vibration amplitude of the structure (modal reduction), as well as changing the vibration distributions of the structure so that the vibration distributions of each structural modes destructively interfere with one another in their associated radiating acoustic field (modal restructuring). Based on these observations, two different but related design strategies, namely the non-volumetric design and the minimum supersonic wavenumber design, were developed for designing efficient ASAC system. The eigenassignment method for feedforward control system serves as the fundamental design tool for both formulations. In this study, the dynamic characteristics of a multiple-input, multiple-output (MIMO) feedforward controlled system was investigated both analytically and experimentally on a simply supported plate under harmonic excitation. It was demonstrated that, when the control system has equal number of control inputs and error sensor outputs, the feedforward controller can effectively modify the system dynamics (i.e., resonance frequencies and mode shapes). This provides the theoretical basis for the eigenassignment method. For the non-volumetric design, the single-input, single-output (SISO) eigenassignment technique is used to modify the eigenproperties of a planar structure using structure actuators and sensors so that all the controlled modes are non-volumetric (inefficient sound radiators at low frequencies, i.e., k_0a << 1), leading large global sound attenuation in the far field. The effectiveness of this formulation was demonstrated through numerical simulations for the control of radiation from simply supported and clamped-free beams. The experimental validation of the non-volumetric design was also carried out on a simply supported beam using PZT actuators and shaped PVDF film as error sensor. The filtered-x LMS algorithm was used in the experiment. Excellent global sound attenuation was achieved in the low frequencies. The minimum supersonic wavenumber design stems from the fact that only supersonic wavenumber components of the structural velocity spectra radiate to the far field. A SISO eigenassignment technique is used to modify the eigenproperties of a planar structure so that the eigenfunctions of the controlled system have minimum supersonic wavenumber in the frequency range of study. The sound pressure or sound power radiated by the structure is therefore reduced. The design was demonstrated on a simply supported beam to minimize the supersonic wavenumber components contributed by the odd-order modes only. Significant global sound attenuation was achieved in the frequency range of study. The main advantage of the proposed design methods is that they do not depend on the characteristics of the external disturbance, such as the form, location and frequency contents. Also, the error sensor and control input are optimized simultaneously, resulting in better acoustic control performance. The practical implementations of the proposed designs require accurate system modeling, this is the major limitation of the proposed designs. / Ph. D.

A general procedure for the optimization of control actuator forces and locations to minimize the total radiated sound power from complex structures has been developed. This optimization procedure interfaces finite and boundary element models with non-linear optimization techniques. The optimization procedure was used to perform parametric studies of Active Structural Acoustic Control (ASAC) on a simply supported plate with various discontinuities such as point mass, line mass, and spring mass systems. These system models were harmonically excited by an off resonance point force of 550 Hz and controlled by piezoceramic actuators. Although the excitation frequency is the same for each of the cases studied, the eigenproperties change with alteration of the physical parameters of the system. Therefore the excitation frequency for each case is effectively different, as is its response. This optimization procedure was very effective in reducing the total radiated sound power from these complex structures. The addition of a second optimized actuator resulted in additional attenuation of varying extent, highly dependent on the discontinuity. The locations of the optimized actuators were also found to be very sensitive to the discontinuity. It was also observed that the optimal location of a single actuator changed very little with the addition of a second actuator. The accuracy of this sophisticated model was verified by comparing solutions from modal based analytical and assumed mode models for simple and complex structures. Some unique aspects of this procedure are that it requires a single implementation of the finite and boundary element solution, and that the finite element forced response solution is not required. Therefore, this ASAC actuator optimization procedure shows potential for application to any structure that can be accurately modeled with finite element software. / Master of Science

