RESUMO
It has been demonstrated recently via simulations in a duct that the sound transmission loss of porous materials can be increased with embedded periodic scatterers [Jena and Qiu, J. Acoust. Soc. Am. 147, 978-983 (2020)]. In this paper, the insertion loss (IL) of two types of finite size structures constructed by installing two parallel porous sheets within rows of periodic scatterers is investigated in free field. One structure uses rigid cylindrical shells, while the other uses split ring shells, which are obtained by making uniform vertical slits in cylindrical shells. The simulation results show that the mixed structure has larger IL than the summation of the ILs of the individual periodic scatterers and two parallel porous sheets in the frequency range after the first bandgap due to Bragg diffraction, and an additional peak in IL is introduced for the split ring scatterers due to their local resonance. The amplitude, frequency, and bandwidth of the resonance peak can be tuned by changing the geometry of the slit for some specific broadband sound absorption. Finally, the experiment results obtained in an anechoic chamber are presented to support the simulation results and analyses.
