Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3843. Adviser: Kenneth S. Suslick. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Phononic crystals and acoustic metamaterials are heterogeneous materials that enable manipulation of elastic waves. An important characteristic of these heterogeneous systems is their ability to tailor the propagation of elastic waves due to the existence of band gaps -- frequency ranges of strong wave attenuation. In this Thesis, I report discoveries of three new types of band gaps: i) Band gaps induced by geometric frustration in periodic acoustic channel networks; ii) Band gap induced by high connectivity in periodic beam lattices; and iii) Topological band gaps in gyroscopic phononic crystals that protects one-way edge waves. The investigations presented here shed new light on the rich dynamic properties of phononic crystals and acoustic metamaterials, opening avenues for new strategies to control mechanical waves in elastic systems. / Engineering and Applied Sciences - Applied Math

The treatment of scattering from submerged objects in an unbounded environment is of considerable interest to both the academic and technological communities. Several approaches have yielded results for different classes of problems and have proven manageable for the free environment case. The problem of scattering in a confined environment has proven more difficult to formulate in a form useful for calculation due to the coupling of effects from the scattered object with that of the boundaries. The purpose of this work is to propose a numerical scheme that will adequately describe scattering from realistic objects in a confined environment. Some of the realistic objects that are of interest are elongated objects such as spheroids and cylinders with rounded end-caps. Boundary conditions of interest range from those associated with rigid objects to those associated with elastic shells. We will start by developing the incident field in terms of Normal Mode theory. We next develop the near field scattered field. This will be obtained using a transition matrix that relates the incident field to the scattered field. The transition matrix which is obtained from the Extended Boundary Condition (EBC) method of Waterman does not account for boundaries other than that of the object. We then couple the solution with a waveguide solution to satisfy all boundary conditions. By appropriately coupling the two solutions we can satisfy all boundary conditions and preserve continuity of the solution throughout all space. The method that couples the solutions is an application of Huygens' principle. This application of Huygens' principle leads to a manageable direct solution of the problem. This method also satisfies all appropriate boundary conditions and yields a continuous solution throughout space. Details of this method will be presented along with examples / acase@tulane.edu

Detection of buried objects with parametric sonar is studied using time-frequency methods. The sonar equation is used as a framework to assess the sonar system characteristics, the sediment response, and the buried object response. Field tests were conducted to measure the source levels of difference frequencies in the nearfield and spectral components at different ranges. The results of these tests were compared with the predicted values from the Moffett-Mellen model in the nearfield. The analysis reveals the nonstationary and nonlinear behavior of the sonar system and environment and shows that a linear systems approach is not realistic. Analysis of sediment variability and attenuation quantifies the nonstationarity of the media and motivates the use of gain correction techniques presented in this dissertation. The response of buried objects was found to exhibit resonances and time-frequency dependence. Acoustic resonance theory is used to explain the target response and is shown to be effective in estimating target size. The use of time-frequency techniques for object detection in such a system is discussed and it is shown that the Wigner-Ville distribution and the wavelet transform are particularly well suited to the interpretation of the data and detection of buried objects. Results are given for several buried objects and gas bubbles in sediment / acase@tulane.edu

<p>Reverberation and clutter are two of the principle obstacles to active sonar target detection in shallow water. Diffuse seabed backscatter can obscure low energy target returns, while clutter discretes, specific features of the sea floor, produce temporally compact returns which may be mistaken for targets of interest. Detecting weak targets in the presence of reverberation and discriminating water column targets from bottom clutter are thus critical to good performance in active sonar. Both problems are addressed in this thesis using the time-frequency interference pattern described by a constant known as the waveguide invariant which summarizes in a scalar parameter the dispersive properties of the ocean environment. </p><p>Conventional active sonar detection involves constant false alarm rate (CFAR) normalization of the reverberation return which does not account for the frequency-selective fading in a wideband pulse caused by multipath propagation. An alternative to conventional reverberation estimation is presented, motivated by striations observed in time-frequency analysis of active sonar data. A mathematical model for these reverberation striations is derived using waveguide invariant theory. This model is then used to motivate waveguide invariant reverberation estimation which involves averaging the time-frequency spectrum along these striations. An evaluation of this reverberation estimate using real Mediterranean data is given and its use in a generalized likelihood ratio test (GLRT) based CFAR detector is demonstrated. CFAR detection using waveguide invariant reverberation estimates is shown to outperform conventional cell-averaged and frequency-invariant CFAR detection methods in shallow water environments producing strong reverberation returns which exhibit the described striations. Results are presented on simulated and real Mediterranean data from the SCARAB98 experiment. </p><p>The ability to discriminate between water column targets and clutter discretes is vital to maintaining low false alarm rates in active sonar. Moreover, because of the non-stationarity of the active sonar return, classification is most typically achieved using a single snapshot of test data. As an aid to classification, the waveguide invariant property is used to derive multiple snapshots by uniformly sub-sampling the short-time Fourier transform (STFT) coefficients of a single ping of wideband active sonar data. The sub-sampled target snapshots are used to define a waveguide-invariant spectral density matrix (WI-SDM) which allows the application of adaptive matched-filtering based approaches for target depth classification. Depth classification is performed by a waveguide-invariant minimum variance filter (WI-MVF) which matches the observed WI-SDM to depth-dependent signal replica vectors generated from a normal mode model. Robustness to environmental mismatch is achieved by adding environmental perturbation constraints (EPC) and averaging the signal replica vectors over the unknown channel parameters. Simulation and real data results from the SCARAB98, CLUTTER07, and CLUTTER09 experiments in the Mediterranean Sea are presented to illustrate the approach.</p> / Dissertation

