Ultra-broad low-frequency forbidden bands in 1D solid–fluid acoustic superlattices with multiple solid constituents

In this paper, a theoretical model of underwater acoustic wave propagation, based on the transfer matrix method, is employed to calculate acoustic wave transmission through a 1D solid–fluid superlattice (SFSL) with multiple solid constituents. The band structure of infinite multi-element SFSL and the transmission coefficient of finite one are obtained, both of which reveal the existence of multiple separated ultra-broad low-frequency forbidden bands, even in the case of solid constituents with very close acoustic parameters. Simulations about the finite multi-element SFSL are carried out to verify the correctness of these findings. In addition, the influence of the acoustic parameters and the structural parameters on the low-frequency forbidden bands (LFBs) of SFSL with two solid constituents (m = 2) is also investigated. It is found that the required two separated LFBs can be achieved by choosing appropriate material parameters. For the situation of m ≥ 2, it is expected to provide more possibilities to engineer low-frequency bandgaps. Our results infer that multiple separated LFBs in multi-element SFSL can be quite useful for making highly performing low-frequency acoustic filters in selective angle ranges, which are useful for the elimination of the side lobes of low-frequency acoustic transducers.