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The development of fibre-optic interferometric sensor arrays for application in underwater acoustics has been an area of active research since the late 1970's. The technology has reached a level whereby prototype arrays have been successfully demonstrated in sea-trials. However, the recent development of several new technologies may significantly increase the size and performance of these arrays. We demonstrate the potential increase in multiplexed array sizes using architectures based on combining dense wavelength division multiplexing and time division multiplexing. These architectures also include erbium doped fibre amplifiers for post, pre, inline and remote amplification in order to increase the standoff distance between the array and electronics unit. We also theoretical investigate the limitations imposed on the number of sensors that can be multiplexed, due to nonlinear transmission effects in the link fibre in the presence of high optical powers and multiple wavelengths. We also demonstrate novel DFB erbium doped fibre lasers as optical sources. These sources exhibit linewidths significantly narrower than semiconductor DFB lasers, which are currently used in many sensor arrays, and thus may provide a significant improvement in sensor resolution. We investigate the intensity and frequency noise properties of these lasers, their modulation properties and successfully develop intensity noise and frequency noise reduction techniques. We also investigate the potential of fibre-optic acoustic vector sensors and demonstrate fibre-optic flexural disk accelerometers. Finally, we demonstrate polymer coated in-fibre Bragg gratings as pressure and temperature sensors and investigate polymer coatings as a means to increase the acoustic responsivity of fibre laser acoustic sensors.

This thesis is an investigation into the nature of aerodynamic noise emitted by wind turbine blades as they rotate. The main aims of this work were a follows: 1. to give an account of theoretical and experimental work on aerodynamic noise emission from aerofoils, 2. to examine experimental data from a variety of different wind turbines, and to extract from these comparisons information about the influence of different blade design parameters on noise emission, 3. to examine experimentally the influence of tip angle of attack on noise using a new method, 4. to use the information gleaned from (1) and (2) to write a noise prediction program which can be used for any design of horizontal axis wind turbine operating under any reasonable conditions, 5. to compare the predictions with experimental data. The first three chapters consist of background information, concerning the development of wind farms, the noise regulations with which they must comply and the theory of aerodynamic noise emission. The following four chapters contain new work, the principal findings being: 1. Comparisons of experimental noise data from different designs of wind turbines indicate that low frequency noise (less than 250 Hz) is the most sensitive to tip speed, while frequencies between 250 and 630 Hz are less affected, and frequencies in the kHz range are almost unaffected by tip speed. This conflicts with existing noise prediction codes. 2. Noise may increase slightly with tip angle of attack, but this is not pronounced for modern blades with sharp trailing edges. However, there is a significant reduction in high frequency noise (above 1.5 kHz) as the tip angle of attack is reduced in constant winds. In addition, for the turbine studied, a broad aerodynamic peak centred on 630 Hz was observed when the tip angle of attack was reduced. 3. A rotor noise prediction code has been written, which gives accurate results over a range of tip speeds.

Two different optical techniques are applied to the measurement of the acoustic particle velocity in velocity fields. Firstly, the measurement of both the acoustic particle velocity and the mean flow velocity using Laser Doppler Anemometry is described. Secondly, the Particle Image Velocimetry method is investigated to determine the possibility of measuring the acoustic particle velocity and then both the acoustic particle velocity and the mean flow velocity. For the first case, consideration is given to the conditions necessary to obtain both of these parameters. The theory was then developed to allow the analysis of the Laser Doppler Anemometry signal. Experiments are then described which test the technique and some successful measurements are made. In the second case, the conditions for the use of Particle Image Velocimetry is discussed and the theory is developed. The theory is compared against computationally constructed images in order to test the range and accuracy of the possible ways of doing the measurement. Experiments are then done which show that the measured values agree well with the Theory.

