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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:303943 |
| Date | January 1992 |
| Creators | Khirnykh, Konstantin |
| Publisher | London South Bank University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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