Low-frequency noise is a health concern for workers in industrial workshops; rooms of highly varying size and dimensions, usually containing obstacles (the ‘fittings’). Low-frequency noise can be generated from sources such as reciprocating or rotating machinery, or ventilation systems. As the exposure time to the noise lengthens, workers are increasingly at risk to harmful effects such as hearing loss, communication difficulty, personal discomfort, and even nausea from induced body vibrations. Passive methods of noise control, such as absorption or barriers, generally perform better at high frequencies, but are inadequate at low frequencies. A proposed solution is active noise control, which relies on destructive interference of sound waves to reduce noise levels. However, this depends on phase, and how it is affected when sound waves encounter diffracting obstacles. In addition, the geometrical configuration of the active-control system must be optimized, which can be done using a prediction model. Sound-prediction models can also estimate the decibel level of sound within a given room configuration created by a source and the attenuation provided by the control system. Therefore, it is of interest to develop a model that predicts sound propagation in fitted rooms with phase. In this thesis, sound-pressure fields were investigated in rooms containing parallelepiped obstacles at low frequencies for which the wavelength is comparable to the obstacle dimensions. The geometric theory of diffraction (GTD) was used to model edge diffraction from an obstacle and, thus, the pressure field in shadow regions. A ray-tracing prediction model was improved to consider both the amplitude and phase of sound fields, and also the effects of edge diffraction. To validate the prediction model, experiments were performed in an anechoic chamber where a source and diffracting objects were located. In collaboration with Dr Valeau at the Université de Poitiers in France, a second model based on the finite element method (FEM) was used to compare prediction results. It was found that the phase depends mostly on the direct unblocked source-to-receiver distance. The FEM and experimental results showed that occluding objects cause phase shifts. The implementation of first-order diffraction into the ray-tracing program was successful in predicting shadow zones, thus producing a better prediction of realistic sound fields in rooms with obstacles.

Thesis (doctoral)--Helsinki University of Technology, 1999. / Includes bibliographical references. Also available on the World Wide Web.

Thesis (M. Econ.(Res))--University of Queensland, 2002. / Includes bibliographical references.

No description available.

Low-frequency noise is a health concern for workers in industrial workshops; rooms of highly varying size and dimensions, usually containing obstacles (the ‘fittings’). Low-frequency noise can be generated from sources such as reciprocating or rotating machinery, or ventilation systems. As the exposure time to the noise lengthens, workers are increasingly at risk to harmful effects such as hearing loss, communication difficulty, personal discomfort, and even nausea from induced body vibrations. Passive methods of noise control, such as absorption or barriers, generally perform better at high frequencies, but are inadequate at low frequencies. A proposed solution is active noise control, which relies on destructive interference of sound waves to reduce noise levels. However, this depends on phase, and how it is affected when sound waves encounter diffracting obstacles. In addition, the geometrical configuration of the active-control system must be optimized, which can be done using a prediction model. Sound-prediction models can also estimate the decibel level of sound within a given room configuration created by a source and the attenuation provided by the control system. Therefore, it is of interest to develop a model that predicts sound propagation in fitted rooms with phase. In this thesis, sound-pressure fields were investigated in rooms containing parallelepiped obstacles at low frequencies for which the wavelength is comparable to the obstacle dimensions. The geometric theory of diffraction (GTD) was used to model edge diffraction from an obstacle and, thus, the pressure field in shadow regions. A ray-tracing prediction model was improved to consider both the amplitude and phase of sound fields, and also the effects of edge diffraction. To validate the prediction model, experiments were performed in an anechoic chamber where a source and diffracting objects were located. In collaboration with Dr Valeau at the Université de Poitiers in France, a second model based on the finite element method (FEM) was used to compare prediction results. It was found that the phase depends mostly on the direct unblocked source-to-receiver distance. The FEM and experimental results showed that occluding objects cause phase shifts. The implementation of first-order diffraction into the ray-tracing program was successful in predicting shadow zones, thus producing a better prediction of realistic sound fields in rooms with obstacles.

No description available.

Thesis (MEng(MechanicalEngineering))--University of South Australia, 2002.

Local active noise control systems aim to create zones of quiet at specific locations within a sound field. The created zones of quiet generally tend to be small, especially for higher frequencies, and are usually centred at the error sensors. For an observer to experience significant reductions in the noise, the error sensors therefore have to be placed relatively close to an observer’s ears, which is not always feasible or convenient. Virtual sensing methods have been proposed to overcome these problems that have limited the scope of successful local active noise control applications. These methods require non-intrusive sensors that are placed remotely from the desired locations of maximum attenuation. These non-intrusive sensors are used to provide an estimate of the sound pressures at these locations, which can then be minimised by a local active noise control system. This effectively moves the zones of quiet away from the physical locations of the transducers to the desired locations of maximum attenuation, such as a person’s ears. A number of virtual sensing algorithms have been proposed previously. The difference between these algorithms is the structure that is assumed to compute an estimate of the virtual error signals. The question now arises as to whether there is an optimal structure that can be used to solve the virtual sensing problem, which amounts to a linear estimation problem. It is well-known that the Kalman filter provides an optimal structure for solving such problems. An optimal solution to the virtual sensing problem is therefore derived in this thesis using Kalman filtering theory. The proposed algorithm is implemented on an acoustic duct arrangement to demonstrate its effectiveness. The presented experimental results indicate that the zone of quiet was effectively moved away from the physical sensor towards the desired location of maximum attenuation. The previously proposed virtual sensing algorithms have been developed with the aim to create zones of quiet at virtual locations that are assumed spatially fixed within the sound field. Because an observer is very likely to move their head, the desiredlocations of the zones of quiet are generally moving through the sound field rather than being spatially fixed. For effective control, a local active noise control system incorporating a virtual sensing method thus has to be able to create moving zones of quiet that track the observer’s ears. A moving virtual sensing method is therefore developed in this thesis that can be used to estimate the error signals at virtual locations that are moving through the sound field. It is shown that an optimal solution to the moving virtual sensing problem can be derived using Kalman filtering theory. A practical implementation of the developed algorithm is combined with an adaptive feedforward control algorithm and implemented on an acoustic duct arrangement. The presented experimental results illustrate that a narrowband moving zone of quiet that tracks the desired location of maximum attenuation has successfully been created. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1291123 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2007.

Thesis (M.S.)--West Virginia University, 2008. / Title from document title page. Document formatted into pages; contains iii, 44 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 42-44).

Thesis (M. Eng. Sc.)--University of Adelaide, Dept. of Electrical and Electronic Engineering, 1986. / Includes bibliographical references (leaves 221-232).