The development of non fibre-filled resonant sound absorbing systems.
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