Samples of porous ceramics Al_2O_3, manufactured by a promising technology of gelcasting of cellural foams by using biopolymers as gel-formers, are examined in the impedance tube using the transfer function method. It is shown that the ceramics of total porosity around 90% forms an excellent sound absorbing material in the frequency range from 500 Hz to 6.4 kHz. Experimentally-determined curves of acoustic impedance and absorption are then used for inverse identification of relevant geometric parameters like: tortuosity, viscous and thermal permeability parameters and characteristic lengths. These parameters are required by some advanced models of sound propagation in rigid porous media, developed by Johnson, Koplik and Dashen, Champoux and Allard, with some variations introduced by Pride et al., and Lafarge et al. These models are utilized to produce curves of acoustic impedance and absorption that are used by the identification procedure which minimizes the objective function defined as a squared difference to the appropriate curves obtained experimentally. As a matter of fact, some experimental data are used for the determination of parameters while the other data - obtained for another sample of the same porous ceramics, yet having different thickness - serve for the validation purposes. Moreover, it is observed that the identified characteristic length for thermal effects corresponds very well to the average radius of pores, whereas the characteristic length for viscous forces is similar with the average size of "windows" linking the pores. The identification procedure minimises the objective function with respect to a set of some independent dimensionless parameters from which the actual model parameters can be calculated. In the definitions of the dimensionless parameters two reference frequencies are introduced - one relevant for viscous effects, the other for thermal effects. Such approach renders the optimization procedure very robust. In general, it is observed that, if the total porosity is known, simultaneous identification of the remaining model parameters is feasible by using only the objective function. In particular, the inverse identification allows to estimate the so-called static thermal permeability which in practice is not easy to determine by direct measurements, and thus, very often because of lack of this parameter, the simplified Lafarge model must be used which approximate this parameter with an analytical result obtained for a porous medium with circular cylindrical pores. Finally, a periodic microscopic cell consisting of a few pores representing an average morphology of porous ceramics is proposed and a finite-element analysis with periodic boundary conditions in order to estimate the static viscous permeability parameter is presented.
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