Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2006. / Ginsberg, Jerry, Committee Member ; Trivett, David, Committee Chair ; Rogers, Peter, Committee Co-Chair.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: ray chaos, eikonal equations, turbulence. Includes bibliographical references. (p.98-102).

Thesis (Elec.E)--M.I.T., Dept. of Electrical Engineering and Computer Science, 1979. / Supervised by Alan V. Oppenheim and George V. Frisk. Bibliography: leaves 132-133.

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

Knowledge of the geoacoustic properties of the ocean bottom is essential for accurate modeling of acoustic propagation in shallow-water environments. Estimates of these properties can be obtained through geoacoustic inversion. Among the various inversion methods, the ones based on matched-field processing (MFP) have been increasingly used due to their relatively easy implementation and their good performance. In matched-field inversion (MFI), the objective is to maximize the match between the measured acoustic pressure field and the modeled field calculated for trial sets of geoacoustic parameters characterizing the environment. This thesis investigates the technique of matched-field tomographic inversion, a recent application of MFI that takes advantage of a multiple array-multiple source configuration to estimate range-dependent geoacoustic parameters. A two-stage inversion method based on the ray approach adopted to calculate the modeled pressure fields is developed to increase the efficiency of the estimation. The first stage consists of matching measured and modeled amplitudes of waterborne rays propagating between each source-array pair to estimate the parameters at the seafloor. The second stage consists of matching measured and replica pressure fields corresponding to rays that penetrate the sediment to estimate deeper parameters. In the first stage, the match is quantified using a least-squares function whereas in the second stage the robust pairwise processor is used. Both stages use a simplex genetic algorithm to guide the search over the parameter space. The inversion method is first applied to the two-dimensional (2-D) problem of vertical-slice tomography where four sets (2 sources x 2 vertical line arrays) of multi-tone pressure fields are used to estimate the depth and range variations of geoacoustic parameters. The method is validated via simulation studies that show its good performance in the ideal case where every model parameter except the ones to be estimated are exactly known, and quantify its limitations in non-ideal cases where noise in the data or errors in the array positions are present. The inversion results show that the parameters to which the pressure field is the most sensitive are well estimated for signal-to-noise ratios greater than or equal to 5 dB or for array position uncertainties less than two wavelengths of the source wavelet. The inversion method is then applied to a 3-D environment problem. From the different array configurations studied, it is found that the accuracy of the parameter estimates increases with decreasing propagation range. Finally, the method is applied to experimental data for a vertical-slice configuration. The relatively poor match obtained between the replica and measured data is attributed to the large uncertainty in the array position and the simplistic parameterization of the environment. / Graduate

Sound production is widespread among teleosts and is usually observed in social contexts. Many fish, such as triglids, produce sounds by contracting a pair of specialised striated muscles attached to both sides of the swimbladder wall, yielding low frequency sounds made up of repeated brief pulses. Temporal features of acoustic emissions can be controlled by the rate of muscle contraction, thus providing a rich means of conveying information. In this study the sound features of the grey (EutrigJa gurnardus), the streaked (Trigloporus lastoviza), the red (Aspitrigla cuculus) and the tub (Trigla Jucerna) gurnards were compared. The grey and the red gurnards emitted 3 different sound types, knocks, grunts and growls, whereas the streaked and the tub gurnards only emitted one sound type, growls and grunts respectively. Interspecific differences of calls were marked and based on the temporal patterning and on the grouping of the pulses. In the grey gurnard, ontogenetic changes in sound production were found. The sound production rate, the proportion of emitted sound type and the physical features of sounds varied with fish size. A study of diel and seasonal variations of sound production in the grey gurnard showed that more sounds were uttered during the day than at night and that grunts were more important and intense during the SpringSummer period. The sonic apparatus was examined in the species mentioned above and also in the large-scaled gurnard (Lepidotrigla cavillone) and the piper (Trigla lyra). The swimbladder and the sonic muscles grew throughout life in all species. Variations in the sonic apparatus with fish gender or time of the year were not detected. This suggests that the ability to vocalise is similar in both male and female gurnards, probably even during courtship. The shape of the swimbladder was species-specific. All species possessed a pair of intrinsic sonic muscles except for the piper whose sonic muscles were extrinsic. The pair of intrinsic sonic muscles of the grey gurnard contracted synchronously and each contraction generated a pulse of sound. The biological Significance of sound production was studied in the grey, the streaked and the tub gurnards. Few other studies have made a comparison of the behavioural context of sound production in different species of fish of the same family. Competitive feeding interactions were examined and compared between species. The different sound types emitted were also correlated with different behavioural categories. Typical sequences of behaviour were found in each species. Disputes over food items were settled by either reaching food first or by being aggressive, but never involved fighting escalation. The knocks of the grey gurnard and the growls of the streaked gurnard were associated with feeding and low levels of aggressive behaviour, and the grunts of grey and tub gurnards were associated with agonistic behaviour, such as approach, chase and frontal displays. The grey gurnard was the most vocal and aggressive species during competitive feeding. Smaller grey gurnards interacted more frequently than did larger fish, and a larger proportion of their interactions were accompanied by grunt calls. The metabolic costs of sound production in fish were calculated theoretically for different types of acoustic activity. If the assumptions made are valid then it seems that producing acoustic signals in fish is cheap. The biological implications of these low energetic costs were discussed. Finally, the acoustic repertoire of several non-triglid species of fish was examined. Sounds from 7 species were described for the first time. These species uttered sounds during territorial defence and courtship; contexts quite different from those studied for triglids.

