Sound absorption of multiscale sorptive permeable media with a distribution of nanopores

Multiscale porous materials have shown remarkable acoustic properties. Among them, activated carbons stand out due to the multitude of physical phenomena, including sorption in nanopores, that strongly influence and enhance their sound attenuating performance. In the present work, an upscaled multiple porosity model for wave propagation in activated carbons exhibiting a bimodal nanopore size distribution was derived by using the two-scale asymptotic method of homogenisation. This model incorporates the different physics of wave propagation in the materials' inter-granular voids, their inner-grain transport pores and in their different types of nanopores, as well. The theoretical results are numerically illustrated to highlight the impact of the nanopore size distribution, in particular. Further, model predictions based on material parameters, obtained from an inverse fitting routine informed by parameters measured with non-acoustic experimental techniques, have been successfully compared with measured acoustic properties. The latter comparisons indicate that it is possible to control and optimise the acoustic properties of activated carbon particles by controlling and manipulating their structure at the nanoscale. In conclusion, this work provides insights on the role of the nanopore size distribution on the sound absorptive properties of multiscale permeable media and the present results can find application in the design of materials for noise control and other acoustic treatments.