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The existence of topologically protected boundary states in the Su–Schrieffer–Heeger (SSH) model has been widely explored in various systems in the past decades. On the other hand, a seemingly irrelevant phenomenon of Tamm mode, a defect mode caused by elaborately introducing perturbation into the trivial structures, has also attracted enormous interest. In this Letter, we bridge the gap between acoustic Tamm mode and its correlation rule with typical topological acoustic mode. Starting from the 1D acoustic cavity-based SSH model, the topologically protected edge modes in a complete bandgap are achieved. Through modulating the height of boundary cavity, we find two types of Tamm edge modes both numerically and experimentally and grasp their distinct variation tendency from the topologically protected edge modes. Furthermore, we extend the study to 2D higher-order topological acoustic systems and the lower-dimensional Tamm corner modes are clearly observed. We foresee that the multi-frequency sound confinement introduced by the Tamm localized modes may open a revealing landscape for acoustic functional devices.

Traditional topological insulators support the topologically protected boundary states that are one dimension lower than the system itself. Recently, higher-order topological insulators have received increasing attention in the field of acoustic wave manipulation due to their unique bulk-boundary correspondence principle, hosting both gapped edge states and in-gap corner states simultaneously. However, for most of the topological acoustic systems, the lack of reconfigurability and the inevitable outer trivial regions with considerable thickness restrict the potential applications of acoustic topological insulators. Here, we experimentally demonstrate a reconfigurable condensed acoustic second-order topological insulator in free space by using subwavelength soda cans whose side length is significantly reduced to 1.89 times of the corresponding wavelength. The topological nontrivial phase is introduced through tunably modulating the interval between cans. Without the typically required outer trivial regions, we observe the topological corner states at the corner of the finite structures in both simulations and experiments. Furthermore, the robustness against the defects induced by dislocations and deformations is discussed. We foresee that the proposal may facilitate the application potentials of topological acoustics in low-frequency sound manipulations.