A slow wave, which has a long period and low propagation speed, is widely utilized in the transmission and modulation of signals in high-frequency communication devices. Recently, with the development of topological theory, topologically protected slow-wave modulation has been successful in photonic crystals. However, due to the complicated fluid-structure interaction in the underwater environment, the realization of an acoustic broadband slow wave is still a challenge. In this paper, we construct a spin-Chern acoustic topological insulator with helical edge states, consisting of a water-filled connected-cavities rigid structure with a bilayer kagome lattice. The exhibited continuous and gapless helical edge dispersion originates from artificial time-reversal symmetry, which is engineered via next-nearest-neighbor chiral intercell interlayer coupling. Through introducing extra adjustable cavity coupling with helical edge states, the dispersion winds many times around the Brillouin zone to be nearly flat. Consequently, the slow-wave effect is perfectly presented in a wide frequency range. Our structure is protected by both time-reversal symmetry and spatial-inversion symmetry, providing an approach for underwater acoustic transmission and the design of industrial delay devices.

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• Edge states
• Phononic crystals
• Topological insulators
• Kagome lattice
• Topological materials