Observation of chiral edge state pairs in an acoustic trimer waveguide

Here, we experimentally investigate the topological state properties in an acoustic trimer waveguide akin to the extended Su–Schrieffer–Heeger model. By integrating a mirror symmetry into the structure, we are able to observe the existence of two pairs of chiral edge states due to a hidden chiral symmetry even though the original structure itself does not exhibit chirality. We uncover the existence of the hidden symmetry and unearth the topology for the origin of chiral edge state pairs using winding number of a subspace Hamiltonian. The measured dispersion and localization of sound on the edges verify the observations in theory and simulations. This study not only enriches our understanding of acoustic topological systems but also provides a framework applicable to other physical contexts.