An integrated modal approach to surface and volume scattering in ocean acoustic waveguides

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1996Acoustic propagation in the ocean can be strongly affected by small random variations in ocean properties, including rough surfaces and volume fluctuations in the ocean or seabed. Such inhomogeneities scatter part of the incident acoustic field, stripping energy from the coherent part of the field. This scattered energy, or reverberation, propagates further in the modes of the ocean waveguide. The distribution of energy among modes is changed and the coherence of the acoustic field is reduced. This thesis introduces several models which describe scattering of low-frequency sound. First, the rough surface scattering theory of Kuperman and Schmidt is reformulated in terms of normal modes. Scattering from rough fluid-fluid interfaces and rough elastic halfspaces is modeled, and statistics of the acoustic field are calculated. Numerical results show the modal formulation agrees well with Kuperman and Schmidt's model, while reducing computation times by several orders of magnitude for the scenarios considered. Next, a perturbation theory describing scattering from sound speed and density fluctuations in acoustic media is developed. The theory is used to find the scattered field generated by volume fluctuations in sediment bottoms. Modal attenuations due to sediment volume scattering are calculated, and agreement is demonstrated with previous work. The surface and volume scattering theories are implemented in a unified modal reverberation code and used to study bottom scattering in shallow water. Numerical examples are used to demonstrate the relationship between volume and surface scattering. Energy distribution among scattered field modes is found to be a complicated function of the scattering mechanism, the scatterer statistics, and the acoustic environment. In particular, the bottom properties strongly influence the coherence of the acoustic field. Examples show that excitation of fluid-elastic interface waves is a potentially important scattering path. Cross-modal coherences are calculated and used to study the loss of signal coherence with range. Finally, earlier work on scattering from the Arctic ice sheet is extended. Simulations of long-range transmissions are compared with data from the April 1994 trans-Arctic propagation test. The results show modal attenuations and group speeds can be predicted reasonably well, indicating that acoustic monitoring of Arctic climate is feasible.I am extremely grateful for the financial support of the Office of Naval Research, under contracts N00014-92-J -1282 and N00014-95-1-0307

Analysis of the Stochastic Electromagnetic Field Coupling to Single and Multiconductor Transmission Line Structures

In dieser Dissertation werden Übertragungsleitungen und ihr Verhalten in einer Modenverwirbelungskammer analysiert. Zu diesem Zweck wird eine zeiteffiziente semianalytische Lösung zur Berechnung der Einkopplung stochastischer elektromagnetischer Felder in ein- und mehradrige Übertragungsleitungsstrukturen vorgestellt. Das Simulationsmodell basiert auf den Baum-Liu-Tesche (BLT) Gleichungen. Diese Arbeit geht über die Untersuchung der Einkopplung einer einzelnen ebenen Welle hinaus und untersucht die Anregung durch ein stochastisches Feld. Die Übertragungsleitungen werden dabei analytisch mit Hilfe der Leitungstheorie beschrieben und zusätzlich werden Leitungsbeläge definiert. Basierend auf den Maxwell-Gleichungen wird eine verallgemeinerte Leitungstheorie vorgestellt, die jedoch in ihrer Struktur der klassischen Leitungstheorie ähnelt. Für die Leitungsparameter wird ein Hochfrequenz (HF)-Modell abgeleitet, sodass die Parameter frequenzabhängig werden. Das semianalytische Modell wird mit der Momentenmethode und durch Messungen in der Modenverwirbelungskammer verglichen und validiert. Verschiedene Konfigurationen der Leitungsstrukturen werden analysiert und die eingekoppelten Spannung an den Enden der Leitungen berechnet, gemessen und diskutiert. Darüber hinaus wird der Einfluss von gebogenen und geknickten Übertragungsleitungen und der Einfluss des Knicks auf die Einkopplung untersucht.This thesis analyzes transmission lines and their behavior in a reverberation chamber. For this purpose, a time efficient closed-form solution for the coupling of stochastic electromagnetic fields to single- and multiconductor transmission line structures is presented. The simulation model is based on the Baum-Liu-Tesche (BLT) equations, but this thesis goes beyond the study of a coupling to a single plane wave and studies the excitation by a stochastic field. The transmission lines are described analytically using transmission line theory and in addition per-unit-length (p. u. l.) parameters are defined. Based on Maxwell’s equations, a generalized transmission line theory is presented which, however, resembles classical transmission line theory in its structure. The classical transmission line theory is extended to include higher order modes. A high frequency (HF) model is derived for the p. u. l. parameters, in that way that the parameters become frequency dependent. The model of the frequency dependent p. u. l. parameters is deduced and validated by the method of moments and by measurements in the reverberation chamber. Different configurations of the transmission line structures are analyzed and the average magnitude of the coupled voltage at the terminals is calculated and discussed. Moreover, the influence of bending transmission lines and its influence on the field-to-wire coupling is investigated

