Multi-tortuous acoustic materials

In this work, a multi-pressure equivalent fluid (MPEF) approach is applied to model non-conventional acoustic materials that combine different, separate pore networks with contrasting tortuosities. A technique for the informed design of such multi-tortuous materials is proposed. It is based on the observation that broadband performance of such a material can be achieved by tuning the quarter-wavelength resonances corresponding to each network. The material design consists therefore in adding and tailoring the separate pore networks to obtain contrasting tortuosities that evenly distribute these resonances over the desired frequency range. Additional improvement is achieved by independent isotropic scaling of the separate networks. The proposed technique is accurate and also very efficient because it is based on semi-analytical calculations. All this is demonstrated on several examples of multi-tortuous materials which, for simplicity, have an essentially two-dimensional structure. The results obtained in the material design process are verified by Navier–Stokes direct numerical simulations as well as by the MPEF numerical model. Final validation was also carried out experimentally on an additively manufactured sample of one of the multi-tortuous materials designed for this study. The multi-resonance phenomenon observed in sound absorption as well as the experimentally demonstrated anomalous behaviour of the multi-tortuous material backed by an air gap are very well predicted by the modelling and explained in detail on physical grounds.