High order finite difference schemes are generally less dispersive, less dissipative and more isotropic than low order schemes. They are, therefore, better suited for the solution of wave propagation problems. High order schemes, however, support spurious numerical waves which have no relationship to the waves of the original partial differential equations. The large stencils associated with the high order schemes also make the implementation of boundary conditions more difficult. A number of fundamental difficulties which occur when high order finite difference schemes are used to solve computational aeroacoustics and flow problems are investigated and resolved. The research work includes: (a) Development of an artificial selective damping technique for the elimination of spurious numerical waves; (b) Formation of a set of solid wall boundary conditions for high order finite difference schemes; (c) Design of a family of multi-domain multiple-time-step high order finite difference algorithms for the solution of acoustics and flow problems with large disparate length scales. A sequence of direct numerical simulations are performed to demonstrate the effectiveness of all the proposed methods. / Source: Dissertation Abstracts International, Volume: 55-07, Section: B, page: 2768. / Major Professor: Christopher K. W. Tam. / Thesis (Ph.D.)--The Florida State University, 1994.

There is now a substantial body of theoretical and experimental evidence that the dominant part of the turbulent mixing noise of supersonic jets is generated directly by the large turbulence structures/instability waves of the jet flow. The relationship between the instability waves and noise of hot jets at moderate supersonic Mach number is examined in Chapters 1 and 2. It is found that the highest sound-pressure-level of the far-field noise occurs at a direction and frequency that closely match the Mach wave radiation direction and frequency of the most amplified instability wave of the jet. The calculations show that for jet Mach number up to 2.0 and jet total temperature to ambient temperature ratio up to 2.5, the Kelvin-Helmholtz instability waves always grow to a higher amplitude than the supersonic instability wave. Numerical results indicate that for hot jets the most amplified wave invariably belongs to the helical mode Kelvin-Helmholtz instability wave. For lower speed hot jets with jet static temperature higher than or equal to the ambient temperature there is also a fair correlation between the Strouhal number at the peak sound-pressure level of the far-field noise and that of the most amplified instability wave. / In Chapters 3, 4, 5 and 6, a broadband jet noise theory is constructed. In this theory, the compressible flow equations with eddy viscosity are used to calculate the wave propagation characteristics of the instability waves. These equations are solved by the method of matched asymptotic expansions. The inner solution is the instability wave solution. The outer solution gives the associated acoustic field. The amplitudes of the instability waves are assumed to be stochastic random functions. The statistical properties of the random amplitude function are determined by the requirement that the wave spectrum at the nozzle exit has no intrinsic length and time scales. The present theory can predict the dominant part of jet mixing noise from first principles up to a single multiplicative constant. The spectra and directivities of a Mach 2 jet at total temperatures of 500K and 1114K are calculated. The numerical results agree favorably with experimental measurements. / Source: Dissertation Abstracts International, Volume: 54-12, Section: B, page: 6317. / Major Professor: Christopher K. W. Tam. / Thesis (Ph.D.)--The Florida State University, 1993.

