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.

[Truncated abstract] The subject of fan noise generating mechanisms and its control has been studied intensively over the past few decades as a result of the ever-increasing demand for more powerful fans. A unique feature of fan noise is that it consists of high-level discrete frequency noise related to the blade passing frequency, and low-level broadband noise due mostly to turbulent airflow around the fan. Of the two types of fan noise, the discrete frequency noise is the more psychologically annoying component. Past research into fan noise has shown that the discrete frequency fan noise are dipole in nature and are caused predominantly by the fluctuating lift acting on the surfaces of the fan blades. Based on this, several theoretical models have been established to correlate these fluctuating lift forces to the far-field sound pressure. However, one general assumption in these models is that the fan blades are assumed rigid, and the consequence of such an assumption is that it is unclear if the far-field sound pressure is caused solely by the aerodynamic lift force, or whether the blade vibration also plays a substantial role in the generation of the far-field fan noise. One of the goal of this thesis was thus to experimentally quantify the contribution of blade vibration to far-field fan noise and it was found that blade vibration, whilst coherent with the far-field fan noise, did not contribute significantly. Aside of this, several experiments aimed at filling knowledge gaps in the understanding of fan noise characteristics were also be conducted, in particular, to understand the relationship between far-field sound pressure level to blade lengths as well as the number of blades on the fan. The experiments showed that for fans with many blades, the dependency of the far-field sound pressure on blade length is stronger than fans with less blades. Furthermore, dipole measurements showed that the dipole characteristics of fan noise does not occur only at the discrete frequencies, but also within a range of broadband frequencies, implying that the source for both discrete and broadband is the same. The second section of this thesis deals with the study of vortex shedding and its active control. When a circular cylinder (or any object) is placed in a flow within a specified Reynolds number range, flow separation and periodical wake motion is formed behind the cylinder, which is known as vortex shedding. It has been found in previous research that this wake motion is affected by acoustic field imposed on it via loudspeakers. This suggests that there is a strong acoustic-vortex relationship. However, little of this relationship is understood as conventional methods of studying vortex centre around the use of hot-wire anemometry, which effectively measures the velocity fluctuation in the flow. This thesis is the first in using a microphone to study the acoustic characteristic of the vortex wake, and experimental results shows that the two parallel shear layers of the wake carry the strongest pressure signals at the vortex shedding frequency, whilst the entrapped region between the layers carries the strongest pressure signals at the first harmonic.

Orientadores: Jose Roberto de França Arruda, Juan Francisco Camino dos Santos / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-09T13:00:18Z (GMT). No. of bitstreams: 1 Siviero_DiegoAzevedo_M.pdf: 1349512 bytes, checksum: dd6f2edf66df39c704de24d42a9ed55c (MD5) Previous issue date: 2007 / Resumo: Dado o contínuo processo de otimização ao no setor aeroespacial é cada vez mais priorizada nos projetos a busca pelo decréscimo de massa dos sistemas secundários ao vôo, como por exemplo, no sistema responsável pelo controle de ruídos internos. Isto tem provocado um aumento de interesse no desenvolvimento de placas inteligentes, ou ¿smart-plates¿, que consistem em elementos estruturais com atuadores e sensores agregados as suas superfícies, para o controle de suas próprias vibrações estruturais, possibilitando, no futuro, a redução nas dimensões dos elementos passivos de contenção de ruído hoje em uso, principalmente no trabalho com sinais de baixa freqüência. Cresce paralelamente a necessidade de se do definir qual a melhor estratégia de controle para estas estruturas inteligentes. Este estudo descreve a implementação de dois tipos distintos de controladores em uma placa de LEXAN com o objetivo de aumentar a perda transmissão de ruídos. O primeiro controlador utilizado é tipo H2, uma estratégia de controle que utiliza a realimentação da saída (feedback) como referencia para a ação de controle. O segundo controlador é o Filtered-X LMS, uma estratégia por alimentação direta (feedforward) que utiliza um sinal correlacionado ao distúrbio como referencia para o controle. A resposta da planta em malha fechada, com cada controlador, é medida por um microfone com a finalidade de s88e determinar o desempenho atingido pelas diferentes metodologias. Um enfoque maior será dado ao controlador Filtered-X LMS, que também será detalhado e aplicado a um sistema numérico de dutos / Abstract: Due to the continuous optimization process in the aerospace industry, the search for lighter secondary flight systems has been intensively investigated in recent years, for instance, the system responsible for the control of internal noise. This leads to a growing interest in the development of smart panels, which consist of structural elements with actuators and sensors attached to their surfaces, in order to control the structural vibration. This leads to a reduction of the members of passive elements used to attenuate noise mainly at low frequencies. The interest in these smart structures grows along with the necessity of defining the best control strategy. This thesis describes the implementation of two distinct controllers on a LEXAN smart plate, with the purpose of increasing the transmission loss. The first is an H2 dynamic output feedback controller, a strategy that uses the system's output as a reference to the control action. The second controller is the Filtered-X LMS, a strategy that uses a signal correlated with the disturbance as a reference to the control. The response of the closed-loop systems, using each controller, is measured using a microphone. This determines the performance achieved by the different methodologies. More emphasis will be given to the Filtered-X LMS controller, which is also applied to a vibroacoustic problem in a duct / Mestrado / Mecanica dos Sólidos e Projeto Mecanico / Mestre em Engenharia Mecânica

