Abstract:

Multi-frequency topological wave transport has been demonstrated in various acoustic and elastic metamaterial systems. However, realizing independently tunable multi-frequency acoustic channels within a single acoustic metadevice remains challenging, particularly when different topological bandgaps need to be selectively opened, closed, and routed on demand. In this work, we propose a hexagonal pendant-type acoustic metamaterial (AM) that implements an accidental-Dirac-cone-based independent tuning mechanism in an acoustic system. The designed unit cell contains two types of cylinder-frame connectors with independently adjustable rotational angles, {\theta }_1 and {\theta }_2, which provide two geometric degrees of freedom for modulating the acoustic band structure. Finite element analysis reveals that the pristine structure supports three accidental Dirac cones in low-, mid-, and high-frequency regimes. By rotating the connectors to break mirror symmetry, the degeneracies of these Dirac cones can be selectively lifted or preserved, thus leading to independently tunable acoustic bandgaps and valley topological phases. The topological characteristics are identified through modal analysis, Berry curvature distributions, and valley Chern numbers. Interfaces between domains with opposite valley Chern numbers support topologically protected interface modes in the corresponding frequency intervals. A supercell analysis confirms the existence of single-, dual-, and triple-frequency interface states with pronounced sound-pressure confinement along the interfaces. Based on these states, frequency-selective acoustic waveguides are constructed to realize directional transport, wave separation, and acoustic demultiplexing under single-, dual-, and triple-frequency excitations. The results show that acoustic waves at different frequencies can be routed along prescribed paths with high selectivity and stable transmission in the presence of typical structural defects. The present numerical study therefore provides an acoustic implementation of independently tunable accidental-Dirac-cone-based topological transport and offers a feasible route for integrated acoustic-waveguiding, filtering, and demultiplexing devices.

Key words: accidental Dirac cone, acoustic-wave demultiplexing, topologically protected interface mode, tunable bandgap