Control of aerodynamic noise generated by high-performance jet engines continues to remain a serious problem for the aviation community. Intense low frequency noise produced by large-scale coherent structures is known to dominate acoustic radiation in the aft angles. A tremendous amount of research effort has been dedicated towards the investigation of many passive and active flow control strategies to attenuate jet noise, while keeping performance penalties to a minimum. Unsteady excitation, an active control technique, seeks to modify acoustic sources in the jet by leveraging the naturally-occurring flow instabilities in the shear layer. While excitation at a lower range of frequencies that scale with the dynamics of large-scale structures, has been attempted by a number of studies, effects at higher excitation frequencies remain severely unexplored. One of the major limitations stems from the lack of appropriate flow control devices that have sufficient dynamic response and/or control authority to be useful in turbulent flows, especially at higher speeds. To this end, the current study seeks to fulfill two main objectives. First, the design and characterization of two high-frequency fluidic actuators ($25$ and $60$ kHz) are undertaken, where the target frequencies are guided by the dynamics of high-speed free jets. Second, the influence of high-frequency forcing on the aeroacoustics of high-speed jets is explored in some detail by implementing the nominally 25 kHz actuator on a Mach 0.9 ($Re_D = 5\times10^5$) free jet flow field. Subsequently, these findings are directly compared to the results of steady microjet injection experiments performed in the same rig and to prior jet noise control studies, where available. Finally, limited acoustic measurements were also performed by implementing the nominally 25 kHz actuators on jets at higher Mach numbers, including shock containing jets, and elevated temperatures. Using lumped element modeling as an initial guide, the current work expands on the previous development of low-frequency (2-8 kHz) Resonance Enhanced Micro-actuators (REM) to design actuators that are capable of producing high amplitude pulses at much higher frequencies. Extensive benchtop characterization, using acoustic measurements as well as optical diagnostics using a high resolution micro-schlieren setup, is employed to characterize the flow properties and dynamic response of these actuators. The actuators produced high-amplitude output a range of frequencies, $20.3-27.8$ kHz and $54.8-78.2$ kHz, respectively. In addition to providing information on the actuator flow physics and performances at various operating conditions, the benchtop study serves to develop relatively easy-to-integrate, high-frequency actuators for active control of high-speed jets for noise reduction. Following actuator characterization studies, the nominally 25 kHz ($St_{DF} \approx 2.2$) actuators are implemented on a Mach 0.9 free jet flow field. Eight actuators are azimuthally distributed at the nozzle exit to excite the initial shear layer at frequencies that are approximately an order of magnitude higher compared to the \textit{jet preferred frequency}, $St_P \approx 0.2-0.3$. The influence of control on the mean and turbulent characteristics of the jet, especially the developing shear layer, is examined in great detail using planar and stereoscopic Particle Image Velocimetry (PIV). Examination of cross-stream velocity profiles revealed that actuation leads to strong, spatially coherent streamwise vortex pairs which in turn significantly modify the mean flow field, resulting in a prominently undulated shear layer. These vortices grow as they convect downstream, enhancing local entrainment and significantly thickening the initial shear layer. Azimuthal inhomogeneity introduced in the jet shear layer is also evident in the simultaneous redistribution and reduction of peak turbulent fluctuations in the cross-plane near the nozzle exit. Further downstream, control results in a global suppression of turbulence intensities for all axial locations, also evidenced by a longer potential core and overall reduced jet spreading. The resulting impact on the noise signature is estimated via far-field acoustic measurements. Noise reduction was observed at low to moderate frequencies for all observation angles. Direct comparison of these results with that of steady microjet injection revealed some notable differences in the initial development of streamwise vorticity and the redistribution of peak turbulence in the azimuthal direction. However, despite significant differences in the near nozzle aerodynamics, the downstream evolution of the jet appeared to approach near similar conditions with both high-frequency and steady microjet injection. Moreover, the impact on far-field noise was also comparable between the two injection methods as well as with others reported in the literature. Finally, for jets at higher Mach numbers and elevated temperatures, the effect of control was observed to vary with jet conditions. While the impact of the two control mechanisms were fairly comparable on non-shock containing jets, high-frequency forcing was observed to produce significantly larger reductions in screech and broadband shock-associated noise (BBSN) at select under-expanded jet conditions. The observed variations in control effects at different jet conditions call for further investigation. / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / September 19, 2017. / Active Flow Control, Actuators, High-frequency excitation, High-speed Jets, Jet Noise Control, Particle Image Velocimetry / Includes bibliographical references. / Farrukh Alvi, Professor Directing Dissertation; M. Yousu Hussaini, University Representative; Rajan Kumar, Committee Member; Jonathan Clark, Committee Member; Jonas P. R. Gustavsson, Committee Member.

by Wong Wan-shing. / Bibliography: leaves 94-95 / Thesis (M.Ph.)--Chinese University of Hong Kong, 1984

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / In previous works, baffled-piston with small ratio of piston radius a over the baffle wall aperture radius b, or say, small gap was discussed on subjects mainly focused on acoustics gains. In this thesis, we focus more on baffled-piston models with large gaps and find out what could be the difference these new models may have on the acoustic gains. First we introduce a general description about the original models and further develop it into cavity-backed models with either closed or open end on one side. We use finite difference approximation to evaluate the influential parameters L and lE on acoustic gain. Afterwards calculate and plot curves for gains in related with piston motion frequency f for closed and open cavity models with different configuration parameters with are a/b, b/L and ζ0/b and analyse them and compare with previous works. / 2031-01-01

