Glegg, Stewart A. L.

Ocean ambient noise has been studied extensively in the past, and several models have been developed to predict its level and to characterize it in different ocean environments. However, most of these studies have been motivated by the need to reduce the negative effect of the ambient noise on measurements. Recently, several studies have used the ambient noise to obtain information about the ocean floor and to achieve passive target detection. This dissertation describes the development of an Ambient Noise Sonar (ANS) which can be used to measure ambient noise directivity to obtain information about the environment. The system was designed for use on an Autonomous Underwater Vehicle (AUV) and has been used to study the ambient noise in the near shore region by investigating different types of noise in shallow water, such as boat traffic, biological noise and naturally occurring noise up to 20 kHz. There are two advantages to the ANS. First, when mounted on an AUV its mobility will allow the ambient noise to be mapped over an extended area. Second, the very small number of transducers (6 total) used in the array, makes this system suitable for AUV operations because it uses a limited amount of space and power. This dissertation presents the theory of the array processing and preliminary results including examples of boat noise, and biological noise. It was found that the noise from biological sources such as snapping shrimp contributed to the anisotropic component of the acoustic field. These biological sources were found to be concentrated around artificial structures such as piers and groins, and also around natural reefs. Finally noise maps of coastal areas are presented to illustrate the potential of this system to measure the noise field in the coastal region and to evaluate acoustic propagation using passive sources such as the biological noise clusters found along the coast. In the future the ANS will be integrated onto the AUV 'Ocean Explorer II'.

The acoustic field in the ocean is difficult to model theoretically, due to the complexity of the environment. This is particularly true if the water depth is range dependent, such as in the coastal region, where a fully three dimensional description is required. Propagation effects result in horizontal refraction, shadow zones, and the existence of regions with strong interference patterns. As a result, all of the existing theoretical models are based on significant simplifying assumptions. One such assumption is to model a region of the ocean as a water column overlying a planar sloping bottom. To test the accuracy of these theories model scale measurements of the acoustic field under highly controlled conditions have been undertaken in this study. Two experiments were performed on models with a sloping bottom. The first model consisted of a fast fluid bottom, and the second model consisted of a thin epoxy layer, to model a sediment, overlying a concrete layer, which modelled the substrate rock. The measurements performed included pulse, CW traverse, and depth profile measurements in both the across slope and down slope directions, in order to demonstrate the three dimensional features of the field. The features of the results are discussed and where possible are compared with existing theories. The results indicated that the three dimensional propagation effects in a fluid bottom wedge are described accurately by a theoretical model which uses an effective depth correction. No three dimensional theory was available for the shear wave supporting bottom case but the fluid bottom theory was found to provide accurate predictions. Down slope propagation over a shear wave supporting bottom was also shown to be accurately predicted using a two dimensional finite element parabolic equation code.

This thesis considers the effect of scattering on the sound radiation from rotating sources. The study is carried out using a combined numerical implementation of ray acoustics and the paraxial ray approximation. A detailed description of the theoretical background to these methods is presented, along with a description of their numerical implementation. Application of the method to classical problems is considered to prove the accuracy and the power of the approach. Application of the method to some typical problems involving scattering of noise from propellers and rotors is presented. It is found that for impulsive acoustic signatures the scattering effects are important especially in the sideline direction from a helicopter fuselage. The effects of sharp edges on the steady loading noise from tilt-rotor configurations indicates that there is a new mechanism for generating impulsive acoustic signatures caused by scattering by sharp edges of the fuselage. The acoustic signatures generated by this mechanism can appear very similar to other types of impulsive source generated by aerodynamic interactions on the blade and therefore must be important. This type of source can be eliminated if the fuselage has rounded edges. Flow effects on scattering problems have also been considered. It was shown that the flow causes a modification and displacement of the lobes of the directivity pattern and the shadow zone, which can be important at Mach numbers greater than 0.2. The main conclusion of this thesis is that scattering effects cannot be ignored for highly directional rotating sources next to rigid scattering objects as is always the case for propellers and helicopters rotors.

A measurement method for the equivalent noise source height of an arbitrary distribution of moving noise sources is developed to investigate the highway vehicle equivalent source heights which are currently used by the FHWA for noise barrier design. The study is intended to provide information required to improve this noise barrier design code. The equivalent point source position is defined for an arbitrary distribution of acoustic sources above a reflecting plane and a method for its measurement using a microphone array is developed. The normalized errors of the measured equivalent source heights are defined including the effects of background noise, the geometric near field, source size, and source directionality. Normalized errors of the measured source heights obtained by a numerical simulation for each parameter lead to optimization of the microphone spacing and to the design of an array of microphones which give the equivalent source height as a function of frequency. The method is then applied to the measurement of the equivalent source height of stationary loudspeakers and is shown to give results which are consistent with theoretical predictions. The effect of the Doppler frequency shift of moving vehicles is investigated using a loudspeaker mounted on the roof of a moving car. If the speed of a moving noise source is less than 97 kph, the error due to the Doppler frequency shift was found to be negligible. Results of measurement on highway vehicles show that the noise source heights for heavy trucks and for automobiles are different from those used currently by the FHWA.