Laser Doppler Anemometry (LDA) has been used to study underwater acoustic signals both from emitting hydrophones and underwater explosions. A dual-beam LDA arrangement was used to capture Doppler signals arising from light scattered from particles suspended at the point of interest in the flow. These Doppler signals are analysed using either Hilbert transforms or wavelets, both of which allow instantaneous frequency information to be obtained. When an acoustic signal propagates through a medium it creates refractive index variations within the medium. The apparent motion of the scattering particles, as observed by the detector, which give rise to the Doppler signal, is therefore made up of two components. Firstly, the particles oscillate due to the sound field and secondly, the interference fringes oscillate due to the refractive index variations. This is termed the acousto-optic effect. A theory has been developed to investigate the effect of these refractive index variations on the analysed Doppler signals of an LDA system. Analysis of experimental Doppler signals using the Hilbert transform technique shows close agreement with the theoretical predictions. LDA has also been used to investigate the acoustic signal emitted by an oscillating explosion bubble. This is generated by an underwater spark which creates a similar situation to an underwater explosion in which a shock wave and an oscillating bubble are produced. Analysis of the Doppler signal using wavelets provides information on the bubble period, radius, energy and particle velocity. Explosive materials have traditionally been used for investigation of underwater explosions but they have the disadvantage of obscuring the area with explosion debris thus making optical investigation difficult. It is shown in this work that the use of LDA and analysis of Doppler signals using wavelets is an accurate technique for the investigation of acoustic signals from underwater explosions. This allows investigation of the area close to the explosion centre where measurements have been difficult to achieve with traditional techniques.

The self-sustained oscillation of a brass wind musical instrument involves a complex aerodynamic coupling between a multimode mechanical vibratory system (the lips of the player) and a multimode acoustical vibratory system (the air column of the instrument). In this thesis the behaviour of the coupled system near the threshold of oscillation is investigated using a simplified model in which a single mechanical lip mode is coupled to a single mode of the acoustical resonator by air flow through the lips. The theoretical threshold behaviour is compared with the measured threshold behaviour of a trombone sounded by an artificial lip reed mechanism. Comparability between theory and experiment is ensured by using model parameter values derived from mechanical response measurements on the artificial lips and input impedance measurements on the trombone. The mechanical response measurement can be used to classify mechanical models of the artificial lips unambiguously as either "inward striking" or "outward striking". Each of the embouchures considered is found to have at least one mechanical mode of each category. The experimentally observed threshold frequencies of the coupled system suggest a behaviour which passes smoothly from "inward striking" to "outward striking" character as the trombone slide is extended or the embouchure parameters changed. It seem unlikely that this type of behaviour can be explained using a lip model with only a single degree of freedom. After a discussion of the theory of laser Doppler anemometry (LDA), the technique is applied to the problem of measuring the instantaneous acoustic particle velocity within a standing wave pipe driven by a loudspeaker. The resulting Doppler signals display quasi-periodic amplitude modulation with a fundamental frequency equal to the frequency of the acoustic field. The phenomenon of amplitude modulation is investigated in some detail. Two different methods of analysing Doppler signals are compared: the digital Hilbert transform and the Disa analogue frequency tracker; the analogue tracker is found to offer the greater signal-to-noise ratio and dynamic range. Experiments are carried out to establish how phase errors introduced by the analogue tracker can be minimised.

The radiation of sound from the open end of a resonant tube is relatively well understood at low acoustic amplitudes in terms of linear acoustics. At high acoustic amplitudes, however, additional non-linear loss mechanisms such as vortex shedding and jet streaming are known to affect the sound field at the open end and the efficiency with which a standing wave is maintained within a tube. This has implications in a number of fields, notably in musical instrument design and in the automotive industry. Particle Image Velocimetry (PIV) is used to acquire full-field instantaneous velocity information of the acoustic particle velocity field at the open end of a cylindrical tube in which a high amplitude should field has been generated by a loudspeaker. Five different tube terminations are used to examine the effect of varying the geometry of the open end on the flow phenomena observed. A qualitative identification of number of flow regimes is made and a number of techniques are used to evaluate the non-linear losses numerically. PIV results are used in combination with pressure measurements to evaluate the termination impedance and a comparison is drawn with results made by conventional methods. Jet streaming is investigated and the associated energy dissipation is estimated. Vortex sound theory is then used to measure the acoustical losses caused by boundary layer separation and the resultant generation of vortex structures.

