O objetivo deste trabalho é estudar o fenômeno da força de radiação acústica produzida por ondas progressivas e estacionárias. Neste trabalho o estudo da força produzida por ondas estacionárias é aplicado na análise de um levitador acústico e o estudo da força de radiação acústica por ondas progressivas é feito visando a futura construção de um separador acústico. Neste trabalho é utilizado o método dos elementos finitos para simular o comportamento de um levitador acústico. Primeiramente, é feita a simulação de um levitador acústico que consiste de um transdutor de Langevin com uma face de emissão plana que opera na freqüência de aproximadamente 20 kHz e um refletor plano. O método dos elementos finitos é utilizado para determinar o deslocamento da face do transdutor e o potencial acústico que atua numa esfera pequena. O deslocamento da face do transdutor obtido numericamente é comparado com o medido experimentalmente por um vibrômetro de fibra ótica e o potencial acústico determinado pelo método dos elementos é verificado experimentalmente colocando pequenas esferas de isopor no levitador. Depois de verificar o modelo numérico, o método dos elementos finitos é utilizado na otimização de um levitador acústico composto de um refletor côncavo e um transdutor com face de emissão côncava. Os resultados numéricos mostram que a força de radiação acústica no novo levitador é aumentada em 604 vezes quando comparada com o levitador composto de um transdutor com face plana e refletor plano. Este trabalho também apresenta um modelo numérico para determinar a trajetória de partículas esféricas na presença de uma onda de ultra-som progressiva. O modelo assume que as seguintes forças atuam na partícula: gravidade, empuxo, forças viscosas e força de radiação acústica devido a uma onda progressiva. Com o objetivo de não restringir o tamanho das partículas que podem ser utilizadas no modelo é empregada uma equação empírica do coeficiente de arrasto, válida para uma grande faixa de número de Reynolds. O modelo proposto requer a distribuição de pressão gerada pelo transdutor de ultra-som. A distribuição de pressão é medida experimentalmente utilizando um hidrofone calibrado. A verificação do modelo é feita soltando-se pequenas esferas de vidro (com diâmetros da ordem de 500 m) em frente a um transdutor de ultra-som de 1 MHz e 35 mm de diâmetro. / The objective of this work is to study the acoustic radiation force produced by progressive and standing waves. In this work, the studies related to the acoustic radiation force generated by ultrasonic standing waves are applied in the analysis of an acoustic levitator and the studies involving the acoustic radiation force generated by progressive waves are conducted aiming the design of acoustic separators. In this work, the finite element method is used to simulate an acoustic levitator. First, an acoustic levitator consisting of a 20 kHz Langevin ultrasonic transducer with a plane radiating surface and a plane reflector is simulated by the finite element method. The finite element method is used to determine the transducer face displacement and the acoustic radiation potential that acts on a small sphere. The numerical displacement is compared with that obtained by a fiber-optic vibration sensor and the acoustic radiation potential determined by the finite element method is verified experimentally by placing small Styrofoam spheres in the levitator. After verifying the numerical method, the finite element method was used to optimize an acoustic levitator consisting of a concave-faced transducer and a curved reflector. The numerical results show that the acoustic radiation force in the new levitator is enhanced 604 times compared with the levitator consisting of a plane transducer and a plane reflector. This work also presents a numerical model to determine the trajectory of sphere particles when submitted to ultrasonic progressive waves. This model assumes that the following forces act on the particle: gravity, buoyancy, viscous forces and acoustic radiation force due to the progressive wave. In order not to restrict the model to a small particle size range, the viscous forces that act on the sphere are modeled by an empirical relationship of drag coefficient that is valid for a wide range of Reynolds numbers. The numerical model requires the pressure field radiated by the ultrasonic transducer. The pressure field is obtained experimentally by using a calibrated needle hydrophone. The numerical model validation is done by dropping small glass spheres (on the order of 500 m diameter) in front of a 1-MHz 35-mm diameter ultrasonic transducer.

