Improved sound absorption by size gradient granular materials due to Brazil-nut effect

Granular material is attracting increasing research momentum due to its prevalence and rich unique properties. Nevertheless, compared with its numerous functions in other areas, the application of granular material in acoustic waves has received less attention. In this paper, we propose that by exploiting Brazil-nut effect induced particle size segregation in granular material, a significant improvement in the sound absorption can be achieved. Firstly, sound absorptions by particles with step-increasing sizes have been analyzed. It is found that size-dependent mechanisms of sound energy dissipation may occur. Secondly, sound absorptions by size-mixed particles with vibration treatment have been studied. As a result of the Brazil-nut effect induced size segregation, the size-mixed particles with initially limited sound absorptivity exhibited remarkably improved sound absorbing capability, with both broadened absorbing band and raised low-frequency absorption. This work demonstrates that simple processing of the granular material may become a promising way to create premium sound absorbers.