The original work presented in this thesis pertains to the design and characterisation of resonant-cavity-based acoustic metamaterials, with a focus on airborne sound. There are five separate experimental chapters, each with a unique approach to the design of periodic structures that can support and manipulate air-bound acoustic surface waves via diffractive coupling between resonant-cavities. The first two chapters concern measurement of the acoustic transmission though various kinds of periodic slit-arrays, whilst the latter three chapters utilise a near-field imaging technique to directly record and characterise the dispersion of trapped acoustic surface waves. The first experimental chapter investigates the effect that thermodynamic boundary layers have on the Fabry-Perot-like cavity resonances that are so often utilised in acoustic metamaterial design. At audio frequencies, these boundary layers have a decay length that is typically more than two orders of magnitude smaller than the width of the resonating slit-cavities, hence it may naively be assumed that their effect can be ignored. However, by studying in detail the effect that reducing slit-cavity width has on the frequency of the measured cavity-resonance, for both a single slit cavity and a slit-cavity array, it is found that these boundary layer effects become significant on a far larger scale than their characteristic thickness. This is manifested in the form of a reduction in the resonant frequency as the slit-width is narrowed. Significant attenuation of the resonance and a 5% reduction in the effective speed of sound through the cavity is measured when the boundary layers form only 5% of the total width of each slit. Hence, it is both shown that the prevalent loss free treatment of acoustic slit-cavities is unrealistic, and that one may control the effective speed of sound through the slit-cavities with a simple change in slit-width. The second chapter explores the effect of ‘compound’ grating structure on trapped acoustic surface waves, a compound grating having a basis comprised of more than one resonating element. The angle dependent acoustic transmission spectra of four types of aluminium slit-array are recorded, and for the compound gratings, it is found that sharp dips appear in the spectra that result from the excitation of a ‘phase-resonance’. This occurs as new degrees-of-freedom available to the acoustic near-field allow the fields of adjacent cavities within a unit-cell to be both out-of-phase and strongly enhanced. By mapping the transmission spectra as a function of in-plane wavevector, the dispersions of the modes supported by each sample are determined. Hence, the origin of the phase-resonant features may be described as acoustic surface waves that have been band-folded back into the radiative regime via diffraction from higher in-plane wavevectors than possible on a simple grating. One of the samples is then optimised via numerical methods that account for thermodynamic boundary layer attenuation, resulting in the excitation of a sharp, deep transmission minimum in a broad maximum that may be useful in the design of an acoustic filter. The third chapter introduces the near-field imaging technique that can be utilised to directly characterise acoustic surface waves, via spatial fast Fourier transform algorithms of high-resolution pressure field maps. The acoustic response of a square-lattice open-ended hole array is thus characterised. It is found that over a narrow frequency band, the lattice symmetry causes the acoustic surface power flow to be channelled into specific, predictable directions, forming ‘beams’ with a well defined width. In chapter four, the existence of the ‘acoustic line mode’ is demonstrated, a type of acoustic surface wave that may be supported by a simple line of open-ended hole cavities. The near-field imagine technique is again used to extract the mode dispersion. This acoustic line mode may be readily manipulated, demonstrated by arrangement of the line of holes into the shape of a ring. The existence of this type of mode offers a great deal of potential for the control of acoustic energy. Chapter five explores the effect of ‘glide-symmetry’ on a pair of acoustic line modes arranged side-by-side. A control sample not possessing glide- symmetry is first characterised, where measurement of the acoustic near- fields show that this sample supports two separate modes at different frequencies, with their phase either symmetric or anti-symmetric about the mirror plane between the lines of holes. One of these lines is then shifted along its periodicity by half of a grating pitch, thus creating glide-symmetry. The resulting sample is found to support a single hybrid mode, capable of reaching a much larger in-plane wavevector than possible on a simple grating with no gaps in its band-structure, and displaying a region of negative dispersion. The third sample demonstrates how one may increase the coupling strength between the two lines of holes via manipulation of the cavity shape, thus enhancing the glide-symmetry effect. The thesis concludes with preliminary investigations into other possible ways of manipulating acoustic surface waves, such as with the use of ‘screw-symmetry’.

<p>The astronauts? ability to communicate easily among themselves or with the ship?s computer should be a high priority for the success of missions. Long-duration space habitats--whether spaceships or surface bases--will likely be larger than present-day Earth-to-orbit/Moon transfer ships. Hence an efficient approach would be to free the crew members from the relative burden of having to wear headsets throughout the spacecraft. This can be achieved by placing microphone arrays in all crew-accessible parts of the habitat. Processing algorithms would first localize the speaker and then perform speech enhancement. The background "noise" in a spacecraft is typically fan and duct noise (hum, drone), valve opening/closing (click, hiss), pumps, etc. We simulate such interfering sources by a number of loudspeakers broadcasting various sounds: real ISS sounds, a continuous radio stream, and a poem read by one author. To test the concept, we use a linear 30-microphone array driven by a zero-latency professional audio interface. Speaker localization is obtained by time-domain processing. To enhance the speech-to-noise ratio, a frequency-domain minimum-variance approach is used.

