Improvement of sound absorption and insulation using a double-layer metamaterial (DM) in the acoustic free field is proposed in the present paper. The front layer of the DM is composed of a flexible micro-perforated plate with periodic local resonators, the backing layer is a flexible plate attached with periodic local resonators too, and an air gap exists between the double plates. Good agreement is gained between the results of the theoretical prediction and finite element simulation for the DM and the original double-layer plate (DP) while considering the vibroacoustic coupling between the sound excitation and the plates. Both theoretical and simulation results verify that the local resonators can improve the sound absorption and insulation of the DP. The underlying mechanism of the DM is investigated using the acoustic impedance and displacement pattern. Filling the gap with the porous material can further improve the sound absorption and insulation of the DM. Finally, the influences of the number and additional mass ratio of local resonators on the acoustic performance of the DM are investigated, and the practical realization of the DM is verified. The present design shows great potential for practical noise reduction in the free field.
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