<p> Every year, many thousands of people worldwide lose the ability to speak due to receiving a laryngectomy, typically for treatment of cancer. At some point in their recovery, most will use an electrolarynx to recover their ability to speak. Typical electrolarynxes utilize a piston to strike a disc pressed to the patient's neck which delivers a pressure wave into the soft tissue. This pressure wave mechanically couples with the vocal tract and generates the fundamental frequency necessary for creating vowels without which speech is not possible. </p><p> Commonly available electrolarynxes suffer from poor frequency control due to the nonlinear character of their impulse driver. They also create a great deal of "self-noise" which is distracting to listeners and makes using voice communication systems difficult. </p><p> We propose a novel electrolarynx implementation which utilizes two interfering ultrasonic waves to generate a fundamental frequency in the vocal tract required for speech restoration. The device is light weight, compact, inexpensive, and offers excellent control of all aspects of the output waveform. In addition, as the primary waveforms are above human hearing, there is little "self-noise" that can be heard by listeners and most communications devices filter such noise as part of their standard digitization process. </p><p> This device offers the potential to greatly improve the lives of those who have lost their voices and must rely on technology to allow them to communicate in the most efficient manner.</p>

Nondestructive testing (NDT) using ultrasound band 1-5 MHz, has been widely used for the early-stage detection of structural failure; however, it fails to detectf material degradation, fatigue, and microcracks. NDT with nonlinear ultrasound (NLU) can detect a microscopic discontinuity or imperfection that may be a source of the second harmonic in the reflected signal. In this research, we focus on creating a metamaterial band filter that filters out nonlinearities induced by the instrument itself. A 1D elastic superlattice (SL) acoustic filter is designed with a bandgap in its frequency spectrum that covers the frequency range of second harmonic. The SL is made of periodically alternating Cu and Sn-Pb solder layers. We conducted analytical and numerical calculations to obtain the appropriate thickness of each layer. The metamaterial in this study has the pass band for the fundamental frequency of 5 MHz and the first stop band centered near the frequency of 10 MHz; 5 MHz was chosen because the second harmonic at 10 MHz can detect 200μm micro-scale damage. Experiments with aluminum as the reference specimen and with SL filter were conducted. A function-generator generates 3 pulses sine signal, within the frequency range from 2.5 MHz to 20MHz. Spectral analysis of the signal through the SL filter shows 100 times voltage suppression of the second harmonic as compared to the signal transmitted through the Al specimen. By filtering out the device's inherent nonlinearity with the SL ultrasonic filter, one can detect microcracks, fatigue and material degradation with much higher accuracy.

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The LOST2 system is a new accurate underwater positioning system that nonlinearly combines parts of dead-reckoning, acoustic-based positioning, and terrain-based positioning. The system is composed of two major subsystems, a system observer and a constrained extended Kalman filter. Inputs to the system are as follows: (1) high resolution bathymetry, (2) measured ocean depth at the position of the vessel, (3) measured or estimated vessel velocity, (4) slant range to and position of a known point, and (5) an initial prediction of the vessel's location. The system development, simulation studies, results from sea trials and some suggestions for future work are presented. The system is capable of providing position estimates with the same degree of accuracy as present methods, with significantly less hardware. These results prove the concept of the system as a new method to position underwater vessels / acase@tulane.edu