308 research outputs found
The effect of a periodic absorptive strip arrangement on an interior sound field in a room
No full textIn this paper we study the effect of periodically arranged sound absorptive strips on the mean acoustic potential energy density distribution of a room. The strips are assumed to be attached on the room's surface of interest. In order to determine their effect, the mean acoustic potential energy density variation is evaluated as the function of a ratio of the strip's arrangement period to wavelength. The evaluation demonstrates that the mean acoustic potential energy density tends to converge. In addition, a comparison with a case in which absorptive materials completely cover the selected absorptive plane shows that a periodic arrangement that uses only half of the absorptive material can be more efficient than a total covering, unless the frequency of interest does not coincide with the room's resonant frequencies. Consequently, the results prove that the ratio of the arrangement period to the wavelength plays an important role in the effectiveness of a periodic absorptive strip arrangement to minimize a room's mean acoustic potential energy density. (C) 2005 Acoustical Society of America
Cochlear -based transducers: Modeling and design.
No full textThree mathematical models are introduced in this thesis for modeling the coupled fluid structure systems, especially the cochlear-based transducers. A full three dimensional FEM model is developed with the use of the 27-noded mixed fluid element coupled with 9-noded structure element. The fluid displacements and pressure are chosen as solution variables and the pressure degrees of freedom are condensed out in the element level. A modified pressure boundary condition is also derived to approximately account for the viscous effect in the boundary layer under several legitimate assumptions to avoid overly intricate formulations. The viscous boundary condition is applied on previously inviscid 2.Sd FEM framework. A model WKB method, was developed with the ability to consider more than one wave mode in the direction of wave propagation. It is proved capable of producing an accurate solution for the fluid structure interaction problems with the structure modeled as a locally reacting impedance, a membrane or a plate. Three cochlear-based transducers were designed in order to mimic the functioning of the passive and active cochlea, mechanically or electronically. A single-channel trapped fluid transducer, with the inspiration drawn from the low-pass filtering behavior of the passive cochlea, was designed and optimized using nondimensional formulas derived from a boundary integral method. The transducer achieves a sensitivity of -200dB re 1V/muPa in a 10kHz frequency range. A radio frequency channelizing filter is developed at 20-90MHz with 20 output channels. The filter topology is derived from an electrical-mechanical analogy of the passive cochlea and can be scaled to any frequency. A preliminary design of active cochlear-based MEMS transducer is also presented in the thesis. The piezoelectric sensing is shown to have better sensitivity with a unique sensing structure. A series of the cantilever beams are connected to the center of the membrane and piezoelectric material is deposited on the beam surface for sensing as well as actuation if necessary. The preliminary design shows 15dB increase in the sensitivity compared to the passive transducer with an improved filtering.PhDApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126897/2/3287481.pd
Modeling, analysis, and experimental verification of a multi-mode vibro-impacting system.
No full textThis thesis presents the model problem of a pinned-free beam impacting a linear spring due to sinusoidal base motion. The beam has been represented as both a single- and multi-degree-of-freedom (DOF) system and the results of these representations have been compared to each other as well as experimental data. It has been found that the multi-DOF model is required in order to capture higher frequency response. In addition, the effect of multiple impacts on the Fourier transform of the response has been examined and suggests a method by which experimentalists can estimate the number of impacts per cycle in a given motion by observation of its frequency response. The analytical expressions for period-1,1 response (i.e., those solutions which repeat with every forcing cycle and undergo one-impact per period) were developed by matching the piecewise linear expressions at the loss- and regaining-of-contact times. The results of these solutions have been compared to experimental responses and are found to agree in qualitative character. Parameter studies show that increasing stiffness and decreasing contact damping reduce the existence of period-1,1 solutions. Both models exhibit behavior characteristic of potentially chaotic systems and the presence of strange attractors. Finally, the system has been modeled using a unified set of modes prescribed by the Craig-Bampton method. This approximate model is found to agree reasonably well with the analytical models. Alternative one-mode models are discussed which could be used for further parameter studies. Modal convergence studies show that the inclusion of two modes in the multi-DOF model is sufficient for the parameter region presently discussed.PhDApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123166/2/3068860.pd
Modeling and analysis of complex structural -acoustic systems.
No full textTechniques are developed for modeling and analyzing the vibration response of complex structural-acoustic systems. A novel application of Lagrange multipliers to couple domains modeled independently with linear elasto-dynamic and reduced-order theories is presented. This method is developed from a variational formulation of the continuous problem and is easily implemented within a finite element code. Applications include modeling of cracks, holes, and junctions within a frame-like structure. Simulations using this method compare well with a full elasto-dynamic discretizations and existing methods for modeling such problems. In an experimental context, it is desirable to have a robust, flexible method for analyzing wave phenomena along a structure and in the surrounding fluid. A general framework is developed for determining the underlying parameters of general signal models through the application of maximum likelihood estimation theory for functions whose variables separate. This method extends previous work in sinusoidal and exponential estimation to include models with other functional bases, such as exponential functions with non-constant amplitudes and Bessel functions. Non-uniform spatial sampling is also possible with this technique. The maximum likelihood method is applied to the identification of wave components along one-dimensional structural elements. Results are given which demonstrate the viability and accuracy of the technique estimating exponential and Bessel function model parameters from noisy simulation data. Algorithms for determining material properties and power flow in vibrating structures are presented. New techniques are achieved by synthesizing wave component identification methods with new methods targeted towards material property and structural intensity estimation. The effectiveness of this nonlinear least-squares approach is investigated through laboratory and numerical experiments on a non-uniform structure, yielding guidelines for spatial sampling and the effects of noise. The study of harmonically driven in vacuo and fluid-loaded T-beams with various joint configurations is presented to demonstrate the ability of these modeling and analysis techniques to investigate complex structures. Numerical experiments are performed which investigate wave propagation in multiply connected beams, transmission of energy through a complex joint, power dissipation in a structure, and the effect of fluid loading on both wave propagation and energy dissipation.PhDApplied SciencesMechanical engineeringMechanicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132583/2/9977169.pd
Large-scale computation and optimization for ultrasound acoustic transducers.
No full textThis thesis is concerned with efficient numerical techniques for the modeling and design of ultrasonic transducer arrays for therapy. The finite element method provides a rigorous modeling technique for ultrasound transducers. However, the very intensive computation required by ultrasonic frequencies presents a great challenge to numerics. The effectiveness of iterative solution strategies for the large sparse complex linear systems arising from the high frequency response of coupled piezoelectric-elastic-fluid interaction problems is extensively investigated. To remedy the ill-conditioning caused by the fine mesh and vastly different spatial scales of the structures and fluid medium, two preconditioning techniques, SSOR preconditioner and Incomplete LU (ILU) factorization preconditioner, are examined and evaluated through a series numerical experiments. The numerical results show that SSOR preconditioner is the most cost efficient strategy though the ILU factorization is generally more effective in reducing the iteration counts. Also, the simple ILU(0) preconditioner is shown to be more efficient and reliable than recently developed ILUT(p,tau) preconditioner. In therapy transducer design, the acoustic power at the operating frequency is a critical figure of merit. A systematic design methodology for enhancing the acoustic power radiated from a fluid-loaded ultrasonic array element at a fixed frequency is developed. A gradient-based optimization algorithm is integrated within the finite element framework to guide the determination of the two design variables, the piezoelectric element thickness and the matching layer thickness, to maximize the acoustic power output. A novel method for avoiding the explicit remeshing in the optimization iterations is presented. Optimized designs are determined numerically and confirmed by the experiments. Cross-talk in therapy arrays decreases the power efficiency and array steering capabilities. To reduce the cross-talk, a density-based design approach is utilized to optimize the topology of kerf fillings in linear phased arrays. Two design schemes, element-by-element design and layer-by-layer design, are developed. The optimized topology of kerf fillings in each design scheme is presented. The radiation of the array is evaluated in both the pressure level and array directivity. Significant improvement of the acoustic field with optimized kerf fillings is demonstrated.PhDApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132822/2/9990925.pd
Passive and active structural acoustic filtering in cochlear mechanics: Analysis and applications.
