The development of a novel type of resonant sound absorber is described, designed specifically for the absorption of sound at low frequencies and at high intensities. A review of previous work on resonant sound absorbers shows that existing theoretical models for describing the phenomena are incomplete and that there remains a need for further extensions to the relevant models to cater for non-linear effects which become particularly important at high intensities. It is also shown that there are limitations to the current methods for testing absorbers which make them less suitable at low frequency and when the signals are nonharmonic. In the present work a theoretical model of a Helmholtz type of absorption resonator working in the nonlinear regime is developed using nonlinear hydrodynamic equations for viscous incompressible fluids. The model is able to predict the input impedance, the resonant frequency and the absorption coefficient of the device under nonlinear conditions. The model is also used to account for "difference frequency" generation, the reflection of signals of any shape from the surface, and the isolation characteristics of this type of absorber when a porous layer is fixed to the back of the resonant cavity. A new method for the development of acoustic characteristic~ of resonant sound absorbers is described, which was developed as part of the present work and which overcomes the limitations of existing methods. Measurements carried out using this and other techniques show that the acoustic characteristics of the absorber described are very close to those predicted from the model. A new type of packless absorber (one without fibrous material) based on the above theoretical model is described. Such an absorber has been constructed and tested in a reverberant sound field and is shown to provide effective sound absorbtion under conditions typical of a working industrial environment.

An inadequate level of noise attenuation provided by a building element is frequently the result of a lack of completeness in the construction. This often invisible fault acts as a noise source in a room, so in order to undertake remedial work the source position must be found. Recently, near field noise intensity measurement has been the popular method for noise source location in buildings. This method of using intensity studies requires a grid of readings to be taken. An alternative method, the one used in this work, employs a different strategy. Here, the source location is identified by direction scanning of time delays at a number of microphones arranged in a regular three-dimensional array. A novel arrangement of seven microphones, in the shape of a wheel-brace, is used to measure the differences in time taken for the sound waves to travel from a source to the various microphones. The magnitudes of these time differences are combined and converted into the coordinates of the source, relative to an origin which is placed at the centre of the wheel-brace array. The mathematics for this conversion is derived and the errors in the experimental arrangement discussed. The use of this airay for the identification of faults in built structures is explored. A significant contribution is made to the knowledge of noise source location in buildings, since the microphone array is used to demonstrate the location of a noise source irrespective of the direction of the incoming noise. The use of computerised data collection is described for a budget system, where time was cheap, but equipment expensive. The accuracy of the technique would be improved considerably if state-of-the-art electronics were used to measure the lime differences. The feasibility, advantages and potential performance of a modem system, that could be assembled today, is described and discussed.

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Active noise control, an alternative to conventional passive control, duplicates an existing noise field in antiphase so that when the noise and its antidote are added together the result is silence. This dissertation describes work aimed at implementing active control techniques on various noise problems. As each one was successfully completed confidence grew that the technique is on the verge of becoming a potential reality. The first project demonstrated the cancellation of the discrete frequency sound produced by a pair of large Electricity board transformers; the noise in a nearby office was reduced by some 20 decibels. Initial attempts at broadband cancellation demonstrated the necessity for an adaptable form of filtering. Digital techniques were developed to optimally control one-dimensional sound waves in an air-conditioning duct: some 20 decibels than three octaves of random sound. Their application to a practical problem at the Royal Signals and Radar Establishment, Malvern demonstrated their simplicity and. flexibility: it took only a day to design, and construct a device that stopped the low-frequency sound produced by a Diesel generator from entering an anechoic chamber. Further development has led to. control algorithms for a broadband adaptive controller. Its rapid convergence was demonstrated in an experiment which stopped the sound of a wind tunnel fan from disturbing sensitive measurements of turbulence in the test section. The control of three-dimensional sound presents many practical problems. One possible approach to these problems was presented by Olson in 1953 who described a device for cancelling local noise fields in order to produce a quiet zone in a noisy environment. We built an optimal form of that device and demonstrated some of its potential but its direct incorporation as one of many in an array was shown to result in an unstable system. Slight modification of that system has lead to the design of a stable array that effectively reflects sound. An investigation was carried out for Rolls-Royce into the possibility of using such an array to stop the broadband, lowfrequency sound, produced by two RB211 engine test-beds, from disturbing people in a large office 250 Metres away. The scale of the. solution was beyond the scope of a PhD. project. Finally a combination of all the techniques developed have been used to assess the possible performance of an active control system in a Fleet Street press room. The predictions are likely to be tested in a demonstration which is currently being planned. This research has helped reveal the major problems inhibiting the application of active noise control methods. Most of these have been overcome so that it should now be possible for the solutions outlined in this thesis to be applied relatively easily to design reliable and effective noise control systems.

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