The health, safety, comfort and productivity of a room’s occupants is greatly influenced by the sound field within it. An acoustical engineer is often consulted during the design of a room to prevent or alleviate unwanted acoustical problems. Prediction models are often used to find the most cost-effective solution to a given acoustical problem. The accuracy of sound-field prediction varies with the particular model, as do the parameters predicted. Most models only predict sound-pressure levels. Many only predict energetic quantities, ignoring wave phase and, therefore, interference and modal effects in rooms. A ray-tracing model, capable of predicting sound-pressure level, reverberation time and lateral energy fraction was translated into MATLAB code and modified to increase accuracy by including phase. Modifications included phase effects due to path length travelled and phase changes imparted by surface reflections as described by complex reflection coefficients. Further modifications included predicting steady-state and transient sound-intensity levels, providing information on the direction of sound-energy flow. The modifications were validated in comparison with free-field theory and theoretical predictions of sound fields in the presence of a single surface. The complex reflection coefficients of four common building materials were measured using two methods—an impedance tube and the spherical-decoupling method. Using these coefficients, the modified program was compared with experimental data measured in configurations involving one or more surfaces made of these materials, in an anechoic chamber, a scale-model room, and a full-scale office space. Prediction accuracy in the anechoic chamber, and in the presence of a single reflecting surface, greatly improved with the inclusion of phase. Further comparison with full-scale rooms is required before the accuracy of the model in such rooms can be evaluated definitively. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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This research thesis involves the measurement of the airborne sound insulation of road traffic noise barriers, with the goal of gaining a more in depth understanding of the factors that influence noise barrier performance. A measurement system is developed, based on EN 1793-6:2012, to quantify the airborne sound insulation of a noise barrier in situ. Validation testing is performed to ensure that the system meets the requirements of EN 1793-6:2012. MATLAB code is developed, incorporating all of the signal processing tasks into a single graphical user interface. The measurement system is then used to measure the airborne sound insulation of eight existing traffic noise barriers located around Auckland, New Zealand. The results from the Auckland field tests show that consistent single number ratings of airborne sound insulation can be achieved on different samples of the same noise barrier. The presence of air gaps and hidden defects will degrade the acoustic performance of a noise barrier, most significantly at the high frequencies. The comparison of single number ratings calculated with differing measurement frequency ranges is discussed, and some comments are made on the measurement standard itself.

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A prominent feature of many practical flows is the hydrodynamic wave system attached to moving bodies or concentrations of vorticity. Sound waves are usually present, and these act as a mechanism for energy and momentum transport. With their source rooted in the unsteadiness of the flow, they can sometimes play an important role in determining the general flow structure, particularly if the flow is unstable. In this thesis we investigate the basic connection between sound, and hydrodynamic waves. By analysing the waves attached to boundaries which are in prescribed unsteady motion, details emerge concerning the linear production of sound from hydrodynamic motions. We show that the abrupt arrest or commencement of a steady hydrodynamic wave causes the production of a quantity of sound energy exactly equalling that of the hydrodynamic wave. For more gentle modulations of the steady state, we identify those aspects of the evolving hydrodynamic field which determine how much sound is produced. These results are used to suggest ways to improve procedures for minimising the noise from vibrating surfaces. According to linear theory, when waves on an infinite fluid boundary travel at sonic speed the fluid response is infinite. We use the ideas developed to cope with the sound generation problem to investigate the effects of unsteady transonic motion. We give a detailed analysis of acoustic 'Cerenkov radiation', which would occur if a body travelled through an inviscid medium supersonically, and decelerated to a subsonic speed. We assess the degree to which non-linear transonic effects are important. Sound waves are known to be a critical factor leading to the destabilisation of line vortices, and we were intrigued to know whether compressibility has a corresponding effect on the stability of a rigid body moving steadily in an irrotational, inviscid flow. Our investigation reveals that the motion is always stable.

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