Active control of sound radiation from vibrating structures has been an area of much research in the past decade. In Active Structural Acoustic Control (ASAC), the minimization of sound radiation is achieved by modifying the response of the structure through structural inputs rather than by exciting the acoustic medium (Active Noise Control, ANC). The ASAC technique often produces global far-field sound attenuation with relatively few actuators as compared to ANC. The structural control inputs of ASAC systems are usually constructed adaptively in the time domain based on a number of error signals to be minimized. One of the primary concerns in active control of sound is then to provide the controller with appropriate ``error'' information. Early investigations have implemented far-field microphones, thereby providing the controller with actual radiated pressure information. Most structure-borne sound control approaches now tend to eliminate the use of microphones by developing sensors that are integrated in the structure. This study presents a new sensing technique implementing such an approach. A structural acoustic sensor is developed for estimating radiation information from vibrating structures. This technique referred to as Discrete Structural Acoustic Sensing (DSAS) provides time domain estimates of the radiated sound pressure at prescribed locations in the far field over a broad frequency range. The structural acoustic sensor consists of a set of accelerometers mounted on the radiating structure and arrays of digital filters that process the measured acceleration signals in real time. The impulse response of each filter is constructed from the appropriate radiation Green's function for the source area associated with each accelerometer. Validation of the sensing technique is performed on two different systems: a baffled rectangular plate and a baffled finite cylinder. For both systems, the sensor is first analyzed in terms of prediction accuracy by comparing estimated and actual sound pressure radiated in the far field. The analysis is carried out on a numerical model of the plate and cylinder as well as on the real structures through experimental testing. The sensor is then implemented in a broadband radiation control system. The plate and cylinder are excited by broadband disturbance inputs over a frequency range encompassing several of the first flexural resonances of the structure. Single-sided piezo-electric actuators provide the structural control inputs while the sensor estimates are used as error signals. The controller is based on the filtered-x version of the adaptive LMS algorithm. Results from both analytical and experimental investigations are again presented for the two systems. Additional control results based on error microphones allow a comparison of the two sensing approaches in terms of control performance. The major outcome of this study is the ability of the structural acoustic sensor to effectively replace error microphones in broadband radiation control systems. In particular, both analytical and experimental results show the level of sound attenuation achieved when implementing Discrete Structural Acoustic Sensing rivaled that achieved with far-field error microphones. Finally, the approach presents a significant alternative over other existing structural sensing techniques as it requires very little system modeling. / Ph. D.

This dissertation presents an exhaustive comparative study on optimization in structural acoustics. A combination of a commercially available finite element software package and additional user-written programs is used to modify the shape of a structure. This is done iteratively and without manual intervention to achieve significant improvements of the objective function. The optimization process continues automatically until the predefined maximum number of function evaluations is reached. The design variables are the structure's local geometry modification values at selected surface key-points. The objective of the optimization includes the minimization of the root mean square level of structure borne sound (a general measure of the vibrational sensitivity of a structure). In addition, the structural mass remains constant and the allowable ranges of design variable values are restricted by prescribed upper and lower limits. The optimization procedure is tested on the finite element model of a rectangular plate made of steel. Twelve different optimization methods are tested against each others. These methods are considered either as approximate or exact. The approximate optimization methods use either an approximated value of objective function, e.g., hybrid design of experiments and hybrid neural networks, or the approximated values of the first and the second derivatives of the objective function, e.g., method of feasible directions, sequential quadratic programming method, Newton method, limited memory Broyden-Fletcher-Goldfarb-Shanno method for bound constrained problems, method of moving asymptotes, mid-range multi- oints method and controlled random search method. The exact optimization methods, e.g., genetic algorithms, tabu search and simulated annealing, are derivative-free methods and they use the exact value of objective function. Furthermore, a statistical approach is followed for the comparison of methods. Advantages and disadvantages of each optimization algorithm are reported in details. The rate of convergence (a measure of optimization speed) and the robustness level of each optimization method are evaluated. Some optimization methods are classified as fast, medium and slow. Method of moving asymptotes and mid-range multi-points method are introduced as the fastest methods. Finally, it is experienced that the use of e ective structural-acoustic analysis methods can drastically reduce the total optimization time. If the powerful optimization methods become equipped with effective (fast and reliable) structural acoustic analysis methods, then they can present more desirable optimization results in a shorter period of computation time. In this case, they can even be considered as a suitable replacement for the complex and the multi-stages hybrid optimization algorithms.

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.

Virtual acoustic prototyping (auralization) is the rendering of a virtual sound field that is created from the calculated acoustic response of a modeled structure. Auralization is useful in the design and subjective evaluation of buildings, automobiles, and aircraft. The virtual acoustic prototyping method in this thesis uses finite element modeling (FEM), the equivalent source method (ESM), and head-related transfer functions (HRTFs). A tradeoff exists between the accuracy of the auralization process and the number of equivalent sources (and thus computational power) that are required. The goal of this research is to validate (numerically and subjectively) a virtual acoustic prototyping method for use in structural design; this thesis illustrates the first attempt to apply the aforementioned methods to a structure that represents a typical building or automobile. The structure's acoustics were modeled using FEM, ESM, and HRTFs. A prototype of the modeled structure was built. A 36% correlation was achieved between the model and prototype. Slight variations in boundary conditions caused significant FEM error, but the data represented a typical structure. Psychoacoustic comparison testing was performed to determine the number of equivalent sources that must be used in an auralization to accurately recreate the sound field. The number was found to be dependent on the type of noise that is played to the test subject. A clear relationship between the numerical correlation of two sounds and the percentage of subjects who could hear a difference between those two sounds was established for impulsive, broadband, and engine noises. / Master of Science

An innovative implementation of the skin concept for the reduction of the radiated sound power from a vibrating structure is proposed. The skin has a periodic structure and continuously covers a vibrating beam. Thus, this skin decouples the vibrating structure from the acoustic field by modifying the wavenumber spectrum of the radiating surface. First, structural acoustics and periodic structure theories are reviewed in order to predict how bending waves propagate along a periodic beam and how this beam radiates sound. These theories are then extended to the case of multi-layered structures in order to understand the behavior of a beam loaded with a periodic skin. In order to design the beam and skin structural periods, two different methods are used: Galois sequences and an optimization process using a real-valued genetic algorithm. Simulations are run for the case of periodic beams and beams coupled with periodic smart skins in both finite and infinite configurations. Results show that periodic beam can radiate less sound than equivalent uniform structures. Results also show the potential of periodic skin for application to the structural radiation problem for frequencies higher than approximately 100Hz with an approximately 10dB of radiated sound power attenuation. / Master of Science

The concept of a weak sound radiating cell is proposed to reduce the low frequency radiated noise from structures. The cell consists of two coupled surfaces such that, when placed on a vibrating structure, the responses of the two surfaces are nearly out-of-phase and of equal strength over a wide frequency range. This structure response leads the cell to behave as an acoustic dipole and thus as a poor sound radiating source. The control of low frequency structurally radiated noise is then achieved by covering the structure with an array of these weak radiating cells, i.e. surface treatment. Thus, the surface treatment essentially transforms the response of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. Theoretical models are developed to predict the performance of the cell. Experimental verification is performed for a single cell applied to a piston-like structure to demonstrate the concept on a simple radiating structure. The results demonstrated an overall sound power level reduction of 5.2 dB between 400-1600 Hz with maximum reductions over 30 dB at discrete frequencies. Finally, an array of weak radiating cells is experimentally applied to a more complex structure, a rectangular plate. The results of the plate experiments reveal an overall sound power level reduction of 10.2 dB between 100-1600 Hz with maximum reductions of 25 dB at discrete frequencies. These results demonstrate the potential of the weak radiating cell concept to reduce low frequency structurally radiated noise. / Master of Science

International Telemetering Conference Proceedings / October 26-29, 1998 / Town & Country Resort Hotel and Convention Center, San Diego, California / A novel concept using the cockpit voice recorder (CVR) as a structural vibration recording device, to aid in structural health monitoring of commercial and military aircraft, is outlined. The unused cables in the CVR wiring harness act as “latent transducers” that respond to structural vibrations, generating vibration signals, which the CVR records. Postprocessing of such data can provide clues to problem areas or changes in the signature of the aircraft. The standards which the CVR must meet to qualify as a instrumentation-quality recorder are discussed and the steps required to assure compliance are outlined.

This dissertation presents an exhaustive comparative study on optimization in structural acoustics. A combination of a commercially available finite element software package and additional user-written programs is used to modify the shape of a structure. This is done iteratively and without manual intervention to achieve significant improvements of the objective function. The optimization process continues automatically until the predefined maximum number of function evaluations is reached. The design variables are the structure's local geometry modification values at selected surface key-points. The objective of the optimization includes the minimization of the root mean square level of structure borne sound (a general measure of the vibrational sensitivity of a structure). In addition, the structural mass remains constant and the allowable ranges of design variable values are restricted by prescribed upper and lower limits. The optimization procedure is tested on the finite element model of a rectangular plate made of steel. Twelve different optimization methods are tested against each others. These methods are considered either as approximate or exact. The approximate optimization methods use either an approximated value of objective function, e.g., hybrid design of experiments and hybrid neural networks, or the approximated values of the first and the second derivatives of the objective function, e.g., method of feasible directions, sequential quadratic programming method, Newton method, limited memory Broyden-Fletcher-Goldfarb-Shanno method for bound constrained problems, method of moving asymptotes, mid-range multi- oints method and controlled random search method. The exact optimization methods, e.g., genetic algorithms, tabu search and simulated annealing, are derivative-free methods and they use the exact value of objective function. Furthermore, a statistical approach is followed for the comparison of methods. Advantages and disadvantages of each optimization algorithm are reported in details. The rate of convergence (a measure of optimization speed) and the robustness level of each optimization method are evaluated. Some optimization methods are classified as fast, medium and slow. Method of moving asymptotes and mid-range multi-points method are introduced as the fastest methods. Finally, it is experienced that the use of e ective structural-acoustic analysis methods can drastically reduce the total optimization time. If the powerful optimization methods become equipped with effective (fast and reliable) structural acoustic analysis methods, then they can present more desirable optimization results in a shorter period of computation time. In this case, they can even be considered as a suitable replacement for the complex and the multi-stages hybrid optimization algorithms.

The search for a convenient connection between vibration patterns on a structure and the sound radiated from that structure is ongoing in structural acoustics literature. Common techniques are wavenumber domain methods, or representation of the vibration in terms of some basis, such as structural modes or elementary radiators, and calculating the sound radiation in terms of the basis. Most choices for a basis in this situation exhibit strong coupling between the basis functions, but there is one choice which does not: Acoustic radiation modes are by definition the basis that orthogonalizes the radiation operator, meaning the radiation modes do not exhibit any coupling in radiation of sound.Acoustic radiation modes are coming up on their 30th anniversary in the literature, but still have not found wide use. This is largely due to the fact that most radiation modes must be calculated through the computationally intensive boundary element method or boundary integral equations. Analytical expressions for radiation modes, or for the radiation resistance matrix from which they are derived, are only available for a few geometries. This thesis meets this problem head on, to develop additional analytical expressions for radiation resistance matrices of cylindrically curved structures.Radiation modes are developed in the context of their use to calculate sound power. Experimental and computational sound power calculations are presented in order to validate the use of the modes developed here. In addition, the properties and trends of the developed modes are explored.

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