Previous work has demonstrated the benefit of grouping actuators to increase the controllability of an active control system, without increasing the number of control channels. By driving two or more secondary sources with the same control input, one is also able to reduce the hardware cost and complexity. In this work, a time domain cost function is developed for on-line actuator grouping and active structural acoustic control (ASAC) of a simply-supported beam excited with a broadband disturbance. Three PZT actuators are mounted on the beam structure to control the wavenumber components corresponding to five radiation angles. The propagation angles are selected to represent the total radiated sound power. The point force disturbance is bandlimited random noise which encompasses the first three modes of beam vibration. Actuators are considered grouped when their compensators are equal. Therefore, the cost function presented here incorporates an additional non-quadratic term which penalizes the controller for differences between the feedforward compensator coefficients. The backpropagation neural network algorithm provides the proper procedure to determine the minimum of this cost function. The main disadvantage of using a stochastic gradient technique, while searching the prescribed control surface, is convergence to local minima. In this thesis, a resolution to this problem is suggested which incorporates using a variety of initial conditions. Two initialization conditions are considered: grouping actuators based upon weights determined by converging the filtered-x LMS algorithm and simultaneously grouping and controlling with the compensator weights initialized to small arbitrary numbers. Test cases of heavy and light grouping parameters were evaluated from both initial conditions. The computer simulations demonstrate the ability of this new form of the cost function to group actuators and control the error response with either initial condition. The heavy grouping cases achieved the same one channel control system from both initial conditions. The performance of the one channel solution was 1.5 dB lower than the performance of the ungrouped filtered-x LMS solution. The ability to select the different levels of grouping was demonstrated when the algorithm was initialized with the filtered-x LMS weights and run with light grouping parameters. For this case, the on-line algorithm grouped two actuators, but allowed the third actuator to exist independently. The performance of the two channel control system was only 0.6 dB less than the performance of the filtered-x LMS solution. In all grouping cases investigated, the convergence times of the grouping algorithm were within the same order as for the filtered-x LMS algorithm. The effect of uncorrelated error sensor noise on the actuator groupings is also briefly discussed. / Master of Science

Driven by the need to reduce the sound transmitted into aircraft cabins from the power plant, this thesis investigates the active control of sound transmitted through a structure into coupled enclosures. In particular, it examines alternatives to conventional microphone and accelerometer error sensors. This study establishes a design framework for the development and analysis of an active noise control system which can be applied to any complex vibro-acoustic system. The design approach has focused on using techniques presently used in industry to enable the transfer of the active noise control technology from the research stage into practical noise control systems. The structural and acoustic sub-systems are modelled using FEA to estimate the in vacuo structural modal response of the structure and the acoustic pressure modal response (with rigid boundary conditions) of the interior cavity. The acoustic and structural systems are then coupled using modal coupling theory. Within this framework, two novel error sensors aimed at overcoming observability problems suffered by traditional microphone and accelerometer sensors are investigated: namely, acoustic energy density sensors and shaped radiation modal vibration sensors. The principles of the measurement of energy density are discussed and the errors arising from its measurement using two and three-microphone sensor configurations are considered for a one-dimensional reactive sound field and a plane wave sound field. The error analysis encompasses finite separation effects, instrumentation errors (phase and sensitivity mismatches, and physical length errors), diffraction and interference effects, and other sources of error (mean flow and turbulence, temperature and humidity, statistical effects). Following the one-dimensional study, four 3-axis energy density sensor designs are proposed and error analysis is conducted over the same acoustic fields as for the one-dimensional study. The design and construction of the simplest arrangement of the 4 three-axis sensors is discussed with reference to design issues, performance and limitations. The strategy of using energy density control is investigated numerically for a purely acoustic system and a coupled panel-cavity system. Energy density control is shown to provide greater local and global control compared to that possible using an equivalent number of microphones. The performance of the control system is shown to be relatively insensitive to the placement of the energy density sensor. For an enclosed cavity system with high modal overlap, the zone of local control achieved by minimising energy density is found to be approximately the same as the zone of local control obtained when min-imising pressure and pressure gradient in a diffuse sound field. It is also shown that if there is only one control source used per energy density sensor, global control will be almost optimum. The addition of further control sources leads to an improvement in global control, however, the control is no longer optimal. The control system is found to be very tolerant of errors in the estimate of the energy density and thus the use of simpler energy density sensor designs is justified. Finally, an experiment is presented in which the global performance achieved by controlling a three-axis energy density sensor is compared with the performance achieved by minimising the acoustic potential energy and minimising the sum of squared pressures at a finite number of microphones. The experimental results are found to reflect the numerical results. The active minimisation of harmonic sound transmission into an arbitrarily shaped enclosure using error signals derived from structural vibration sensors is investigated numerically and experimentally. It is shown that by considering the dynamics of the coupled system, it is possible to derive a set of "e;structural radiation"e; modes which are orthogonal with respect to the global potential energy of the coupled acoustic space and which can be sensed by structural vibration sensors. Minimisation of the amplitudes of the "e;radiation modes"e; is thus guaranteed to minimise the interior acoustic potential energy. The coupled vibro-acoustic system under investigation is modelled using Finite Element Analysis which allows systems with complex geometries to be investigated rather than limiting the analysis to simple, analytically tractable systems. Issues regarding the practical implementation of sensing the orthonormal sets of structural radiation modes are discussed. Specific examples relating to the minimisation of the total acoustic potential energy within a curved rectangular panel and a coupled cavity are given, comparing the performance offered using vibration sensing of the radiation modes on the structure with the more traditional error sensing; namely, the discrete sensing of the structural kinetic energy on the structural boundary and the acoustic potential energy in the enclosed space approximated by the mean squared pressures at several locations. / Thesis (Ph.D.)--Mechanical Engineering, 1999.

Given the increasing awareness of indoor air quality (IAQ) and the direct correlation to human health, passive removal materials (PRM) have become known as a potential strategy for reducing occupant exposure to indoor air pollutants (Lu 2013). In recent studies, untreated natural wool fiber has been recognized as a PRM for removing formaldehyde, sulfur dioxide and nitrogen dioxide. These are common volatile organic compounds (VOCs) emitted from sources, such as building materials, fixtures, furnishings and cleaning supplies (Darling et al. 2012). Test chamber studies have shown that wool fiber can irreversibly remove up to 67% of these VOC’s in an interior environment (Curling et al 2012). When the toxins come in physical contact with the fiber, a chemical reaction occurs due to the amino-acid side chains within the keratin molecule. Increase in air-tight buildings has recently become a concern with the rising popularity of sustainable building practices, causing occupant exposure to these indoor air pollutants to rise (Weschler 2009). Beyond known adverse health effects, such as eye irritation and respiratory issues, formaldehyde has been designated by the International Agency for Research on Cancer (IARC) as a human carcinogen and is the leading cause for Sick Building Syndrome (SBS) (World Health Organization 2010). The interior cortex of a wool fiber is hydrophilic – highly water absorbent, and can absorb 1/3 its weight in moisture. Wool fiber has a unique wicking property that allows the fiber to absorb water vapor from the air in a regulating sense; absorbing when there is an excess moisture level and releasing the gained moisture when the surrounding atmosphere is less humid. This provides passive humidity regulation in an indoor environment, stabilizing the comfort level of 20-50% relative humidity (RH) without requiring higher air-conditioning or ventilation rates (Bingelli 2010). Wool also has excellent properties for optimizing indoor acoustics, as it absorbs acoustic energy via the friction of air being moved through the tiny spaces between fibers and reduces traveling noise and reverberation (Bingelli 2010). In an untreated, natural roving state the density of wool is ideal for acoustic control in conversational speech situations where 70dB or lower is present, such as meeting rooms, lobbies and restaurants. With the consideration of these properties, wool has the capability to improve the indoor environment quality (IEQ) and the health of occupants through the absorption of indoor air pollutants, humidity regulation and acoustic control. As Australia and the USA are among the top 3 wool producing countries, I will be working specifically with locally sourced wool from New South Wales and Texas as a basis for a sustainable IEQ intervention installation model that may be applied to future projects. The wool was obtained from local, small-scale fiber farms that implement hand processing in an effort to reduce toxins, in addition to lowering the manufacturing energy and transportation emission requirements. The local supply chain model provides increased environmental, social, economic and human health benefits to the design. Individual installations based on the vernacular wool fiber atrributes and interior climate needs will greatly increase the overall spatial environment, while also serving as an aesthetically pleasing piece of art. / text

Cylindrical shells are common structures that are often used in industry, such as pipes, ducts, aircraft fuselages, rockets, submarine pressure hulls, electric motors and generators. In many applications it is desired to attenuate the sound radiated from the vibrating structure. There are both active and passive methods to achieve this purpose. However, at low frequencies passive methods are less effective and often an excessive amount of material is needed to achieve acceptable results. There have been a number of works regarding active control methods for this type of structure. In most cases a considerable number of error sensors and secondary sources are needed. However, in practice it is much preferred to have the fewest number of error sensors and control forces possible. Most methods presented have shown considerable dependence on the error sensor location. The goal of this dissertation is to develop an active noise control method that is able to attenuate the radiated sound effectively at low frequencies using only a small number of error sensors and secondary sources, and with minimal dependence on error sensor location. The Weighted Sum of Spatial Gradients control metric has been developed both theoretically and experimentally for simply supported cylindrical shells. The method has proven to be robust with respect to error sensor location. In order to quantify the performance of the control method, the radiated sound power has been chosen. In order to calculate the radiated sound power theoretically, the radiation modes have been developed for cylindrical shells. Experimentally, the radiated sound power without and with control has been measured using the ISO 3741 standard. The results show comparable, or in some cases better, performance in comparison with other known methods. Some agreement has been observed between model and experimental results. However, there are some discrepancies due to the fact that the actual cylinder does not appear to behave as an ideal simply supported cylindrical shell.