Yes. While the results presented in this thesis declare that listeners prefer the recording of an acoustic instrument, there is still a positive response about virtual violins, especially when compared to an unfiltered electric instrument. In this context a virtual violin is the result of convolving the raw signal from an electric instrument with the impulse from a real violin. A key part of this process is the characterisation of this impulse response, of which there are several methods. This thesis explores the use of virtual violins in acoustics research and music performance. The opening chapters provide an overview of the literature about violin acoustics and previous uses of virtual violins. A significant portion of the thesis details the development of a system that is used to produce digital characterisations of violins. The method used involved the measurement of sound radiation from the violin body after it had been excited by an impulse. This impulse is provided by an instrumented hammer, which strikes the violin on the bridge. One of the pitfalls of this method is the imperfect frequency response of the strike, which is corrected using a deconvolution algorithm. Deconvolution is an important part of the process and is discussed at length in the thesis. The described characterisation system utilised a bespoke frame that could hold the violin by the neck and rotate it to a specific angle on a single plane. This enabled the characterisation of a violin at incremental angles. Not only can these characterisations be used for the development of virtual violins, but they can also be used to analyse the spectral properties of the instrument. These characterisations at different angles allowed an examination of the violin's directivity, which explored how the sound energy is distributed from the instrument. Analysis determined that in the mid-range (C4-B5) the sound distribution is fairly isotropic. Outside of these ranges the sound distribution is significantly anisotropic. Finally, the thesis details three psychoacoustic experiments where participants listened to different audio samples and rated them according to their personal preference. The first of these had five virtual violins, of various manufacturers and ages, along with an unfiltered electric instrument. Listeners of mixed musical ability were asked to listen to recorded samples of these instruments and to provide preference scores for each. The experiment found that listeners preferred the sound of virtual violins to the unfiltered electric. The next experiment presented musically trained participants with samples of a virtual violin with varying impulse response lengths. This revealed that there was a sigmoid shaped relationship between the number of coefficients of the violin's impulse response and the listener's preference score. The final experiment was designed to examine the preference scores towards a recording of a real violin and its emulated equivalent. Participants of two groups, one with musical training and one without, took part in the study. The results showed that the real violin was preferred to the emulated violin and that musical training did not have any effect on the preference scores. The statistical analysis also determined that there was no interaction between violin treatment and musical training. An additional question was also posed to the participants post-experiment: Which of the presented samples is the real instrument? The results showed that those with musical training were significantly better at identifying the real instrument compared to those without such training. Speculation is drawn as to why this might be, though it is speculated that the spatial effects, such as those described by Weinreich, had no effect.

Advances in computational power and expanded access to computing clusters has made mathematical modeling of complex wave effects possible. We have used multi-core and cluster computing to implement analytical and numerical models of ultrasonic wave scattering in fluid and solid media (acoustic and elastic waves). We begin by implementing complicated analytical equations that describe the force upon spheres immersed in inviscid and viscous fluids due to an incident plane wave. Two real-world applications of acoustic force upon spheres are investigated using the mathematical formulations: emboli removal from cardiopulmonary bypass circuits using traveling waves and the micromanipulation of algal cells with standing waves to aid in biomass processing for algae biofuels. We then move on to consider wave scattering situations where analytical models do not exist: scattering of acoustic waves from multiple scatterers in fluids and Lamb wave scattering in solids. We use a numerical method called finite integration technique (FIT) to simulate wave behavior in three dimensions. The 3D simulations provide insight into experimental results for situations where 2D simulations would not be sufficient. The diverse set of scattering situations explored in this work show the broad applicability of the underlying principles and the computational tools that we have developed. Overall, our work shows that the movement towards better availability of large computational resources is opening up new ways to investigate complicated physics phenomena.

In order to perform useful tasks for us, robots must have the ability to notice, recognize, and respond to objects and events in their environment. This requires the acquisition and synthesis of information from a variety of sensors. Here we investigate the performance of a number of sensor modalities in an unstructured outdoor environment, including the Microsoft Kinect, thermal infrared camera, and coffee can radar. Special attention is given to acoustic echolocation measurements of approaching vehicles, where an acoustic parametric array propagates an audible signal to the oncoming target and the Kinect microphone array records the reflected backscattered signal. Although useful information about the target is hidden inside the noisy time domain measurements, the Dynamic Wavelet Fingerprint process (DWFP) is used to create a time-frequency representation of the data. A small-dimensional feature vector is created for each measurement using an intelligent feature selection process for use in statistical pattern classification routines. Using our experimentally measured data from real vehicles at 50 m, this process is able to correctly classify vehicles into one of five classes with 94% accuracy. Fully three-dimensional simulations allow us to study the nonlinear beam propagation and interaction with real-world targets to improve classification results.