Thesis (M.S. in Engineering Acoustics)--Naval Postgraduate School, June 2004. / Thesis advisor(s): Thomas Hofler, Bruce Denardo. Includes bibliographical references (p. 35). Also available online.

Hydrophones used in the ocean produce spurious outputs due to vibration sensitivity which can severely degrade measurement accuracy. Sources of these vibration inputs are ocean surface waves, flow turbulence, and induced mechanical vibration. The hydrophone response to these vibrations is a noise voltage output. This can leave to a signal-to-noise problem particularly when measurements of small sound pressure levels are to be made. The objective of this thesis is to analyze the vibration response of three typical piezoelectric hydrophone sensor elements configurations and give design methods and constraints for reducing the problem of vibration sensitivity to an acceptable level. The sensor element configurations analyzed are the radially polarized cylindrical shell, radially polarized spherical shell, and axially polarized cylindrical shell. The analysis is carried out due to two causes. An electromechanical analysis is given of the voltage sensitivity of each of the three sensor configuration to the inertial effect of acceleration inputs. The second effect analyzed is the voltage sensitivity of a pressure sensitive sensor element to the hydrostatic pressure amplitude caused by periodic vertical displacement of a hydrophone. Results of the analyses show that the radially polarized cylindrical and spherical shell configurations have zero acceleration sensitivity to inputs on the axes analyzed. An equation is derived for the axial acceleration sensitivity of the axially polarized cylindrical shell in terms of the equivalent sound pressure. The analysis of hydrophone sensor response to periodic vertical displacements shows high voltage sensitivity to very small displacement amplitudes. Data is given for the maximum permissible vertical displacement amplitude to produce a 20dB signal-to-noise ratio. Based on these analyses, design considerations are given to minimize hydrophone vibration sensitivity.

Approved for public release; distribution is unlimited / The MiniCan hydrophone is a small, easy to build, preamplified hydrophone with similar characteristics in sensitivity and self noise to larger and more expensive commercial devices. Previous work on the design showed a very promising performance, though it proved to have a flat sensitivity response of only up to 14 kHz. Unknown were also the effects that the aluminum housing parts produced on the overall response and whether the cable of the hydrophone had some influence on the sensitivity. A new design was built and tested changing the dimensions of the aluminum housing for the hydrophone, resulting in an increase in the region of flat sensitivity response up to 20 kHz and acceptable response up to 30 kHz, due to an increase of the lowest mechanical resonance of the hydrophone. A resonance testing device was built to investigate the mechanical characteristic of the components of the design, discovering that the first resonance of the aluminum base of 34.6 kHz caused the first overall resonance of the assembled device. Measurements of the influence of the cable showed an acoustic variation of about 1 dB in relative response in the range of interest, which is up to 30 kHz. The measurements proved that better performance can be achieved on the basic MiniCan design by increasing the resonant frequency of the aluminum body housing component. / Lieutenant, Chilean Navy

Dans ce travail, nous proposons une étude générale des hydrophones, leur fonctionnement suivant le principe de la piézoélectricité, les éléments les constituant ainsi que les brevets et les hydrophones existants. Nous modélisons les courbes de sensibilité à la réception et à l'émission avec le logiciel COMSOL que nous comparons avec des hydrophones en notre possession. Avec ces modèles, nous proposons le design d'un hydrophone potentiel, large bande, viable à de grandes immersions (>6000 m). Nous testons deux nouvelles méthodes de calibration d'hydrophone à basses fréquences (< 2 Hz) que nous appliquons aux hydrophones des flotteurs MERMAID. Dans la méthode «dynamique», la réponse de l'hydrophone est étudiée suite à l'application d'une brève surpression (1000 Pa avec tτ < 1 s), réalisée par le déplacement vertical de l'hydrophone dans l'eau. La méthode «statique» permet d'étudier la réponse du système d'acquisition complet. L'hydrophone est placé dans un caisson étanche dans lequel une surpression est générée par l'ajout d'une colonne d'eau placée au-dessus. Nous déterminons les pôles et zéros de la chaîne d'acquisition des flotteurs. La correction des sismogrammes enregistrés par trois flotteurs MERMAID déployés en mer Méditerranée, nous permet d'estimer la pression générée par le séisme de Barcelonnette à environ 400 Pa (7 avril 2014, Mw = 4,8). Nous validons les données acquises par les MERMAID dans une étude sismologique. Nous étudions les données de six mois d'enregistrements par les trois flotteurs déployés en mer Ligure, développons un protocole de pré-traitement des données que nous validons avec une étude tomographique. / In this work, we propose a general study of hydrophones focusing on their operation, based on the piezoelectric principle, the different elements that compose them and the available hydrophones and patents. We model reception and emission sensibility curves with COMSOL software and compare with hydrophones at our disposal, allowing us to qualitatively estimate the sensitivity. We propose a design for a potential broadband hydrophone, viable at large depths (> 6000 m). We test new methods of hydrophone calibration at low frequencies (< 2 Hz) and apply them to the MERMAID floats hydrophone. In the «dynamic» method, the hydrophone response is studied with a brief pressure variation (1000 Pa with tτ < 1 s), performed by a winch, which vertically moves the hydrophone into water. The «static» method allows us to study the full system response. The hydrophone is placed in a calibration chamber in which a pressure variation is performed with an additional water column. We have determined poles and zeros applicable to the MERMAID. The correction of seismograms recorded by three MERMAID floats, deployed in the Mediterranean Sea, allows us to estimate the pressure variation produced by the Barcelonnette earthquake which is around 400 Pa (April 7, 2014, , Mw = 4,8). We validate the data acquired by the MERMAID in a seismological study. We study data of six months of acquisition of the three floats deployed in the Ligurian Basin, develop a preprocessing method of these data and validate with a tomographic study.