High quality noise and vibration measurements outside of a laboratory environment on real life structures and applications are not trivial. True boundary and operating conditions enforce unique challenges on the measurements. Measurements in hazardous situations such as high magnetic fields, and high temperature environments, etc., where ordinary measurement equipment and methods may not be employed, require further precautions. Post measurements objectives such as analysis, design and strategic decisions, e.g., control, rely heavily on the quality and integrity of the measurements (data). The quality of the experimental data is highly correlated with the on-field expertise. Practical or hands-on experience with measurements can be imparted to prospective students, researchers and technicians in the form of laboratory experiments involving real equipment and practical applications. However, achieving expertise in the field of sound and vibration measurements in general and their active control in particular is a time consuming and expensive process. Consequently most institutions can only afford a single setup, resulting in the compromise of the quality of expertise. In this thesis, the challenges in the field of sound and vibration measurements in high magnetic field are addressed. The analysis and measurement of vibration transferred from an operational magnetic resonance imaging (MRI) scanner to adjacent floors is taken as an example. Improvised experimental measurement methods and custom-made frequency analysis techniques are proposed in order to address the challenges and study the vibration transfer. The methods may be extended to other operational industrial machinery and hazardous environments. To encourage and develop expertise in the field of acoustic/vibration measurements and active noise control on practical test beds, remotely controlled laboratory setups are introduced. The developed laboratory setup, which is accessed and controlled via the Internet, is the first of its kind in the active noise control and acoustic measurements area. The laboratory setup can be shared and utilized 24/7 globally, thus reducing the associated costs and eliminating time restrictions.

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.

No description available.

Active noise control (ANC) is a real-time process in which a system measures an external, unwanted sound source and produces a canceling waveform. The cancellation is due to destructive interference by a perfect copy of the received signal phase-shifted by 180 degrees. Existing active noise control systems process the incoming and outgoing audio on a sample-by-sample basis, requiring a high-speed digital signal processor (DSP) and analog-to-digital converters (ADCs) with strict timing requirements on the order of tens of microseconds. These timing requirements determine the maximum sample rate and bit size as well as the maximum attenuation that the system can achieve. In traditional noise cancellation systems, the general assumption is that all unwanted sound is indeterminate. However, there are many instances in which an unwanted sound source is predictable, such as in the case of a song. This thesis presents a method for active acoustic cancellation of a known audio signal using the frequency characteristics of the known audio signal compared to that of a sampled, filtered excerpt of the same known audio signal. In this procedure, we must first correctly locate the sample index for which a measured audio excerpt begins via the cross-correlation function. Next, we obtain the frequency characteristics of both the known source (WAVE file of the song) and the measured unwanted audio by taking the Fast Fourier Transform (FFT) of each signal, and calculate the effective environmental transfer function (degradation function) by taking the ratio of the two complex frequency-domain results. Finally, we attempt to recreate the environmental audio from the known data and produce an inverted, synchronized, and amplitude-matched signal to cancel the audio via destructive interference. Throughout the process, we employ many signal conditioning methods such as FIR filtering, median filtering, windowing, and deconvolution. We illustrate this frequency-domain method in Native Instruments’ LabVIEW running on the Windows operating system, and discuss its reliability, areas for improvement, and potential future applications in mobile technologies. We show that under ideal conditions (unwanted sound is a known white noise source, and microphone, loudspeaker, and environmental filter frequency responses are all perfectly flat), we can achieve a theoretical maximum attenuation of approximately 300 dB. If we replace the white noise source with an actual song and the environmental filter with a low-order linear filter, then we can achieve maximum attenuation in the range of 50-70 dB. However, in a real-world environment, with additional noise and imperfect microphones, speakers, synchronization, and amplitude-matching, we can expect to see attenuation values in the range of 10-20 dB.