Gray whales (Eschrichtius robustus), while engaged in underwater signalling, circumvent noise in the acoustical channel by the structure and timing of their calls. Data yielding this conclusion were collected during an acoustical study on gray whales and their habitats (1981-1984). Sonographic analyses of tape recordings were used to quantify the acoustical repertoire, the ambient noise characteristics of the area, and the relationship between the animals' calls and the environment. The acoustical responses of whales to artificially increased levels of noise were documented during playback experimentation in Mexico. Nine sound parameters were inspected and compared between control and experimental conditions: calling rates, call types, frequency range of signals (Hz), emphasized frequencies (Hz), received levels of sounds (dB re 1 μPa), call duration (sec), percentage of calls exhibiting frequency modulation, number of pulses per series, and repetition rates of signals. The observed surface behavior of gray whales in response to noise (i.e., dive durations, movements and abundance) was also investigated. Analyses yielded: a description of gray whale call types; a characterization of the acoustical habitats occupied by this species, including a list of sources contributing to the ambient noise and a profile of the propagation characteristics of the study area; a determination of the relationship between whale calls and their habitats; and the acoustical capabilities and strategies of whales in response to noise. The plasticity observed in the overall behavior of this whale is of adaptive significance when considering the dynamic nature of noise in the environment. Typically, the multiple strategies employed by the whales when faced with various noise situations enable them to minimize the detrimental effect that noise has on their underwater signalling. Gray whale responses varied with the sound source and may also differ relative to the geographical range and/or general behavior of the animal. It is concluded that ambient noise (both natural and man-made) has a profound effect on the behavior of this coastal species and that acoustical calling is modified to optimize signal transmission and reception. / Science, Faculty of / Zoology, Department of / Graduate

<p>The Ocean Environment was explored to determine the effects of temperature, absorption and leakage on acoustic propagation. Having modeled the acoustic region, a cylindrical sonar is evaluated to determine its array gain as a function of frequency. The Detection Threshold is analyzed and its impact on signal detectability discussed in detail. utilizing the Passive Sonar Equation, the sonar's effectiveness is quantitatively measured as a function of frequency for a typical noise source. Correlation techniques, modeled on an IBM PC, are then introduced to improve the sonar's effectiveness.</p> / Master of Science

Research on the propagation of acoustic waves in ocean bottom sediment is of interest for active sonar applications such as target detection and remote sensing. Currently, all seabed scattering models available in the literature are based on the full solution of the wave equation, which sometimes leads to mathematically intractable problems. In the electromagnetics community, an alternative formulation that overcomes some of this complexity is radiative transfer theory, which has established itself as an important technique for remote sensing. In this work, radiative transfer (RT) theory is proposed for the first time as a tool for the study of seabed acoustic scattering. The focus of this work is the development of a complete model for the interaction of acoustic energy with water-saturated sediments. The general geometry considered in this study consists of multiple elastic layers containing random distributions of inhomogeneities. The accuracy of the proposed model is assessed by rigorous experimental work, with data collected from random media in which acoustic properties such as the concentration and size of scatterers, background material, and the presence of elastic boundaries are controlled parameters. First, the ultrasound RT model is implemented for layers of finite thickness. The range of applicability of the proposed model is then illustrated using scaled experiments conducted at the Northwest Electromagnetics and Acoustics Research Laboratory (NEAR-Lab). Next, the model is applied to field data collected in a region with gassy sediments and compared to the formulation originally used to explain these data. Finally, insight into the emerging area of study of the time-dependent RT formulation is presented, and its role in the representation of finite broadband pulses is discussed.

In recent years the United States Navy has concentrated most of its Anti-Submarine Warfare (ASW) research and development efforts toward passive sonar. Its ability to locate enemy targets without being detected gives the passive sonar system a supreme strategic advantage over its active counterpart. One aspect of passive sonar signal processing is the time delay estimation of an underwater acoustic wavefront. From this estimation the location and velocity of the radiating source (target) can then be determined. This report compares two popular methods of estimating time delay utilizing computer simulations of each: the cross correlator and the beamformer.