The plasma jet, a commonly used ignition device, has been investigated as a source of acoustic energy suitable for sub-bottom profiling. Named the plasma gun, the device discharges electrical energy in a cylindrical arc ignited in a gaseous environment surrounded by water. When the arc energy evaporates water, it produces a rapidly expanding vapor bubble that creates the acoustic pressure wave. Acoustic properties of the device are similar to small explosives, and to electric sparkers. Multiple bubble oscillations, a problem of explosive-type sources, are generally less troublesome for the plasma gun than with the sparker sources. Some degree of frequency control of the acoustic pulse is possible if proper values are selected for the electrical circuit components and for the total stored electrical energy. Peak acoustic pressures are controlled both by the total electric energy and by the rate it is delivered to the arc. These quantities are determined by capacitance, inductance, and charging voltage. Frequency components of the primary pressure pulse depend on the arc discharge frequency and on the immersion depth of the device. The bubble period depends primarily on the amount of energy discharged into the water; this in turn is proportional to the total stored electrical energy. The plasma gun has been compared to small air guns, pingers, sparkers, and boomers. Sub-bottom profiles obtained show penetration less than the 1 in3 air gun but with more resolution. Stored energy in the plasma gun, however, was nearly five times less. Penetration was equal and resolution better than electric sparkers of the same energy. Penetration was better and resolution poorer than the pinger, and resolution poorer and penetration slightly better than the boomer source. Except for the sparkers, which used the same power supply, the plasma gun has a decided advantage in equipment size and ease of deployment. / Graduate

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Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Michaels, Thomas; Committee Co-Chair: Michaels, Jennifer; Committee Member: Degertekin, Levent; Committee Member: Qu, Jianmin; Committee Member: Ruzzene, Massimo; Committee Member: Scott, Waymond. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Diss. (sammanfattning) Uppsala : Sveriges lantbruksuniv., 2003. / Härtill 5 uppsatser ; 1 CD.

The primary goal of this thesis is to utilize acoustical radiation from the Arctic ice cover to infer the response of sea ice to environmental forcing, and to sense remotely the mechanical properties of the ice. The work makes use of two experiments in the Canadian arctic undertaken by the Ocean Acoustics Group of the Institute of Ocean Sciences, which resulted in an extensive body of acoustical and related environmental data. Cracking sounds originating from both first and multi-year ice fracturing processes are analyzed. Data used in this thesis also include sound made by artificial sources. The survey of in situ ice conditions by air photography and synthetic radar imaging, and a crack distribution map based on observations made with a 3-D hydrophone array, reveal, for the first time, a close correlation between thermal cracking events and ice type. It is shown that most of the thermal cracks occur in irregular multi-year ice where there are exposed, snow-free surfaces. The study shows that acoustical radiation from some cracks implies a slip-stick seismic movement over the faults, and some cracks tend to radiate more high frequency sound downwards rather than sideways. This phenomenon is most clearly apparent in sounds made by artificial sources. Another interesting finding from this study is that the sound of cracking ice does not always exhibit a vertical dipole radiation pattern, and some cracks due to thermal tension on smooth first year ice radiate more energy horizontally. The observations have motivated the development of various analytical models. These models allow the observed acoustical features to be related to the length and depth of a crack, the thickness of the ice cover and its Young's modulus. The models also show that maximum sound radiation from a crack is in the direction of external forcing. Finally, it is found that noise due to rubbing between ice floes exhibits a narrow band spectrum. This phenomenon is investigated and a linear model derived shows that the observed peak frequency is that of the first mode horizontal shear wave triggered by frictional effects at the ice floe edge. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Vita. / Bibliography: leaves 378-399. / by David H. Johnston. / Ph.D.

Interrogating metals with ultrasonic waves can be used to evaluate their microstructural and mechanical properties. These techniques analyze ultrasonic wave features in order to make inferences on the medium of interest. Current research is being conducted to determine higher order elastic properties and characterize material degradation of 7075-T651 aluminum with ultrasonics. This thesis topic will use acoustoelasticity, the stress dependency of acoustic velocity, to accomplish these goals. Acoustoelasticity is a manifestation of the inherent nonlinearity in the interatomic binding energy, which appears mathematically as higher order elastic terms in the stress strain constitutive relation. The acoustoelasticity will be determined for longitudinal and shear waves propagating through a sample under uni-axial stress. Experimentally, specific techniques and tooling will be designed to insure accurate measurements of acoustic wave velocity as a function of stress. Using acoustoelasticity the third order elastic constants of 7075-T651 aluminum will be determined. Further, Al samples will be fatigue damaged and acoustoelasticity and third order elastic constants will be mapped versus damage. Literature will be used to verify measured values of acoustoelasticity as well as provide theoretical models for acoustoelastic dependence on damage.

Numerical models will be developed for the prediction of silencer transmission loss under the operating conditions present in gas turbine exhausts. In these systems the large diameter ducts and high operating temperatures produce a challenging acoustic environment due to the unverified behaviour of fibrous materials at high temperatures and the existence of complex sound fields. To understand the behaviour of fibrous materials at high temperatures their bulk acoustic properties are measured using a modified impedance tube which can heat material samples up to a temperature of 500 C. It will be demonstrated that the high temperature material properties can be extrapolated from room temperature measurements given knowledge of the temperature dependant flow resistivity. Finite element numerical models using point collocation and mode matching techniques to predict the transmission loss of silencers are developed and successfully validated. Dissipative silencer designs with various cross-sectional designs are explored numerically and experimentally according to common industry standards. It is demonstrated that transmission loss may be optimised by the arrangement of the fibrous material across the cross-section. The accurate numerical models allow for effe cient silencers to be designed reducing silencer size and cost. A new hybrid silencer is presented combining dissipative and reactive elements with the aim of increasing the low frequency attenuation of large silencers while maintaining an effective broadband spectrum. Measurements and predictions show this innovative design to be successfull. Application of the hybrid silencer allows for more flexible noise control solutions when design is limited by low frequency noise.

[by] R.D. Maurer [and] Melvin A. Herlin. / Bibliography: p. 6. / Army Signal Corps Contract No. W-36-039 sc-32037, Project no. 102B. Dept. of the Army Project No. 3-99-10-022.

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