Thèse (Ph.D.)--Université de Sherbrooke (Canada), 2007. / Titre de l'écran-titre (visionné le 1 février 2007). In ProQuest dissertations and theses. Publié aussi en version papier.

Thesis (M.S. in Electrical Engineering and M.S. in Engineering Acoustics)--Naval Postgraduate School, June 2003. / Thesis advisor(s): Cristi, Roberto ; Rice, Joseph. "June 2003." Includes bibliographical references (p. 107-108). Also available online.

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The methods of investigation in the present work deal with: firstly the calculation of the frequencies of vibration of the different modes of extensional vibrations both radial and torsional of circular, solid and annular, plates as predicted by Love's theory. These are experimentally examined and empirical rules governing the frequencies are evolved. Secondly, the physical form of the vibration at the different calculated resonant frequencies is observed. This is carried out either by investigating the position of the nodal and antinodal regions on the vibrating surface or by investigating the distribution of stress inside the body of the vibrator. Three methods of observations have been used, namely, the interference method and this gives a measurement of displacements in the normal direction to the plate surface, together with an exact measurement of the surface amplitudes of the vibrating plate. Chapter IX is dealing with amplitude measurements. The powder method and it will be shown that it gives indication of both vertical and horizontal displacements. The third method used for the present investigation gives the stress distribution within the specimen provided the specimen is sufficiently thin. It is shown that it is generally unreasonable to expect any one method of observation to give a reliable solution to the type of motion and to the mode of vibrating body. The three methods together should be used in order to give a sound information about the vibrating motion; otherwise a misleading representation of the state of vibration is likely to arise. A practical agreement has been found between the theoretical and observed patterns for the second and third methods of investigation. The interference method did not offer a good correlation between theoretical and observed patterns; since it is always coupled to flexural modes of particular symmetry. However, the method gives an exact measurement of the surface amplitudes. The measurements require no calibration.

This thesis describes the theory and implementation of a system for the active control of acoustic noise in a small enclosure ("small" inferring that only a small number of acoustic modes dominate the reverberant field in the en c1osure). The theory for a multichannel active noise control system is developed. A system could consist of a number of detectors and cancellation sources controlling the field at a number of monitor positions. The simplest system comprises a single detector and a single source and is capable of controlling the field at one position. The controller for this system could consist of a pair of electronic filters, one between the detector and the source in parallel with another cancelling the feedback from the source to the detector. It is shown that for any configuration of transducers the required controllers can be realised by repeatedly using the same filter pair as described above. A particular study is made of the active control of reverberant fields. A system was successfully implemented to partially control the reverberant field in a small enclosure (0-5 x 0-6 x 0.7m). This consisted of a single detector microphone and a single control loudspeaker controlling the field at a single monitor microphone. The system was controlled with two finite impulse response filters realised using a Texas Instruments TMS32020 microprocessor accessed via a Ferranti PC360XT personal computer. The first two modes of the reverberant field were successfully attenuated. In summary, the theory for a multichannel controller has been developed and the simplist case tested. The importance of this is that the basic unit of a multichannel controller has been successfully implemented and this unit could be replicated as the basic building block for more complex controllers. This facilitates the implementation of controllers to attenuate the sound field at a number of points in a practical enclosure.

The surface acoustic wave properties of Berlinite (a-AlPO4) have been investigated theoretically and experimentally, for a variety of crystallographic orientations, to evaluate its possible use as a substrate material for temperature compensated surface acoustic wave devices. A computer program has been developed to calculate the surface wave properties of a material from its elastic, piezoelectric, dielectric and lattice constants and their temperature derivatives. The program calculates the temperature coefficient of delay, the velocity of the surface wave, the direction of power flow and a measure of the electro-mechanical coupling. These calculations have been performed for a large number of orientations using a modified form of the data given by Chang and Barsch for Berlinite and predict several new temperature compensated directions. Experimental measurements have been made of the frequency-temperature response of a surface acoustic wave oscillator on an 80° X axis boule cut which show it to be temperature compensated in qualitative agreement with the theoretical predictions. This orientation shows a cubic frequency-temperature dependence instead of the expected parabolic response. Measurements of the electro-mechanical coupling coefficient k gave a value lower than predicted. Similar measurements on a Y cut plate gave a value which is approximately twice that of ST cut quartz, but again lower than predicted. The surface wave velocity on both these cuts was measured to be slightly higher than predicted by the computer program. Experimental measurements of the lattice parameters a and c are also presented for a range of temperatures from 25°C to just above the alpha-beta transition at 584°C. These results are compared with the values obtained by Chang and Barsch. The results of this work indicate that Berlinite should become a useful substrate material for the construction of temperature compensated surface acoustic wave devices.

Active noise control (ANC) is concerned with achieving some degree of cancellation between unwanted acoustic noise and the output from one or more actively controlled secondary sources. The minimum configuration for an A.N.C. system is a detecting microphone to detect the unwanted acoustic noise and a single loudspeaker to act as a secondary source. The basic problem is to specify the electronic system needed between the detecting microphone and the loudspeaker to achieve cancellation. The system has an inherent feedback pathbetween the loudspeaker and the detecting microphone that may cause instability. The system described in this thesis uses the principle of parallel feedback to ensure system stability. The basic idea of this is to use a second electronic system in a feedback path parallel to the electroacoustic feedback path to, in effect, remove this feedback. Incorporating both this feedback modelling and the appropriate forward transfer function between thedetecting microphone and the loudspeaker should produce a practical stable working system. This thesis considers implementation of these two electronic systems as sampled data systems employing finite impulse response (FIR) filters. Measurements on an experimental duct are used as input data for the study of system performance, this data being analysed using aleast squares approach to find best fit. FIR filters. The system is assessed in terms of the lengths of FIR filters required in the feedforward and feedback paths to achieve satisfactory operation, i.e. that the system produce a significant level of attenuation and be stable. The predicted level of performance of the ANC system indicates that the use of two independent FIR filters to model the feedforward and feedback paths produces a stable working system, capable of producing a significant level of attenuation over a broad frequency range. However, the length of FIR filter needed in each path has to be long in order to achieve a satisfactory level of performance.

<p> Passive acoustic monitoring (PAM) is a more effective method of monitoring cetaceans&rsquo; distribution and abundance than conventional visual surveys. Cetaceans are highly vocally active and produce identifiable acoustic signals during echolocation and communication. Three different PAM platforms recorded data in overlapping time periods in the vicinity of the 2010 Deepwater Horizon oil spill site: bottom-moored buoys (EARS), Unmanned Surface Vehicle towed arrays (USV), and subsurface glider-mounted hydrophones. Detection rates of the EARS and USV were compared to investigate their efficiency in detecting marine mammals. Detection events were obtained using independent detectors for each platform and then compared by feeding data through a common detector. Results from both detectors and platforms were compared, and a comparable trend of detection rates was found. The purpose of this study is to aid in the development of cost-efficient PAM methodology for mitigation and environmental impact assessment purposes.</p><p>

A means of assessing the quantity of exploitable fish in the sea is a requirement for effective management of the resource. Sonar is widely used in this regard, as it provides a rapid means of assessment. Two acoustic assessment techniques currently used are the echo counting and echo integration· methods. The echo counting method requires that only single fish echoes are present in the backscatter from the shoal, while the echo integration technique requires an a-priori knowledge of the average target strength of the fish in the shoal. A novel method of assessment has been proposed. It relies on the relationship between the statistics of the backscatter from a volume distribution of scatterers and the number of scatterers contributing to the backscatter at any one time. The attraction of the method when applied to the estimation of number density of fish, is that estimates can be produced in the presence of overlapping echoes, and that knowledge of the target strength of the fish is unnecessary. The application of this method to acoustic fish stock assessment is investigated in this work. Current methods of assessment are reviewed and the theory of the statistical method is given. A computer simulation of the scattering problem gives a useful insight into the effects of sample size and density on the accuracy of the method. The method has been applied to the assessment of fish at sea, where it was run in tandem with an echo integrator. The results obtained with the two techniques are compared. Reasons for discrepancies are proposed and problems in the application of the method are identified.

The acoustic radiation force, as seen in apparati such as acoustic levitators, continues to find applications in materials science, manufacturing, and medical fields. One example of the utilization of an acoustic levitator is measuring the progress of clotting blood droplets. One of the largest advantages of using acoustic levitation is that the process is a minimal contact method. In some biological and chemical processes, surface contact can corrupt measurements, and acoustic levitation avoids these issues by using the acoustic radiation force to contain and manipulate the blood drop. The deformation of Newtonian liquid droplets via acoustic levitation has been well studied. In that case, the shape of the droplet is governed by the surface tension and shape curvature (Young-Laplace equilibrium). The quasi-static deformation of elastic droplets in acoustic levitators, however, has not yet been investigated. In this thesis, we explore the application of acoustic levitation to the characterization of the deformation of soft elastic droplets. This thesis consists of three main efforts. To start, the history of the acoustic radiation force and its applications are discussed. Next, a generalized theory for the acoustic radiation pressure acting on droplets of sizes similar to the acoustic wavelength is presented. We model the droplet as an incompressible, isotropic, linear elastic solid undergoing small deformations, under the conditions that the deformation is axisymmetric, with a purely radial traction condition, where the traction condition is derived from the acoustic radiation pressure. The quasi-static displacement and stress within the droplet is then solved for utilizing two potentials developed by Love (1926). We conclude by testing the validity of the theory by measuring the deformation and location of soft alginate gel spheres with known material properties in an acoustic levitator.