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It is often assumed that interference or noise signals are Gaussian stochastic processes. Gaussian noise models are appealing as they usually result in noise suppression algorithms that are simple: i.e. linear and closed form. However, such linear techniques may be sub-optimal when the noise process is either a non-Gaussian stochastic process or a chaotic deterministic process. In the event of encountering such noise processes, improvements in noise suppression, relative to the performance of linear methods, may be achievable using nonlinear signal processing techniques. The application of interest for this thesis is maritime surveillance radar, where the main source of interference, termed sea clutter, is widely accepted to be a non-Gaussian stochastic process at high resolutions and/or at low grazing angles. However, evidence has been presented during the last decade which suggests that sea clutter may be better modelled as a chaotic deterministic process. While the debate over which model is more suitable continues, this thesis investigates whether nonlinear processing techniques can be used to improve the performance of maritime surveillance radar, relative to the performance achievable using linear techniques. Linear and nonlinear prediction of chaotic signals, sea clutter data sets, and stochastic surrogate clutter data sets is carried out. Volterra series filter networks and radial basis function networks are used to implement nonlinear predictors. A novel structure for a forward-backward nonlinear predictor, using a radial basis function network, is presented. Prediction results provide evidence to support the view that sea clutter is better modelled as a stochastic process, rather than as a chaotic process. The clutter data sets are shown to have linear predictor functions. Linear and nonlinear predictors are used as the basis of target detection algorithms. The performance of these predictor-detectors, against backgrounds of sea clutter data and against a background of chaotic noise data is evaluated. The detection results show that linear predictor-detectors perform as well as, or better than, nonlinear predictor-detectors against the non-Gaussian clutter backgrounds considered in this thesis, whilst the reverse is true for a background of chaotic noise. An existing, nonlinear inverse, noise cancellation technique, referred to as Broomhead’s filtering technique in this thesis, is re-investigated using a sine wave corrupted by broadband chaotic noise. It is demonstrated that significant improvements can be obtained using this nonlinear inverse technique, relative to results obtained using linear alternatives, despite recent work which suggested otherwise. A novel bandstop filtering approach is applied to Broomhead’s filtering method, which allows the technique to be applied to the cancellation of signals with a band of interest greater than that of a sine wave. This modified Broomhead filtering technique is shown to cancel broadband chaotic noise from a narrowband Gaussian signal better than alternative linear methods. The modified Broomhead filtering technique is shown to only perform as well as, or.

The propagation of ultrasonic waves in single crystals of mercury telluride has been studied on a broad front between 1.2ºK and 380ºK. Measurements have been made by the pulse-soho technique for ultrasonic waves (frequency 10 MHz to 300 MHz) directed along the [100], [110] and [111] crystallographic directions. The large mercury telluride single crystals, required for the ultrasonic measurements, have been grown by the Bridgman technique from either stoichiometric melts or from off-stoichiometric, tellurium-rich, melts. The elastic constants C(_11), C(_12) an C(_44) of mercury telluride have been measured as a function of temperature between 1.2ºK and 300ºK, attention being paid to possible effects of non-stoichiometry in the crystals, and the results correlated with ultrasound attenuation data. Results are discussed in terms of the crystalline interatomic forces and are compared with those of other II-VI and III-V compounds with the zinc blende structure, together with group IV, elemental semiconductors and I-VII compounds: elastic properties of mercury telluride correspond closely to those of cubic zinc sulphide and fall into the general scheme presented by the related compounds. From the Szigeti relationship, the ionicity e* is estimated as 0.65 ± 0.05e and the fundamental lattice absorption (restrahlen) frequency as (4.1 ± 0.1) x 10(^12) Hz. The Debye temperature, calculated from the elastic constant data, is 141 ± 4 K. Anelastic properties of mercury telluride have been deduced from the temperature (1.2 K to 300 K), frequency (10 MHz to 300 MHz) and applied stress dependences of ultrasound attenuation measurements. The important ultrasound dissipation mechanisms include the viscous drag of lattice phonons and forced dislocation motion. Theoretical assessments of piezoelectric coupling on sound attenuation and thermoelastic loss, show that the effect of both are negligible. One of the main sound energy dissipation mechanisms is due to the lattice phonon-ultrasonic phonon interaction. Attenuation due to this effect exhibits characteristic features at low temperatures. These have been found in mercury telluride. The effect is larger than observed in other materials because the Debye velocity is lower and can therefore be observed at relatively low frequencies. Another loss mechanism arises from forced vibration of dislocation segments. At 4.2 K a maximum has been observed in the frequency dependence of attenuation. The results have been accounted for by the vibrating string model. The resonance frequency is 220 MHz. The dislocation drag coefficient is 2.3 x 10(^-5) dyn.sec.cm(^-2) at 4.2 K and loop length is about 3 x 10(^-4) cm. Data for the ultrasonic wave velocity and attenuation before and after annealing and under stress are in agreement with the dislocation mechanism. In the region 170 K to 260 K peaks are found in the attenuation which show characteristics of those of Bordoni. The activation energy is about 0.15 eV and the attempt frequency about 4 x 10(^9) Hz.