Smart foam is a composite noise control treatment that consists of a distributed piezoelectric actuator, known as polyvinylidene fluoride (PVDF), embedded within a layer of partially-reticulated polyurethane foam. The principal function of smart foam is to yield broadband sound attenuation. Passive acoustic foams are a very reliable high-frequency sound reduction method. With regard to smart foam, the embedded piezoelectric actuator is introduced to overcome the limitations of the passive foam in the low-frequency region. The piezoelectric actuator excites the structural and acoustic phases of the foam when driven by an externally supplied control voltage. This generates a secondary acoustic field which destructively interacts with the acoustic field created by a primary noise source. Initial experiments employ the composite "active/passive" treatment to yield attenuation of piston sound radiation. For this simple source, the global farfield pressure is minimized according to the feedforward, Filtered-x LMS control algorithm using one error sensor. Significant broadband sound attenuation is obtained. A more advanced noise control problem is investigated which minimizes plate radiation. The vibrating plate has a distributed modal response requiring a collective array of independently-phased smart foam actuators to yield reduction of the radiated sound power. This is accomplished by minimizing the sound pressure at an array of nearfield microphones. Good broadband sound power reduction is obtained using a MIMO (multiple-input/multiple-output) Filtered-x LMS control scheme. Various techniques for improving smart foam's acoustic control authority are identified during manufacturing and finite element modeling. of the actuator. These improved smart foam actuators are employed as an active/passive liner to suppress the transverse propagating acoustic modes within an anechoically-terminated rectangular duct. A section of a duct wall is lined with an array of smart foam and the sound downstream of the control actuators is minimized at several error microphones. Successful harmonic and broadband noise control is achieved. A full-scale numerical model of the duct acoustic control application is presented based on the finite element method. The purpose of the model is to study the sensitivity of this active/passive control approach relative to the spatial distribution of control channels and error sensors. A comparison of the numerical and experimental results yields similar trends. / Ph. D.

En äldre industribyggnad är oftast byggd i hårda material så som tegel, stål och plåt vilket alla ger upphov till hög andel reflektioner av ljudenergi. Absorptionsnivån i dessa material är låga vilket leder till långa efterklangstider. Att montera in akustikreglerande åtgärder i dessa byggnader står i konflikt med att äldre industribyggnader ofta har ett kulturhistoriskt värde.  Syftet med arbetet var att undersöka möjligheten till att genomföra akustikreglering i en kulturhistoriskt värdefull byggnad utan att väsentligt påverka detta värde. Avseende ambitionsnivån ur akustisk synpunkt var målet att halvera efterklangstiden.  De genomförda efterklangstidsmätningarna visar på en relativ lång efterklangstid i mellanregistret och en kortare vid låga och höga frekvenser. Resultatet av de beräkningar som är genomförda visar att i den mindre, av två stycken, hyttan kan målet med en halverad efterklangstid uppnås för majoriteten av oktavbanden. Åtgärdsförslaget för den större hyttan skulle däremot inte leda till att målet uppfylls fullt ut, utan för detta skulle ytterligare åtgärder behöva sättas in. Hänsyn till hyttans kulturvärde motiverar dock att en längre efterklangstid kan accepteras. / An older industrial building is usually built in hard materials such as brick, steel and sheet metal, all of which give rise to a high proportion of reflections of sound energy. The absorption level in these materials is low, which leads to long reverberation times. Installing acoustic regulatory measures in these buildings conflicts with the fact that older industrial buildings often have a cultural-historical value.  The purpose of this work was to investigate the possibility of implementing acoustic regulations in a culturally and historically valuable building without substantially affecting its cultural value. Regarding the level of ambition from an acoustic perspective, the objective was to halve the reverberation time.  The measured reverberation time shows a relative long reverberation time in the mid frequencies and a shorter one at low and high frequencies. The results of the calculations that have been carried out show that in the smaller production hall, of two, the objective of a halved reverberation time can be achieved in the majority of the octave bands. The proposed measures for the larger production hall would not lead to the objective being fully met, but further measures would need to be implemented. Consideration of the cultural value of the glass factory justifies that a longer reverberation time can be accepted.