Acoustic radiation efficiency is defined as the ratio of sound power radiated to the surface vibration power of a piston with equivalent surface area. It has been shown that the radiation efficiency is maximized and may exceed unity when the structural and acoustic wavelengths are approximately equal. The frequency at which this occurs is called the critical frequency and can be shifted with structural modifications. This has proven to be an effective way to reduce noise. The standard radiation efficiency measurement is comprised of an intensity scan for sound power measurement and accelerometer array for spatially averaged vibration determination. This method is difficult to apply to lightweight structures, complicated geometries, and when acoustic sources are in close proximity to one another. Recently, robust particle velocity sensors have been developed. Combined with a small microphone in the same instrument, particle velocity and sound pressure can be measured simultaneously and at the same location. This permits radiation efficiency to be measured using a non-contact approach with a single sensor. A suggested practice for measuring radiation efficiency has been developed and validated with several examples including two flat plates of different thickness, an oil pan, and components on a running small engine.

<p>Cardiovascular disease is the leading cause of death for developed countries, including the United States. In order to diagnose and detect certain cardiac diseases, it is necessary to assess myocardial performance and function. One mechanical property that has been shown to reflect myocardial performance is myocardial stiffness. Acoustic radiation force impulse (ARFI) imaging has been demonstrated to be capable of visualizing variations in local stiffness within soft tissue. </p><p>In this thesis, the initial investigations into the visualization of myocardial performance with ARFI imaging are presented. <italic>In vivo</italic> ARFI images were acquired with a linear array placed on exposed <italic>canine</italic> hearts. When co-registered with the electrocardiogram (ECG), ARFI images of the heart reflected the expected changes in myocardial stiffness through the cardiac cycle. With the implementation of a quadratic motion filter, motion artifacts within the ARFI images were reduced to below 1.5 &mu m at all points of the cardiac cycle. The inclusion of pre-excitation displacement estimates in the quadratic motion filter further reduced physiological motion artifacts at all points of the cardiac cycle to below 0.5 &mu m. </p><p>In order for cardiac ARFI imaging to more quantitatively assess myocardial performance, novel ARFI imaging sequences and methods were developed to address challenges specifically related to cardiac imaging. These improvements provided finer sampling and improved spatial and temporal resolution within the ARFI images. <italic>In vivo</italic> epicardial ARFI images of an <italic>ovine</italic> heart were formed using these sequences, and the quality and utility of the resultant ARFI-induced displacement curves were examined.</p><p><italic>In vivo</italic> cardiac ARFI images were formed of <italic>canine</italic> left ventricular free walls while the hearts were externally paced by one of two electrodes positioned epicardially on either side of the imaging plane. Directions and speeds of myocardial stiffness propagation were measured within the ARFI imaging field of view. In all images, the myocardial stiffness waves were seen to be traveling away from the stimulating electrode. The stiffness propagation velocities were also shown to be consistent with propagation velocities measured from elastography and tissue velocity imaging as well as the local epicardial ECG.</p><p>ARFI-induced displacement curves of an <italic>ovine</italic> heart were formed and temporally registered with left ventricular pressure and volume measurements. From these plots, the synchronization of myocardial stiffening and relaxation with the four phases (isovolumic contraction, ejection, isovolumic relaxation, and filling) of the cardiac cycle was determined. These ARFI imaging sequences were also used to correlate changes in left ventricular performance with changes in myocardial stiffness. These preliminary results indicated that changes in the ARFI imaging-derived stiffnesses were consistent with those predicted by current, clinically accepted theories of myocardial performance and function.</p><p>These results demonstrate the ability of ARFI imaging to visualize changes in myocardial stiffness through the cardiac cycle and its feasibility to provide clinically useful insight into myocardial performance.</p> / Dissertation

<p>Acoustic radiation force impulses (ARFI) have been used to generated transverse-traveling mechanical waves in various biological tissues. The velocity of these waves is related to a medium's stiffness and thus can offer useful diagnostic information. Consequently, shear wave velocimetry has the potential to investigate cardiac disease states that manifest themselves as changes in tissue stiffness (e.g., ischemia).</p><p> The work contained herein focuses on employing ARFI-based shear wave velocimetry techniques, similar to those previously utilized on other organs (e.g., breast, liver), for the investigation of cardiac tissue. To this end, ARFI excitations were used to generate slow-moving (under 3 m/s) mechanical waves in exposed myocardium (with access granted through a thoracotomy); these waves were then tracked with ultrasonic methods. Imaging techniques to increase frame-rate, decrease transducer/tissue heating, and reduce the effects of physiological motion were developed. These techniques, along with two shear wave velocimetry methods (i.e., the Lateral Time-to-Peak and Radon sum transformation algorithms), were utilized to successfully track shear wave propagation through the mid-myocardial layer <italic>in vitro</italic> and <italic>in vivo</italic>. <italic>In vitro</italic> experiments focused on the investigation of a shear wave anisotropy through the myocardium. This experimentation suggests a moderate shear wave velocity anisotropy through regions of the mid-myocardial layer. <italic>In vivo</italic> experiments focused on shear wave anisotropy (which tend to corroborate the aforementioned <italic>in vitro</italic> results), temporal/spatial stability of shear wave velocity estimates, and estimation of wave velocity through the cardiac cycle. Shear wave velocity was found to cyclically vary through the cardiac cycle, with the largest estimates occurring during systole and the smallest occurring during diastole. This result suggests a cyclic stiffness variation of the myocardium through the cardiac cycle. A novel, on-axis technique, the displacement ratio rate (DRR) method, was developed and compared to conventional shear wave velocitmetry and ARFI imaging results; all three techniques suggest a similar cyclic stiffness variation.</p><p> Shear wave velocimetry shows promise in future investigations of myocardial elasticity. The DRR method may offer a means for transthoracic characterization of myocardial stiffness. Additionally, the future use of transesophageal and catheter-based transducers presents a way of generating and tracking shear waves in a clinical setting (i.e., when epicardial imaging is not feasible). Lastly, it is hoped that continued investigations into the physical basis of these ARFI-generated mechanical waves may further clarify the relationship between their velocity in myocardium and material stiffness.</p> / Dissertation

Turbulent combustion processes are inherently unsteady and, thus, a source of acoustic radiation, which occurs due to the unsteady expansion of reacting gases. While prior studies have extensively characterized the total sound power radiated by turbulent flames, their spectral characteristics are not well understood. The objective of this research work is to measure the flow and acoustic properties of an open turbulent premixed jet flame and explain the spectral trends of combustion noise. The flame dynamics were characterized using high speed chemiluminescence images of the flame. A model based on the solution of the wave equation with unsteady heat release as the source was developed and was used to relate the measured chemiluminescence fluctuations to its acoustic emission. Acoustic measurements were performed in an anechoic environment for several burner diameters, flow velocities, turbulence intensities, fuels, and equivalence ratios. The acoustic emissions are shown to be characterized by four parameters: peak frequency (Fpeak), low frequency slope (beta), high frequency slope (alpha) and Overall Sound Pressure Level (OASPL). The peak frequency (Fpeak) is characterized by a Strouhal number based on the mean velocity and a flame length. The transfer function between the acoustic spectrum and the spectrum of heat release fluctuations has an f^2 dependence at low frequencies, while it converged to a constant value at high frequencies. Furthermore, the OASPL was found to be characterized by (Fpeak mfH)^2, which resembles the source term in the wave equation.

This dissertation introduces a new approach to measure the mechanical properties of soft tissues. A laser-induced microbubble, created by focusing a single nanosecond laser pulse with a custom-made objective lens, was created at desired locations inside a tissue sample. An acoustic radiation force was generated by a low frequency transducer to displace the microbubble. A custom-built high pulse repetition frequency (PRF) ultrasound system, consisting of two 25 MHz single element transducers, was used to track the dynamics of the microbubble. Reconstruction of the mechanical properties at the specific location in a tissue sample was performed using a theoretical model, which calculated the dynamics of a microbubble under an externally applied force in a viscoelastic medium. The theoretical model and the high PRF ultrasound system were successfully validated in both gelatin phantoms and ex vivo bovine crystalline lenses. Age-related sclerosis of the crystalline lenses from bovine was clearly detected, which might be linked to changes in the crystalline. Location-dependent variation explained that the outer cortex and the inner nucleus had different mechanical properties. In the old and young porcine vitreous humors, age-related changes were not found. However, local variations of the mechanical properties were discovered, which may coincide with the different distributions of the molecular compositions. The laser-induced microbubble approach shows potential for future research into the origin of physiological phenomena and the development of inherent disorders in the eye. I hope that further studies – in the development of a more suitable theoretical model for the microbubble dynamics, in extension to in vivo applications, and in defining the relationship of the mechanical properties to molecular components in the eye – may provide a plan for the therapeutic treatment of eye-related diseases. / text

xi, 265 leaves : photos., diags ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1978

O objetivo deste trabalho é estudar o fenômeno da força de radiação acústica produzida por ondas progressivas e estacionárias. Neste trabalho o estudo da força produzida por ondas estacionárias é aplicado na análise de um levitador acústico e o estudo da força de radiação acústica por ondas progressivas é feito visando a futura construção de um separador acústico. Neste trabalho é utilizado o método dos elementos finitos para simular o comportamento de um levitador acústico. Primeiramente, é feita a simulação de um levitador acústico que consiste de um transdutor de Langevin com uma face de emissão plana que opera na freqüência de aproximadamente 20 kHz e um refletor plano. O método dos elementos finitos é utilizado para determinar o deslocamento da face do transdutor e o potencial acústico que atua numa esfera pequena. O deslocamento da face do transdutor obtido numericamente é comparado com o medido experimentalmente por um vibrômetro de fibra ótica e o potencial acústico determinado pelo método dos elementos é verificado experimentalmente colocando pequenas esferas de isopor no levitador. Depois de verificar o modelo numérico, o método dos elementos finitos é utilizado na otimização de um levitador acústico composto de um refletor côncavo e um transdutor com face de emissão côncava. Os resultados numéricos mostram que a força de radiação acústica no novo levitador é aumentada em 604 vezes quando comparada com o levitador composto de um transdutor com face plana e refletor plano. Este trabalho também apresenta um modelo numérico para determinar a trajetória de partículas esféricas na presença de uma onda de ultra-som progressiva. O modelo assume que as seguintes forças atuam na partícula: gravidade, empuxo, forças viscosas e força de radiação acústica devido a uma onda progressiva. Com o objetivo de não restringir o tamanho das partículas que podem ser utilizadas no modelo é empregada uma equação empírica do coeficiente de arrasto, válida para uma grande faixa de número de Reynolds. O modelo proposto requer a distribuição de pressão gerada pelo transdutor de ultra-som. A distribuição de pressão é medida experimentalmente utilizando um hidrofone calibrado. A verificação do modelo é feita soltando-se pequenas esferas de vidro (com diâmetros da ordem de 500 m) em frente a um transdutor de ultra-som de 1 MHz e 35 mm de diâmetro. / The objective of this work is to study the acoustic radiation force produced by progressive and standing waves. In this work, the studies related to the acoustic radiation force generated by ultrasonic standing waves are applied in the analysis of an acoustic levitator and the studies involving the acoustic radiation force generated by progressive waves are conducted aiming the design of acoustic separators. In this work, the finite element method is used to simulate an acoustic levitator. First, an acoustic levitator consisting of a 20 kHz Langevin ultrasonic transducer with a plane radiating surface and a plane reflector is simulated by the finite element method. The finite element method is used to determine the transducer face displacement and the acoustic radiation potential that acts on a small sphere. The numerical displacement is compared with that obtained by a fiber-optic vibration sensor and the acoustic radiation potential determined by the finite element method is verified experimentally by placing small Styrofoam spheres in the levitator. After verifying the numerical method, the finite element method was used to optimize an acoustic levitator consisting of a concave-faced transducer and a curved reflector. The numerical results show that the acoustic radiation force in the new levitator is enhanced 604 times compared with the levitator consisting of a plane transducer and a plane reflector. This work also presents a numerical model to determine the trajectory of sphere particles when submitted to ultrasonic progressive waves. This model assumes that the following forces act on the particle: gravity, buoyancy, viscous forces and acoustic radiation force due to the progressive wave. In order not to restrict the model to a small particle size range, the viscous forces that act on the sphere are modeled by an empirical relationship of drag coefficient that is valid for a wide range of Reynolds numbers. The numerical model requires the pressure field radiated by the ultrasonic transducer. The pressure field is obtained experimentally by using a calibrated needle hydrophone. The numerical model validation is done by dropping small glass spheres (on the order of 500 m diameter) in front of a 1-MHz 35-mm diameter ultrasonic transducer.