This thesis has described the development of an artificial listener model capable of predicting a number of different perceived spatial attributes at arbitrary locations in the listening area for reproduced sound. Previous research·into modelling the perceived spatial attributes of sound reproduction systems has concentrated primarily on the sweet spot in the centre of the listening area. However, good audio reproduction is ideally required at multiple points in the listening area, for example for a family living room with a home cinema system. A framework for modelling the perception of reproduced audio was developed, including the capture of the original sound-field, modelling the signals at the ears and the translation of the binaural signals to the perceptual domain. Explicitly modelling the binaural signals meant that the same principal cues as human listeners are used and also allowed existing binaural models to be incorporated into the system.

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It is shown theoretically that there should be a frequency dependence of the velocity of a Rayleigh wave propagated along the edge of a thin plate. This velocity can be expressed in terms of an effective Poisson's ratio, of which the pseudo Poisson's ratio of Oliver, Press and Ewing (1954) is a special case. Rayleigh waves have been generated and detected on the edge of a metal disc. Results from the apparatus show that the velocity shift of the Rayleigh wave is very much smaller than expected over the frequency range used. The measured velocities are corrected for the curved propagation path. The velocity deviation occurs at the frequency where the theoretical value begins to show a rapid change, i.e. where the effective Poisson's ratio should become negative. The discrepancies between the theoretical and experimental values for the Rayleigh wave velocities are explained, and the elastic constants of the material in which propagation occurs (brass 60% Cu. 40% zn.) calculated for a temperature of 220 C.

The object of this thesis is to investigate experimentally the effects of surface waves on time reversal in a shallow-water waveguide and to use chaotic signals to detect resonant objects. These two areas of research are separate but complementary. Chapters 2. to 4 will explain the background of acoustic time reversal techniques, and the associated experiments performed during this thesis. Chapters 5 and 6 will then focus on chaotic signals, arid how they can be used to detect resonant objects underwater. The synergy of these two separate approaches is explored in Chapter 7, which summarises the findings for both areas of research and makes recommendations for further work in these areas. 1.2 Overview of research into the effects of waves on time reversed signals. Time reversal is a powerful self-adapting technique capable of focusing (or refocusing) energy in media. The strength of time reversal is that it is an adaptive technique that can compensate for errors caused by both geometric distortions within an array of sensors (hydrophones in this case), and wave field distortion caused by inhomogeneities within a media WithOlit any prior knowledge ofeither the array or the medium. This ability makes time reversal a very attractive technique for focusing energy in areas such as waveguides, where the performance of conventional methods (such as beamforming) will be significantly reduced by multiple reflections, signal scattering etc. In the field of underwater acoustics, this can be applied to communications in complex environments and to mine hunting, especially in areas where other techniques may fail such as shallow water channels and littoral zones. One of the assumptions of time reversal is that the medium can be considered !., invariant over the time of the process. In shallow-water channels and littoral zones, water depth fluctuations due to surface waves can become a significant proportion of the total water depth, and in these conditions the assumption of a static medium breaks down. My research is to determine what effect surface waves have in shallow water waveguides, so that it might be used to determine the feasibility of using time reversal for mine-hunting and/or communication in these conditions. 1.3 Overview of research into the use of chaotic pulses for acoustic detection. This research was carried out in collaboration with (and with partial funding from) QinetiQ. Dr. Alan Fenwick (QinetiQ) and his team had carried out simulations of chaotic (acoustic) pulses and their use in detecting resonant objects. The research presented in this thesis aimed at providing experimental validation of these simulations. As with the research into surface waves on time reversal, this area of research has applications in the field of mine hunting. In conditions of harmonic resonance, chaotic signals produce sporadic higheramplitude responses (called 'bursts') from a target. The present research aimed at using this characteristic to produce a method assisting in the detection of resonant objects. My research provided the necessary experimental validation and research was then taken further to design a method of detection.