Previous work has shown that active noise control (ANC) can be applied to axial cooling fans. Optimization of the control source and error sensor placement is desired to maximize the attenuation using ANC. A genetic algorithm was developed to find the optimal placement of control sources for a given primary source. The optimal configuration of control sources around a single primary source was shown to be a linear arrangement of the sources. This holds true for both two-dimensional as well as three-dimensional configurations. The higher-order radiation of the linear arrangement has also been verified experimentally, but the improvement in the experimental apparatus was not as dramatic as the theoretical model. Multiple flow visualization techniques have been used to find optimal near field error sensor locations. When there is little obstruction to the flow field of the fan, minimal airflow is found along the near field null that is created by minimizing the sound power of the system. Surface mounting of the error sensors can lead to a small increase in the signal-to-noise ratio of the error sensors if vortices exist in the near field of the fan due to obstructions in the main flow. It has also been shown that the introduction of the ANC system does not affect the flow field of the fan.

The Filtered-X Least-Mean-Square (FXLMS) algorithm is widely used in active noise control due to its robustness, simplicity, and ability to be implemented in real time. In a feedforward implementation of the FXLMS algorithm, a reference signal that is highly correlated with the noise to be controlled is filtered with an estimate of the transfer function of the secondary path. The convergence characteristics of the FXLMS algorithm have been well studied. A convergence parameter is used to optimize the convergence of the algorithm. However, the optimal value for the convergence parameter is frequency dependent. Thus for noise containing multiple tones at different frequencies the convergence parameter can only be optimized for one of those tones. Other tones will have slower convergence rates and in general less attenuation than they would have if they were treated singly and parameters could be optimized for those frequencies separately. A method is developed to modify the magnitude response of the secondary path estimate while maintaining the original phase response, which equalizes the convergence characteristics over multiple frequencies, giving more uniform convergence and attenuation for all tones being controlled. Stability of the algorithm is not compromised. The modification to the FXLMS algorithm is relatively simple to implement and has been shown to increase overall attenuation of a signal containing multiple tones by an additional 6-9 dB.

The properties of acoustic kinetic energy density and total energy density of sound fields in lightly damped enclosures have been explored thoroughly in the literature. Their increased spatial uniformity makes them more favorable measurement quantities for various applications than acoustic potential energy density (or squared pressure), which is most often used. In this dissertation, a new acoustic energy quantity, the generalized acoustic energy density (GED), will be introduced. It is defined by introducing weighting factors, α and 1 − α, in the formulation of total acoustic energy density. With the additional degree of freedom, the GED can conform to the traditional acoustic energy density quantities, or be optimized for different applications. The properties and applications of the GED are explored in this dissertation. For enclosed sound fields, it was found that GED with α = 1/4 is spatially more uniform than the acoustic potential energy density, acoustic kinetic energy density, and the total acoustic energy density, which makes it a more favorable measurement quantity than those traditional acoustic energy density quantities for many indoor measurement applications. For some other applications, such as active noise control in diffuse field, different values of α may be considered superior. The numerical verifications in this research are mainly based on a hybrid modal expansion developed for this work, which combines the free field Green's function and a modal expansion. The enclosed sound field is separated into the direct field and reverberant field, which have been treated together in traditional modal analysis. Studies on a point source in rectangular enclosures show that the hybrid modal expansion converges notably faster than the traditional modal expansions, especially in the region near the source, and introduces much smaller errors with a limited number of modes. The hybrid modal expansion can be easily applied to complex sound sources if the free field responses of the sources are known. Damped boundaries are also considered in this dissertation, and a set of modified modal functions is introduced, which is shown to be suitable for many damped boundary conditions.