Side branch-based acoustic metamaterials with a broad-band negative bulk modulus

We present theoretical investigations and numerical simulations of one-dimensional (1-D) acoustic metamaterials that exhibit wide negative bulk modulus bands down to zero. The metamaterials consist of double or quadruple branch openings in a 1-D waveguide. A lumped model is developed for theoretical analysis and the bandwidths of negative bulk modulus for different structures are calculated and compared. As much as 100 % increase over the traditional single branch opening structure in bandwidth can be achieved. The proposed metamaterials can be utilized to further achieve double negativity, which could facilitate applications such as acoustic cloaking and superlensing.