This project aims to evaluate the zoo soundscape from the animals and visitors perspective. A complete acoustic environment study in zoological parks should involve these two different, but equally important, characters. This thesis presents the results of the influence of the sound components of the zoo environment in the welfare and behaviour of mammals and in the visitors’ experience. Firstly, a critical literature review was made concerning the impact of noise on wildlife. Several papers were evaluated regarding some topics such as the target species, the sound source studied, and the methodology applied. The aim was to assess the reliability of the articles and to propose a guideline for future studies in this area. The results of the literature review have shown that only seven per cent of the published papers used suitable equipment and acoustic metrics to investigate the sound effect on wildlife and confirmed the importance of a complete and well-described methodology for studies replicability. The influence of sound on zoo mammals was explored by direct recordings of animal behaviour and sound measurements, and by the collection of faecal samples for glucocorticoid metabolites analysis in two zoos, Chester Zoo and Twycross Zoo. The results show that animals express some behavioural and hormonal responses to different environmental sound amplitudes. Therefore, zoos could use these findings for a better animal management and enclosures planning. The zoo soundscape perception by the public was investigated by the application of questionnaires with the soundwalk methodology around Chester Zoo. The objective of this part of the study is to understand how the zoo visitors perceive the environmental sound around the zoo and how different aspects of an area can influence the individual perception of the sound. The results show, among other important variables, that technological sounds can have a negative on the visitors’ perception and evaluation of the soundscape. For this reason, zoos should be more careful about the environmental sound of places with predominant technological sounds. In conclusion, for the animals, sound levels and the visitors can be a source of stress that causes variations in the expression of behaviour and in physiological stress levels. For the visitors, the influence of sound is caused mostly by the noise sources and less by the sound levels.

Several problems concerning wave propagation in a stratified fluid under the influence of gravity are considered. These may be divided into two major groups: the first group considered pertaining to the motions of the earth's atmosphere resulting from explosive disturbances, the second pertaining to the dissipation of energy of waves generated by bodies moving within an incompressible fluid. For the first group of problems attention is concentrated on the properties of the created wave motions, whereas for the second group attention is concentrated on the energy losses experienced by a moving body which are known to correspond to the generation of gravity waves within the fluid. The first three chapters are a study of wind effects upon acoustic-gravity waves caused by atmospheric nuclear explosions. In Chapter I experimental evidence is presented which shows that winds have a pronounced effect on groundpressure waves. Chapter II contains a derivation of a theoretical expression for ground-pressure waves from a point source in a particular type of model atmosphere containing winds. In Chapters II and III computations based on this expression are presented, some theoretical aspects of the results are considered, and comparison is made between theoretical wave-trains and actual microbarographic recordings associated with nuclear explosions. In Chapter IV is considered the determination of the vertical wave energy flux due to a disturbance at ground level in an atmosphere containing winds, An expression is derived for vertical energy flux and numerical estimates are presented for a particular case. Problems in the aforementioned second group are studied in Chapters V and VI, The motion of a body in a medium which may be regarded as the superposition of two uniform semi-infinite layers of fluid of differing densities is considered in Chapter V. The energy lost at the fluid interface is determined for a body which moves steadily through the medium at various inclinations and speeds and in various directions. Some numerical results are presented in connection with the motion of a body which is caused to oscillate freely at the interface. In Chapter VI wave resistances to the steady motion of a body through a fluid of varying density are presented for various inclinations of the body and various speeds and directions of the motion. The work described in this thesis was done at the Department of Mathematics, Imperial College of Science and Technology, under the supervision of Professor R.S. Scorer between October 1965 and January 1968. Computer calculations were performed at the Institute of Computer Science of the University of London. Here the author wishes to express his gratitude to Dr. P.W.G. Warren, Imperial College, for his guidance and encouragement during the preparation of this thesis, Any merit possessed by the work presented herein is a result of the many hours he gave to discussions with the author. Acknowledgment is gladly given to R.A. Hamilton, former Superintendent, Kew Observatory for his invaluable aid in providing the author with the opportunity to study the microbarographic records of the Meteorological Office. The author is also grateful to the Director, Meteorological Services of Eire and to Cmdr. Thoday of the Science Museum for their cooperation. During his studies the author was recipient of a scholarship from the Defence Research Board of Canada.

In this thesis, a review is given of the development of the theory and experimental work on acoustic wave propagation in water containing air bubbles. Then: the various processes of attenuation of sound for distributions of air bubbles in a liquid, which are regarded as simulating the properties of foams, are considered theoretically by the author. The measurements of the attenuation of sound in air-liquid foams, whose parent liquids have viscosities in the range 1-10 centipoise, show an order of magnitude agreement with a theory which suggests that the attenuation is due to viscous dissipation in the bubble walls. The possibility of energy loss due to pressure relaxation in an atmosphere of saturated vapour is rather discounted by the smallness of the calculated value as compared with the overall measured attenuation. The measured values of the velocity of sound in air-liquid foams, for those with very large volume air concentrations, lie in the neighbourhood of the computed adiabatic values. This contrasts with the measurements of Karplus(14) in water containing air bubbles, of lower volume air concentrations, which agreed well with computed values for the isothermal velocity of sound.