A model for the simulation of sidescan sonar

This thesis presents the development of a computer model for the simulation of the sidescan sonar process. The motivation for the development of this model is the creation of a unique and powerful visualisation tool to improve understanding and interpretation of the sidescan sonar process and the images created by it. Existing models tend to generate graphical or numerical results, but this model produces synthetic sidescan sonar images as the output. This permits the direct visualisation of the influence of individual parameters and features of the sonar process on the sidescan images. The model considers the main deterministic aspects of the underlying physical processes which result in the generation of sidescan sonar images. These include the propagation of the transmitted pulse of acoustic energy through the water column to its subsequent interaction and scattering from the rough seafloor. The directivity and motion characteristics of the sonar transducer are also incorporated. The thesis documents the development of the model to include each of these phenomena and their subsequent effect on the sidescan sonar images. Finally, techniques are presented for the investigation and verification of the synthetic sidescan images produced by the model.Defence Research Agenc

A NUMERICAL AND EXPERIMENTAL VALIDATION OF THE DIFFUSION EQUATION APPLIED TO ROOM ACOUSTICS

The recently developed room-acoustics diffusion model relies on the basic as- sumptions of the Fick’s law of diffusion, relating the acoustic intensity and the energy density inside a room, through a constant diffusion coefficient. This study investigates the relationship between these two acoustic quantities in the station- ary state, for the particular case of long rooms with different amounts of boundary scattering, by means of numerical simulations and experimental measurements. The numerical study was performed with a sound particle-tracing code. The ex- periments consist in measurements inside the scale model of a long room, where a three-dimensional Microflown➤ probe (calibrated and equalized with digital filters) was employed to collect data in terms of pressure and axial velocity components. Then, for each receiver position, the intensity and the energy density gradient were derived. Both numerical and experimental results show that inside long rooms the diffusion coefficient is not a constant but increases with the distance from the source, with a slope depending on the scattering coefficient of the walls. This re- sult implies that, for such long enclosures, the diffusion model should consider a space-varying diffusion coefficient to be more consistent with real phenomena

Airborne sound insulation of wall structures : measurement and prediction methods

Protection against noise is one of the six essential requirements of the European Construction Product directive. In buildings, airborne sound insulation is used to define the acoustical quality between rooms. In order to develop wall structures with optimal sound insulation, an understanding of the physical origins of sound transmission is necessary. The purpose of this thesis was, firstly, to study and compare the validity of existing physical models to predict the sound insulation of wall structures, and, secondly, to study the benefits of the sound intensity measurement method for determining the sound insulation. To develop the kind of knowledge that is applicable to the improvement of real wall and door structures was the motive behind this study. Five main results are summarized in the following. 1. It was possible to measure wall structures with a considerably, up to 22 dB, higher sound reduction index with the intensity method than with the pressure method. Thus, the intensity method enables the determination of sound insulation in the presence of strong flanking where the pressure method gives only an underestimate. 2. The sound transmission through doors was modelled by two separate paths: a structural path through the door leaf and a leaking path through the door slits. The structural path was predicted using Sharp's model. The agreement with measurements was reasonably good except at high frequencies where overestimations were obtained. The leaking path was predicted using the model of Gomperts and Kihlman. The agreement with measurements was good for free apertures. 3. Thirteen existing prediction models of double panels were compared. The variations in predicted sound reduction indices were high, 20 ... 40 dB. Further work is needed to rank different models according to their reliability for practical structures. In addition, there is an obvious need to develop a hybrid model where all the important parameters are considered. 4. A new flanking mechanism could be observed in situ for a floating floor covering over a concrete slab. Identical floor structures in adjacent dwellings led to strong flanking transmission at the double panel resonance frequency of the floors. Strong flanking could be avoided by modifying the double structure in one dwelling. 5. In general, the most typical design fault of sound insulating double structures was strong mechanical connections, either in the form of rigid interpanel connections (studs) or in the form of bonded cavity absorbent (sandwich structures). In the case of door structures, efforts are usually wasted on the development of the structure, while the leak transmission may be the main transmission path. The results of this study are useful when the intensity method is used in the presence of strong flanking sound, the sound insulation of wall and door structures are predicted or improved and when prediction models are developed.reviewe

Evaluation of the acoustical performance of louvre by impulse response analysis /

Tese (Doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico