Acoustical engineering

A Motion Compensated (MC) Ultra Short Baseline (USBL) Acoustic Positioning
System (APS) operable in shallow water and port environment has been implemented at
Florida Atlantic University. Multi-tones signal modulation and log-likelihood
maximization enable this APS to operate in volumes of water of less than 10 cubic
meters. Standard deviations of the acoustic source elevation and azimuth estimates were
computed to be 3 degrees in an 8 cubic meters test tank, and reduce to 0.9 degree in a 2
meters deep marina. The motion compensating system estimates the array position and
orientation while merging noisy measurements from a Magnetic, Angular Rate, and
Gravity (MARG) sensor and a Differential Global Positioning System (DGPS) using
Kalman filters. Experiments show 0.67 and 2.67 degrees of error for the array tilt and
heading estimates, and 0.74 meter for the array position estimate.

Over recent years, the trend in Autonomous Underwater Vehicle (AUV) design has been to reduce vehicle size and cost. On board navigation systems are both large and expensive so alternate solutions for vehicle positioning are required. The thesis explores the performance of a passive platform, the Ambient Noise Sonar (ANS), in remotely detecting, localizing and tracking submersible vessels. This task is achieved by exploiting communication signatures emitted by the moving submersible. The utility modem integrated on the AUV can be operated in a PSK and a MFSK mode. It was demonstrated that the ANS successfully tracks AUVs in both cases. First, the thesis presents the sonar beamformer and shows its potential for tracking by using the AUV communication signals. It describes a scheme developed to enhance the processor performance in a multi-target configuration and clutter. Then, it discusses promising tracking results from experiments conducted in summer and fall 1998, off the coast of South Florida.

Having the ability to dock an Autonomous Underwater Vehicle (AUV) can significantly enhance the operation of such vehicles. In order to dock an AUV, the vehicle's position must be known precisely and a guidance algorithm must be used to drive the AUV to its dock. This thesis will examine and implement a low cost acoustic positioning system to meet the positioning requirements. At-sea tests will be used as a method of verifying the systems specifications and proper incorporation into the AUV. Analyses will be run on the results using several methods of interpreting the data. The second portion of this thesis will develop and test a fuzzy logic docking algorithm which will guide the AUV from a location within the range of the sonar system to the docking station. A six degree of freedom simulation incorporating the Ocean Explorer's hydrodynamic coefficients will be used for the simulation.

Laboratory scale measurements were conducted to validate numerical prediction models used to predict the acoustic field in a shallow water ocean environment. Experimental measurements were conducted in a range independent environment which included the effects of shear in marine sediments and in a depth varying range dependent environment. Good agreement between the experimental measurements and the numerical prediction codes were obtained using optimized values for the input parameters of the environmental model. In comparing experimental measurements to the numerical prediction codes it became apparent that the codes were very sensitive to the input parameters describing the bottom boundary of the ocean waveguide.

The interaction between vibrating structures and fluids can have a profound influence upon the natural frequencies of the structure's vibration. This study examines one specific structure; a thin circular plate with the rarely studied free edge condition. It starts by considering a completely free plate in a vacuum and then, using receptance matching, utilises this result to determine the effects, on the natural frequencies, of a centrally located driving rod. Then, using the same technique, a result for the drive admittance of the fluid loaded plate is adapted to predict the natural frequencies of the same structure when subjected to significant fluid loading. All these results are then compared to those obtained from experiments.

The human speech production system is reviewed through
general acoustic theory. Based upon that, the
characteristics of helium speech is compared to normal
speech. The Linear Prediction algorithm is derived
for computer implementation by recursive formulas.
The correction factors for the vocal tract area
functions are found from simulated helium speech and
normal speech data for four vowels. By the correction
factors, new corrected area functions are applied to
the Linear Prediction algorithm so that new synthesis
filters can be built. The output of the algorithm is
enhanced helium speech.

The operation of unmanned underwater vehicles requires communications with other nearby vehicles as well as accurate positioning to prevent duplication of work, collisions and other mishaps. This thesis details the integration of an ultra-short baseline positioning system with four transducers arranged as a tetrahedron for use with the FAU Dual Purpose Acoustic Modem. The source position is estimated by processing coherently a series of frequency-hopped pulses to obtain a set of bearings, optimally combined through maximum likelihood estimation of the azimuth and elevation. A simulation has been implemented and experiments have been performed in a calibration tank. Model and experiments confirm that the accuracy of this system improves with the number of pulses and the signal-to-noise ratio. A mean positional error of 5.51% can be obtained with an SNR of 20 dB and a single processed pulse, the error decreases to 2.84% using six processed pulses.

The vibrational behavior of inhomogeneous beams and circular plates is studied, utilizing the semi-inverse method developed by I. Elishakoff and extensively discussed in his recent monograph (2005). The main thread of his methodology is that the knowledge of the mode shape is postulated. The candidate mode shapes can be adopted from relevant static, dynamic or buckling problems. In this study, the exact mode shapes are sought as polynomial functions, in the context of vibration tailoring, i.e. designing the structure that possesses the pre-specified value. Apparently for the first time in the literature, several closed-form solutions for vibration tailoring have been derived for vibrating inhomogeneous beams and circular plates. Twelve new closed-form solutions for vibration tailoring have been derived for an inhomogeneous polar orthotropic plate that is either clamped or simply supported around its circumference. Also, the vibration tailoring of a polar orthotropic circular plate with translational spring is analyzed. There is considerable potential of utilizing the developed method for design of functionally graded materials.

This thesis expands upon an existing noise cleansing technique, polishing, enabling it to be used in the Software Quality Prediction domain, as well as any other domain where the data contains continuous values, as opposed to categorical data for which the technique was originally designed. The procedure is applied to a real world dataset with real (as opposed to injected) noise as determined by an expert in the domain. This, in combination with expert assessment of the changes made to the data, provides not only a more realistic dataset than one in which the noise (or even the entire dataset) is artificial, but also a better understanding of whether the procedure is successful in cleansing the data. Lastly, this thesis provides a more in-depth view of the process than previously available, in that it gives results for different parameters and classifier building techniques. This allows the reader to gain a better understanding of the significance of both model generation and parameter selection.

In shallow water or fluid half-space, the acoustic scattering from a target is significantly different from that of an unbounded medium, due to the multiple reflections occurring between the target and the boundaries. The purpose of this thesis is to investigate the influence of the boundaries on the acoustic scattering of a rigid sphere by means of a superposition method. A minimum number of point sources necessary to accurately model the scattered field is determined in the case of a free medium, a fluid half-space and a waveguide. The free field symmetry vanishes due to the presence of boundaries and, at particular frequencies or scatterer depths, a significant change in the magnitude and spatial distribution of the scattered field occur. In an unbounded medium or fluid half space, the superposition method is shown to give similar results to analytical formulations found in the literature, provided enough point sources are used.