Orientador: José Roberto de França Arruda / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T12:19:39Z (GMT). No. of bitstreams: 1 Coser_LucasFernando_M.pdf: 3895849 bytes, checksum: 203a0d7be9958aaa628e2b404ad566a9 (MD5) Previous issue date: 2010 / Resumo: Duas abordagens para a simulação do campo sonoro produzido por um arranjo esférico de alto-falantes são apresentadas e comparadas a resultados experimentais. Na primeira, modos estruturais obtidos da analise modal experimental da membrana são usados em simulações vibroacusticas por elementos de contorno no software LMS Virtual.Lab®. Na segunda, adota-se uma solução analítica baseada na expansão dos harmônicos esféricos de um padrão de velocidades sobre uma calota esférica. Os resultados são apresentados em termos de potencia sonora e de padrões de diretividade para o arranjo. No primeiro caso, são observadas as mesmas tendências na faixa de baixa freqüência em todas as curvas analisadas, havendo distorções consideráveis na faixa de alta freqüência para a solução analítica devido ao fato desta não incluir os efeitos dos modos estruturais da membrana. Por outro lado, os padrões de diretividade demonstram um alto grau de similaridade em todos os casos analisados e não são fortemente afetados pelos modos estruturais da membrana. As diferenças observadas nos resultados e as amplificações não realistas nas curvas de potencia sonora das simulações são causadas por três fatores principais: modos de cavidade acústica do arranjo esférico, desconsideração do acoplamento acústico entre os alto-falantes durante seus funcionamentos e utilização de um mesmo conjunto de FRFs para todos os alto-falantes. De uma forma geral, pode-se dizer que a simulação usando o modelo de membrana flexível melhora consideravelmente a previsão da potencia sonora na alta freqüência, o que não pode ser obtido com o modelo analítico comumente usado na analise de arranjos esféricos de alto-falantes / Abstract: Two approaches for sound field prediction of a spherical loudspeaker array operation are presented and compared to experimental measurements. In the first, real membrane modes from experimental modal analysis are used as input for BEM vibroacoustic simulations using LMS Virtual.Lab® software. In the second, an analytical solution based on the spherical harmonic expansion of an idealized velocity pattern over the spherical array is used. Results are presented in terms of sound power and directivity patterns, showing that the former has the same trend in all comparisons for the low frequency range, and that the analytical solution cannot be used for the high frequency range since it does not include the effect of the flexible membrane modes. Directivity patterns, however, show a good degree of similarity in all cases, and are not strongly affected by the flexible membrane modes. The differences found in the results and the unrealistic amplifications in the sound power curves from the simulations are caused mainly by three factors: acoustic cavity modes of the array, neglecting the acoustic coupling among loudspeakers for the operating condition and utilization of the same set of FRFs for all loudspeakers in the array. In a general way, it can be said that the flexible membrane modeling improves considerably the prediction of radiated sound power in the high frequency range, which cannot be obtained by the analytical model commonly used in the analysis of spherical loudspeaker arrays / Mestrado / Mecanica dos Sólidos e Projeto Mecanico / Mestre em Engenharia Mecânica

In this paper a psychoacoustic sound field synthesis (pSFS) system for musical applications is presented. It recreates the radiation patterns of musical instruments for an extended listening area. For this purpose several psychoacoustic effects are utilized: Considering the critical bandwidth, the amount of data to be processed can be reduced massively. Applying a “precedence fade” allows for wave fronts to arrive from angles opposing to the virtual source position when regarding the integration time of the auditory system and phenomenons known from the field of auditory scene analysis. A listening test demonstrates that the approach results in a precise source localization together with a spatial sound impression which is known to be the most crucial aspect of the subjective acoustical quality of concert halls. This is achieved even in a reverberant room and with a relatively large distance between loudspeakers compared to other wave field synthesis systems. Implementations in audio systems such as 5.1 or wave field synthesis systems with a large number of loudspeakers are possible.