A NOVEL PTF-BASED EXPERIMENTAL CHARACTERISATION FOR PORO-ELASTIC LINERS: METHOD AND SAMPLING CRITERION

The vibro-acoustic behaviour of structures coupled with poroelastic trims and fluid cavities can be predicted by means of the Patch Transfer Function (PTF) approach. The PTF is a sub-structuring procedure that allows to couple different sub-systems via impedance relations determined at their common interfaces. The coupling surfaces are discretised into elementary areas called patches. Since the patch impedances can be determined in either computational or experimental manner, the PTF approach offers high degrees of modularity. For sub-systems, which allow for an efficient numerical characterisation, simulation schemes should be adopted, whereas for sub-systems, which involve highly complex models, measurements might offer a cost-efficient alternative. In the first part of the work the PTF approach is presented. Next, the sampling criterion for the PTF description of the vibratory field and sound radiation from structures is discussed. When studying the sound radiating from vibrating structures the surface is discretised into elemental areas also referred to as patches. Here the special case of a simply supported baffled plate excited by a broadband point force is considered. It is shown that accurate approximation of the radiated power may be obtained well beyond the frequency limit imposed by the structural Nyquist sampling criterion, provided the complex-valued vibration field is averaged over each patch. This is due to the fact that the averaging process not only reduces the structural velocity but simultaneously determines an increased radiation efficiency. An analytical expression is provided allowing the optimisation of the patch size given a desired accuracy level for the assessment of the radiated power. In the third part of the work a novel PTF-based experimental method for the charac- terisation of poroelastic materials is proposed. The novelty of the methodology proposed consists in the fact that the trim characterisation no longer relies on a material micro- model (i.e. Biot), whose parameters are often difficult to be acquired. The specimen is considered as a single component, therefore no separation of layers is necessary. An air layer correction for surface impedance measurement is discussed. Finally, the PTF approach proposed has been applied on a laboratory test case, consisting of a trimmed plate coupled to a rigid acoustic cavity. The PTF reconstruction has been successfully validated through a measurement conducted on the fully assembled system.