Thesis (M.S. in Applied Physics)--Naval Postgraduate School, June 2004. / Thesis advisor(s): Thomas J. Hofler, Bruce Denardo. Includes bibliographical references (p. 57). Also available online.

Thesis (M.S. in Engineering Acoustics)--Naval Postgraduate School, December 2003. / Thesis advisor(s): Thomas J. Hofler, Andres Larraza. Includes bibliographical references (p. 45). Also available online.

Thesis (M.S.)--University of Florida, 2004. / Title from title page of source document. Document formatted into pages; contains 38 pages. Includes vita. Includes bibliographical references.

La vitesse du son dans les écoulements cavitants varie considérablement en fonction du taux de vide local. Par exemple, elle est proche de 1500 mis dans l'eau pure, 400 mis dans la vapeur d'eau et peut diminuer bien en dessous de ces valeurs dans le mélange liquide-vapeur. Les écoulements cavitants sont donc quasiment incompressibles dans les zones purement liquides, et fortement compressibles dans les zones diphasiques. Il est nécessaire de prendre en compte la compressibilité du fluide dans les simulations numériques afin de reproduire certains mécanismes complexes associés aux instabilités de cavitation. Cela nécessité une estimation correcte de la vitesse du son dans un milieu cavitant. Ce travail de thèse avait donc pour principal objectif la création d' une zone de cavitation homogène afin d'y mesurer la célérité du son en fonction du taux de vide. Un dispositif expérimental a donc été développé pour créer une telle zone. Deux méthodes pour mesurer la vitesse du son sont examinées: J- avec 3 capteurs de pression équidistants le long de la veine d'essais TI- avec 2 hydrophones en vis-à-vis dans une section de la veine. Le taux de vide est mesuré à l 'aide d' une sonde optique. L'étalonnage de la sonde optique et la validation des mesures des capteurs et des hydrophones, sont fait en écoulement eau-air. La méthode J étant plus adaptée pour la mesure de la vitesse du son, est appliquée pour un écoulement cavitant. Pour chaque écoulement (eau-air et cavitant), les résultats sont comparés avec les modèles théoriques existants et une discussion est menée. Un bon accord se révèle entre les expériences et la théorie.

M.Ing. / A study was done on optical fibre hydrophones. Integrated optic hydrophone components as well as optical fibre components were investigated. An optical fibre coupler was successfully manufactured with an etch process. The coupling ratio can be varied mechanically or by means of refraction index variation. Experimental procedures and results will be shown. Bending losses excited in an optical fibre coil were investigated as a function of different coil diameters. The results were extrapolated to determine the amount of losses arising in a hydrophone coil.

An interferometer is used for phase detection. Thus, the hydrophone which is based on interferometric optic fiber sensor has extremely high sensitivity. Sagnac interferometric hydrophone has low noise of marine environment, which is more suitably used to detect underwater acoustic signal than that of a Mach-Zehnder interferometer. In this paper, a configuration of dual Sagnac interferometer was proposed. The mathematical model was derived and analyzed. The fiber optic sensor head contains mandrel type and planar one. The acoustic window are made of silicon rubbers. It was should that we can increase their sensitivities by increase number of wrapping fiber coils. In our experiment, the result shows that among all the mandrel sensor heads, the most highest dynamic range is up to 37.6¡Ó1.4 dB, and its sensitivity is -223.3¡Ó1.7 dB re V/1&#x00B5;Pa ; while the dynamic range of the planar sensor head is 42.1¡Ó1.5 dB, and its sensitivity is -213.8¡Ó1.3 dB re V/1&#x00B5;Pa¡C As for the configuration of the optical interferometers, the intensity of the dual Sagnac interferometer is 20 dB larger than its Sagnac counterpart. Its dynamic range is above 66 dB, which is 24 dB larger than that of the Sagnac interferometer with the sensitivity of -192.0 dB re V/1&#x00B5;Pa. In addition, by using software simulation to design optimal lengths of delay fibers , we can increase interferometer¡¦s the dynamic range of underwater acoustic detection. This paper verify that, by means of adjusting the length of these two delay fibers, we can actually increase the dynamic range of acoustic signal detection.