A compact low-frequency projector is of crucial importance especially in underwater acoustics due to the frequency dependence of the absorption. To improve the efficiency of an omnidirectional acoustic source at low frequencies, parametric amplification may be used by adding a thin layer of nonlinear medium around a spherical transducer. The parametric effect is based on the interaction of two acoustic waves propagating through a nonlinear medium to produce a difference frequency wave. If both primary frequencies are sufficiently close enough, the result is the creation of a low frequency wave. Investigation is required to find the optimal medium, that is to say, one with a large nonlinear coefficient and a low sound speed. Such a source has already been built using a medium composed of a gel and microsphere mixture. In this case, the nonlinear coefficient is highly pressure dependent reaching a maximum when the microspheres buckle. The need is to optimize the material layer to increase the range of hydrostatic pressures over which the projector is useful.

Thesis (M.Sc.)--Memorial University of Newfoundland, 2000. / Bibliography: leaves 160-165.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, September 1990. / Thesis Advisor(s): Miller, James H. Second Reader: Tummala, Murali. "September 1990." Description based on title screen as viewed on December 17, 2009. DTIC Identifier(s): Acoustic tomography, inverse problems, Fast Hadamard Transforms, theses. Author(s) subject terms: Acoustic tomography, Fast Hadamard Transform, maximal-length sequences, Doppler processing. Includes bibliographical references (p. 95-96). Also available in print.

Thesis (M.S. in Computer Science)--Naval Postgraduate School, September 2003. / Thesis advisor(s): Geoffrey Xie, John Gibson. Includes bibliographical references (p. 65-66). Also available online.

Thesis (Ph. D.)--University of Hong Kong, 1968. / Mimeographed.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Rogers, P. H.; Committee Member: Ferri, A. A.; Committee Member: Ruzzene, M.; Committee Member: Smith, M. K.; Committee Member: Trivett, D.; Committee Member: Zinn, B. T. Part of the SMARTech Electronic Thesis and Dissertation Collection.

This thesis describes the development and testing of the Combustive Sound Source (CSS), which is a broadband underwater sound source. The CSS is being developed as a clean, safe, and cost effective replacement to underwater explosive charges, which exhibit an inherent danger to marine life and researchers using the charges. The basic operation of the CSS is as follows. A combustible mixture of gas is held below the surface of the water in a combustion chamber and ignited with an electric spark. A combustion wave propagates through the mixture and converts the fuel and oxidizer into a bubble of combustion products, which expands due to an increase in temperature, and then ultimately collapses to a smaller volume than before ignition, producing a high intensity, low frequency acoustic signal. The thesis begins by discussing the background, history, and purpose of developing the CSS. It continues by describing the current apparatus and the essential components and convenient features added to the latest mechanical design. The general operation is discussed along with a description of an experiment conducted to determine the acoustic output and robustness of the current CSS. The results of this experiment are presented in terms of the effect of volume, ignition depth, oxidizing gas, combustion chamber size, and repeatability of acoustic signatures. Discussion of apparatus robustness is presented to suggest improvements for future CSS designs. / text

A transponder for underwater acoustic data communications is prototyped. The mobile transponder emits a data sequence whenever it detects a ping from a base station. The data sequence includes GPS coordinates and UTC time sent over a conservative and brief 12 kbps turbo-coded BPSK link, and a 6 kB JPEG image sent over an ambitious 67 kbps turbo-coded 16-QAM link. The range of the transponder from the base station can also be accurately derived. Several challenges exist in decoding the underwater signals at the base station receiver, including Doppler distortion and multipath. While experimental results show that the ranges for decoding the 16-QAM signals with a single hydrophone are limited to less than 25 m, the BPSK signals prove to be much more robust, decoding at ranges of up to 625 m. Experiments with delays and transducer tether length indicate methods for improving reliability in the presence of reverberation and thermocline. This transponder uses mostly off-the-shelf parts and is anticipated to be improved when paired with advanced sonar array devices. / text

No description available.

This thesis develops a Bayesian inversion algorithm for autonomous underwater vehicle localization, and carries out a study of several factors contributing to localization accuracy in an underwater acoustic positioning system. Specifically, a ray-based algorithm is described that estimates target position through the linearized inversion of transmission arrival time differences, and provides linearized uncertainty estimates for model parameters. Factors contributing to source localization uncertainty considered here included: (1) modelling transmission paths accounting for refraction due to a depth-varying SSP instead of using a constant sound-speed approximation and straight-line propagation, (2) inverting for a potential bias in the measured sound-speed profile, (3) accounting for errors in hydrophone position by including these positions as unknown parameters in the inversion, and (4) applying path-dependent timing correction factors to account for lateral variability in the sound-speed profile. In each case, nonlinear Monte Carlo analysis is applied in which a large number of noisy data sets are considered, to obtain statistical measures of the localization improvement that results by addressing these factors. / Graduate