Measurements of ultrasound wave velocity and attenuation are used to investigate the structure of Na(_2)O-B(_2)O(_3)-SiO(_2) glasses. The propagation characteristics of longitudinal and shear waves between 1.3 K and 400 K at frequencies between 12 MHz and 60 MHz are dominated by a broad and intense acoustic loss peak whose height and position are frequency sensitive. Of the previously proposed models for the mechanism of the acoustic loss, which also occurs in other inorganic glasses, the ultrasound absorption is most consistent with a thermally activated structural relaxation involving the transverse vibration of an oxygen atom between two potential wells of equal or nearly equal depth in the plane of a non linear cation-oxygen-cation bond. An Akhieser type acoustic phonon-thermal phonon interaction is shown no to be responsible for the observed loss. An attempt frequency of l0(^13)Hz and a distribution of activation energies out to 12 K cal/mole but with a mean value of about 3 K cal/mole are found for the relaxation mechanism in the Na(_2)O-B(_2)O(_3)-SiO(_2) glasses. The absolute value and the temperature coefficient of ultrasound velocity, and the maximum acoustic loss are strongly dependent on the total Na(_2)O content of the glasses. Ultrasound propagation characteristics are also affected by phase-separation inducing heat treatment: the steady rise in the height of the loss peak and the complex behaviour of the ultrasound velocity with time of treatment suggest that structural rearrangement is still taking place in the individual glassy phases even after long periods of heat treatment. Also reported is the existence of a small acoustic loss peak at liquid helium temperatures in the Na(_2)O-B(_2)O(_2)-SiO(_2) glasses. This feature of the ultrasound absorption spectrum is characteristic of many tetrahedrally coordinated inorganic glasses. On the assumption of an Arrhenius activation process for this loss peak, an activation energy of 60 ± 15 cal/mole and an attempt frequency of 10(^10) to 10(^12) Hz is indicated.

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High-Intensity Focused Ultrasound (HIFU) has been applied as an acceptable therapeutic technique and various HIFU systems have achieved clinical approval for general use in numerous countries. To ensure that an HIFU system is used in a safe and reliable application, its acoustic field needs to be characterized accurately. However, the traditional methods mostly are based on hydrophones, these sensors are found short of sufficient temporal and spatial resolutions, especially, they can only tolerate small acoustic pressures. This work is concerned with the feasibility of optical interferometry in measuring focused acoustic fields and in predicting HIFU fields.

A Laser Doppler Instrument has been developed for the measurement of vibrations on moving objects. A particular interest has been taken in rotating turbine blades. The technique used is similar to the well-known microwave Doppler radar system, but the many elements in an optical counterpart have been investigated. The analysis of the coupling network between photodetector and following amplifier, and the optimization of the optical receiver para-meters such as beam splitting and current gain are amongst the main subjects of this work. The signal and noise performance has been fully specified by means of a newly proposed Generalized System Noise Figure, Fs*, which is expressible as a function of a few simple, measurable parameters. Using this Noise Figure a comparison between different systems and photodetectors such as photomultiplier, p-i-n and avalanche photodiodes, has been carried out and in conclusion the avalanche photodiode is recommended. The minimum measurable vibration velocity depends on the Doppler broadening, and the optimum optical arrangement is calculated to minimize this effect. Very closely related to the main work is the analysis of the performance of an avalanche photodiode used as an electronic mixer. The results of measurements in the laboratory prove that it is feasible to measure vibrations on moving objects using laser Doppler techniques and that the analysis developed satisfactorily describes the performance of the system.

In the pages that follow I have attempted to give a systematic description of some findings made during my investigation into the acoustic equations in flows at low Mach number. The practical justification for this study is the need to understand how noise produced by aircraft at take-off or landing propagates from sources, such as the engines, to people living around airports. We first construct a method for deriving a single acoustic equation which is correct to any desired order in the Mach number of the flow around the aircraft. Then we discuss the difficulties which exist when we try to express the acoustic pressure, for example, as a power series in the Mach number at high frequencies. We illustrate the problems involved by a detailed analysis of scattering by a region of fluid in solid body rotation. We go on to show how these difficulties can be circumvented in the case of potential flow by the application of a novel transformation. The considerable simplification achieved by the transformation enables a straightforward connection to be made between the acoustic field with flow to that without flow. This result leads me to question the generally accepted use oi emission time coordinates for describing aircraft noise measurements. I argue strongly that such coordinates far from clarifying the effect of flow on aircraft noise will in fact make it harder to interpret the changes brought about by the aircraft's flow field. My final conclusion is that emission time coordinates should be replaced by coordinates fixed in the aircraft body for the presentation of noise data.