The natural frequencies of piano strings depart somewhat from the harmonic series and the degree of inharmonicity has important implications for tone quality, tuning and the electronic synthesis of piano sounds. Apart from effects due to the finite compliance of the supports, the stiffness of the steel wire from which piano strings are made accounts almost entirely for the inharmonicity of the plain wire strings. It has been shown, however that the string stiffness is not the only source of inharmonicity of the overwound piano strings. Not only the effects of wave-reflection at the terminations of the various copper covering layers of overwounds strings, but also the effects of nonuniformity may contribute. Weak partial cannot be explained by string stiffness alone. Some discussions on the stepped string have appeared over the last few years by Levinson, Sakata and Sakata, and Gottlieb, but their analyses have not incorporated the stiffness of the stepped string. In this thesis, an expression for the frequencies of vibration of a stepped overwound string was described, and numerical calculations have been undertaken to compute theoretical mode frequencies for strings with varying degrees of overwinding. The numerical results of the frequency equation were compared with data from the experiment. The experiments of the inharmonicities of overwound string on a rigid monochord have been measured. The rigid monochord has been designed in order to control the parameters and to reduce external effects disturbing the vibration of the strings. It is evident from the comparison that the theory presented here gives a better fit to measured inharmonicities than Fletcher's analysis for a uniform string. The original motivation for this study was to determine the extent to which the non-uniformity of the overwinding on a bass piano string affected the inharmonicity of its mode frequencies. To examine the extent to which this work was relevant to the behaviour of overwound piano strings with the end support conditions typical of normal use, a series of measurements was performed on the bass strings of a Broadwood grand piano. It is evident from the results that the major cause of the discrepancy between the Fletcher prediction and the measurement is indeed the non-uniformity of the winding.

This thesis is concerned with sonar signal processing, in particular the statistical characterisation of sonar data. It proposes a number of signal processing methods that are suitable for sonar data. More specificity it deals with the signal processing of time series, from a sonar system, which were collected during a number of experiments in the Baltic Sea. The return signals in sonar can be viewed as a mixture of deterministic and stochastic parameters. Consequently, instead of trying to model the wave propagating environment itself this thesis will illustrate a variety of characteristic properties of the signals and propose suitable methods for solving the problems. In Chapter 3 a statistical characterisation of sonar data is presented, different aspects of statistical properties of sonar data are addressed. The focus is on the level of stationarity of both active and passive sonar. For active sonar the issue of ping to ping stationarity, for both the actual ping and for the reverberation tail, are examined and found to be non-stationary. However, for some cases it is seen that there are consecutive pings that are stationary. This leads to the conclusion that it can be beneficial to use several pings from the same target. However it is necessary to remember that there is a large variation in the number of consecutive stationary pings. For passive sonar both ambient noise and tonals emitted from surface vessels are investigate. It is found that for all cases tested the sonar data is non-stationary. That is speaking of stationarity in the strict sense. There are however parts in the data that exhibit more stationary behaviour then the data in general. The stationarity length of the ambient noise data is also examined, using data from a multisensor trial (almost 800 data files, see Section 2.5.1). A large proportion of the data set had a stationarity time of roughly 0.4 seconds, or slightly longer. This is also seen in data from the fibre glass boat trial (Section 2.5.6). The data seems to be stationary for about 0.4 seconds. Testing the passive data for symmetry and linearity show that the data is mostly linear, and symmetric.

Non-linear effects are known to occur in high intensity sound fields and have important consequences in the study of a diverse range of applications from medical imaging to musical instruments. Particle Image Velocimetry (PIV) is used to obtain full-field instantaneous velocity information without physical intrusion into the acoustic field. After a review of the theory of PIV, the technique is applied to the investigation of non-linear effects on acoustic streaming in a closed-end cylindrical tube. A distortion of the streaming motion is observed with increasing sound intensity and a comparison to theoretical work is presented. PIV is then used in conjunction with pressure measurements to examine the non-linear behaviour at the termination of a open-ended cylindrical tube. The results are compared to acoustic measurements of the non-linear radiation impedance using conventional techniques. The work is then extended through the introduction of side holes into the open tube. PIV measurements of the streaming motion as a function of frequency and intensity are made and used to draw comparisons with the streaming behaviour present at the toneholes of a modern Boehm flute under normal playing conditions. The results from this work clearly show that PIV has the potential to provide a better understanding of acoustical fields in situations where non-linear effects can have a significant influence, such as that found on musical instruments.