No full textA linear physiologically-based finite element model is developed for analyzing the global mechanical-electrical-acoustic (active) filtering in the mammalian cochlea. The model consists of a two-duct fluid-filled rectangular geometry, the micro-mechanical structural network interacting with the fluid, electrical circuit equivalent of cells and fluid in every cross-section connected by longitudinal cables representing the conductivity of the cochlear fluids and includes the mechano-electrical and electro-mechanical transduction at the outer hair cells. The acoustic pressures, structural displacements, and electrical potentials are determined numerically and compared with experiments. For the first time, the response of the cochlea to both acoustic and electrical excitation are predicted using the same physiological model and compared with experiments. Reducing the amount of activity present in the model reduces the gain and lowers the frequency of peak BM velocity response compared to a fully active model, in accordance with experimental data. This model also possesses near invariance to click induced noise at different gain levels. Using the same model parameters, the predictions of the local BM velocity response to electrical stimulation match available experimental data, providing an independent test of the model capability. Predictions of electrically evoked otoacoustic emissions are found to match experimental results as well. Roughness introduced into BM stiffness is found to result in fine structures in a fully-active model and have little effect on a model with reduced activity. A new kind of passive hydraulic and pneumatic silencer called the structural acoustic silencer for broadband passive noise control is designed based on analogy with passive mechanics of the cochlea and compared with physical tests from experiments. The design of the silencer is done numerically using three dimensional finite element method. The structural acoustic silencers indeed result in broadband transmission loss. The relation between transmission loss and plate dispersion in the silencer is shown for the first time.PhDApplied SciencesAudiologyHealth and Environmental SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124539/2/3150070.pd
Biomimetic trapped fluid microsystems for acoustic sensing.
No full textAn innovative architecture for acoustic sensing using a trapped fluid as an acoustic transmission medium has been developed. This architecture was inspired by the structure of the mammalian cochlea, the most successful natural acoustic sensor design. Micromachining technology was used to fabricate the sensors in order to preserve the physiological size scale of the mammalian cochlea, and to aid in the integration of sensing elements. Mathematical models for microscale acoustics, including fluid-structure interaction, are developed in support of these designs. Three microsystems are described. First, a lifesize hydromechanical cochlear model is discussed. This model is used to explore the effects of structure orthotropy and fluid viscosity on the mechanics. It is demonstrated that achievable orthotropy ratios of 8:1 in tension do not result in the sharp filtering observed in animal experiments. It is also demonstrated that high viscosities (20 cSt) must be used to introduce enough damping to avoid nonphysiological standing waves. These results underscore the importance of the active mechanisms present in the cochlea which appear to be critical for sharp filtering. Second, a design for a single-output-channel acoustic sensor with the trapped fluid architecture and a capacitive sensing scheme is described. This device achieves sensitivities (-170 to -200 dB re 1V/muPa in a 30 kHz band) competitive with commercial piezoelectric hydrophones in a compact MEMS design. The system also demonstrates a novel fabrication process for producing trapped-fluid microsystems with integrated capacitive sensors. Third, a microscale cochlear analogue transducer (muCAT) is described. This system is the first demonstration of integration of multiple sensing elements into a lifesize cochlear-like mechanical structure to produce a multiple-output-channel acoustic sensor capable of mechanical spectral analysis. Electrical output from the 32 integrated capacitive sensors demonstrates competitive sensitivities (-200 to -186 dB re 1 V/muPa) and bandwidth (100 kHz) for some channels. Laser vibrometry results demonstrate the presence of cochlear-like traveling waves and a frequency position map. Electrical output from the integrated sensors does not follow the vibration pattern measured with laser vibrometry.PhDApplied SciencesElectrical engineeringMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125538/2/3192812.pd
Efficient finite element methods/reduced-order modeling for structural acoustics with applications to transduction.