A series of diffraction problems in acoustics and electromagnetism are considered, and solved by various integral equation techniques. A general introduction to the method of approach is given in Chapter 1. The major problem under investigation is that of a vibrating piston mounted in a finite, acoustically rigid baffle, radiating sound, from one face only. This is solved for low frequencies in Chapter 2 by reducing the problem to the solution of a Fredholm integral equation of the second kind. Numerical techniques are used to solve this equation. The following three methods for high frequencies are then employed, in Chapter 3: (i) an approach leading to an integral equation of the second, kind, which is suitable for iterative solution at high frequencies, (ii) a Wiener-Hopf technique, and (iii) an application of Keller's geometrical diffraction theory. Far field amplitude and radiation impedance results are presented. Chapters 4 and 5 contain three cases of high frequency diffraction by a circular disc of various incident fields. These are the problems of (i) a point source of sound situated above acoustically rigid disc. (ii), an oscillating electric dipole situated above a perfectly conducting disc and oriented (a) perpendicular to the plane of the disc, and (b) parallel to the plane of the disc. The solutions are obtained by the first high frequency method mentioned above for the baffle problem. Asymptotic expansions for the far field amplitudes are given in each case. Finally, Chapter 6 is concerned with the problem of diffraction of a plane acoustic wave by a general, three-dimensional, rigid body. Certain methods are employed in an attempt to obtain the first two terms of the scattering cross section involving only the solution to one simple potential problem. The particular case of a rigid ellipsoid is discussed in detail.

Interaural Timing Differences (ITDs) are a cue for sound localisation. In response to low-frequency sounds, sensitivity to ITDs can be conveyed by the fine-structure of the sound waveform. In response to high-frequency sounds, sensitivity to ITDs can only be conveyed by the amplitude modulated envelope of the sound waveform. Sensitivity to ITDs within high-frequency sounds has classically been described as poorer than in response to low-frequency sounds. However, using a "transposed" sound stimulus, it has been shown that human sensitivity to ITDs in high-frequency sounds can be equivalent to sensitivity to ITDs in low-frequency sounds. In the present study, sensitivity to ITDs was investigated in the responses of neurons from the Inferior Colliculus of the guinea pig using transposed, and conventional, stimuli. A neural correlate of the improvement in sensitivity to ITDs provided by transposed tones was found. ITD-tuning functions had greater depths of modulation in response to transposed tones as compared to conventional stimuli, and neural discrimination thresholds for ITDs in transposed tones were similar to those obtained in response to low-frequency tones. Neural coding of ITDs at low frequencies has been shown to depend on a neuron's frequency tuning. Therefore, the responses of neurons were examined for evidence of frequency-dependent tuning to ITDs in the envelope of high-frequency stimuli. The frequency-dependent ITD-tuning that was found contradicts a model of ITD coding proposed in 1948 by Jeffress. ITD-coding at high-frequencies, similarly to at low- frequencies, may use a population of neurons which are broadly tuned to ITDs. It is suggested that sensitivity to ITDs in the envelope of high-frequency sounds is restricted both by peripheral processing and also by an upper fm above which sensitivity to ITDs does not occur. For these reasons, the physiological relevance of sensitivity to ITDs in the envelope of high-frequency may be limited.

This Ph.D. is concerned with wave propagation problems. The main focus is on nonlinear acoustics, looking at sonic boom propagation in a physically realistic atmosphere, whilst a secondary part will look at the problem of landmine detection and how to improve the target detection rates. The work on nonlinear acoustics emerged as a desire to model the behaviour of the sonic booms formed by supersonic aircraft in the atmosphere to see what environmental impact they would have on people and animals on the ground, in terms of the form of the sound waves once they reach the ground. The work on landmine detection originated from a Knowledge Transfer Partnership between the University of East Anglia (UEA) and Cobham Technical Services (CTS) organised through the Knowledge Transfer Network (KTN). This partnership took the form of a six month internship with work undertaken afterwards to publish the findings of the internship.