To simplify the measurement of energy-based structural metrics, a general registration process for the scanning laser doppler vibrometer (SLDV) has been developed. Existing registration techniques, also known as pose estimation or position registration, suffer from mathematical complexity, instrument specificity, and the need for correct optimization initialization. These difficulties have been addressed through development of a general linear laser model and hybrid registration algorithm. These are applicable to any SLDV and allow the registration problem to be solved using straightforward mathematics. Additionally, the hybrid registration algorithm eliminates the need for correct optimization initialization by separating the optimization process from solution selection. The effectiveness of this approach is demonstrated through simulated application and by validation measurements performed on a specially prepared pipe. To increase understanding of the relationships between structural energy metrics and the acoustic response, the use of structural energy density (SED) in active structural acoustic control (ASAC) has also been studied. A genetic algorithm and other simulations were used to determine achievable reduction in acoustic radiation, characterize control system design, and compare SED-based control with the simpler velocity-based control. Using optimized sensor and actuator placements at optimally excited modal frequencies, attenuation of net acoustic intensity was proportional to attenuation of SED. At modal and non-modal frequencies, optimal SED-based ASAC system design is guided by establishing general symmetry between the structural disturbing force and the SED sensor and control actuator. Using fixed sensor and actuator placement, SED-based control has been found to provide superior performance to single point velocity control and very comparable performance to two-point velocity control. Its greatest strength is that it rarely causes unwanted amplifications of large amplitude when properly designed. Genetic algorithm simulations of SED-based ASAC indicated that optimal control effectiveness is obtained when sensors and actuators function in more than one role. For example, an actuator can be placed to simultaneously reduce structural vibration amplitude and reshape the response such that it radiates less efficiently. These principles can be applied to the design of any type of ASAC system.

Este documento trata do estudo de sistemas de controle de ruído e qualidade sonora em aplicações próprias da indústria da mobilidade, e da avaliação inovadora do seu desempenho com base em métricas da psico-acústica relevantes para esta área como Loudness, Roughness e Tonality. Uma metodologia de projeto e implantação de sistemas de controle ativo com base em métricas da psico-acústica é estabelecida. Os desafios se encontram, primeiramente, na definição do conjunto de atributos temporais e espectrais do distúrbio de entrada, responsável pelas condições auditivas indesejáveis que são identificados a partir da análise da emissão acústica sintetizada de um motor de combustão interna de um veículo automotivo. Algoritmos e metodologias computacionais são usados de forma extensiva para simulação e análise, integrando as áreas de projeto de sistemas de controle e psico-acústica. Um algoritmo adaptativo de controle de qualidade sonora é desenvolvido para solucionar problemas gerados pela interação em amplitude e/ou fase relativa das componentes de banda estreita em distúrbios de característica harmônica. Finalmente, os conceitos avaliados de forma numérica são validados experimentalmente, permitindo não só a análise critica do desempenho psico-acústico dos controladores, como também da própria metodologia de projeto adotada, o que é realizado com auxílio de um mock-up de um veículo automotivo sujeito a distúrbios de características psico-acústicas relevantes para o problema proposto. / This master thesis deals with the study of active noise and sound quality control systems in automotive applications, and the innovative assessment of their performance based on relevant psychoacoustic metrics in this area such as Loudness, Roughness and Tonality. A methodology for designing and implementing active control systems based on psychoacoustic metrics is established. The main challenges reside in defining the set of time-frequency attributes of the disturbance, responsible for the undesirable auditory stimuli. Computer algorithms are used extensively for simulating and analyzing the disturbance, which is a synthesized internal combustion engine noise, integrating both the control system design and psychoacoustics areas. An adaptive active sound quality control algorithm is proposed to resolve the amplitude and/or relative phase interactions among the narrowband components of the multi-harmonic disturbance. Eventually, the numerical results are correlated with real experiments, allowing not only a critical analysis of the psychoacoustic performance of the controllers, but also of the proposed design methodology, which is accomplished with the aid of a vehicle\'s mock-up, subject to disturbances of relevant psychoacoustic characteristics to the proposed problem.