No full textEfficient finite element techniques for time harmonic structural acoustics are developed and examined in the context of applications where high frequencies/wavenumbers are present. In the first portion of this thesis, discrete dispersion analysis on a regular rectangular mesh is developed for the Helmholtz equation and applied to four finite element methods (basic Galerkin method, two Galerkin least-squares methods, and a related residual-based method). The goal of the modified FEM techniques is to reduce problem size by allowing coarse mesh modeling of the fluid domain while retaining solution accuracy. The efficacy of these schemes is assessed analytically then verified numerically for high frequency applications in a therapeutic medical ultrasound model problem. Numerical experiments on a simple 2D beam-forming problem show that GLS methods can reduce matrix storage requirements by a factor of five and solution time by a factor of eight over basic Galerkin FEM. The remainder of this dissertation deals with development and validation of an improved FEM technique for structural acoustics. A novel hybrid analytical/FEM method (known as 2.5D FEM) for fluid-coupled, variable-width plates (linear and exponential variation) is presented. This modeling technique achieves computational savings by reducing the physical dimensionality of a problem via a priori specification of a limited set of structural and fluid modes in the condensed dimension. Consequently, 2.51) FEM models can be 10--20 times smaller than comparable full 3D FEM models. This method, in conjunction with an analytical technique (WKB), is applied to the design of a transducer which employs mammalian cochlea-like behavior. The design process of this so-called cochlear-based transducer is illucidated with discussion of design guidelines. Finally, experimental results are shown which validate the 2.51) FEM method for both isotropic and orthotropic plate models. To facilitate the comparison of numerical results and experimental data, an alternate modal basis (utilizing a static mode approximation) for the lateral plate structural modeshape was implemented. Experimental and numerical data correlated well across the operational range of the experimental setup with average L2-error levels of 17% for the isotropic plate material (aluminum) and 32% for the orthotropic plate material (graphite-epoxy). Various other composite plate materials are examined experimentally to explore plate dynamics of orthotropic materials.PhDApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131177/2/3000944.pd
Micro and macro-mechanics of the mammalian cochlea.
No full textThis thesis tackles issues in cochlear mechanics both at the microscopic and macroscopic level. Insights gained from the microscopic models are used to mathematically represent the microstructures in a macroscopic model of the cochlea. A comprehensive two-state Boltzmann model is developed for outer hair cell (OHC) motility and is validated by comparing its predictions with experimental findings. Issues with modeling outer hair cells (OHCs) are also discussed and their impact on in vivo behavior of OHCs is explored through simpler OHCs model that retain fidelity over the small voltage and strain variations seen in vivo. Models and parameters for other micro-structures are developed or adapted based on an extensive literature search on their properties. These models for the micro-structures are combined with a two-duct fluid model and cable models for electrical conduction to produce a global mathematical representation of the physiology of the cochlea. The equations are solved using the finite element method. The parameters used in the model are almost entirely based on guinea pig data. The model predictions match a number of experimental results, both quantitatively and qualitatively. Model results for acoustic simulation and electrical stimuli are presented. Analysis of the model and its results gives fresh insight into the mechanics of the cochlea. Results indicate that the characteristic frequency at a location is determined predominantly by properties of the tectorial membrane, and not of the basilar membrane. This finding contradicts he conventional view in cochlear mechanics. The model is used to reinterpret certain experimental findings in past publications to provide experimental evidence for the new theory. The model results also show that the high frequency voltage roll-off of OHCs does not preclude force production from OHCs in the cochlea. The model necessitates transducer currents that are possibly higher than currents in vivo to achieve physiologically similar amplification. This leaves open the possibility that HB motility might be aiding force production. The model however rules out the possibility of HB force production being the sole active mechanism in the organ.PhDAudiologyBiological SciencesBiophysicsHealth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126136/2/3237943.pd
- …
