We experimentally demonstrate boundary-induced helical bulk states (BI-HBSs) for rf acoustic transport in LiNbO3 thin-film phononic crystals ($\sim 175-200\mathrm{MHz}$). A boundary-symmetry selection rule at an accidental $\mathrm{\Gamma }$-point fourfold degeneracy creates interior bulk channels that couple to wide-aperture interdigital transducers without edge-aperture mismatch. Near-field vibrometry and two-port rf $S$ parameters confirm low-loss propagation with strongly suppressed backscattering through wavelength-scale defects. The helical band also provides slow-wave, low-dispersion delay and phase control on chip.

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• Acoustic metamaterials
• Acoustic wave phenomena
• Mechanical & acoustical properties
• Mechanical metamaterials
• Microstructure
• Phononic crystals
• Piezoelectric devices
• Dirac semimetal
• Nanomechanical devices
• Thin films
• Finite-element method
• Microwave techniques
• Surface acoustic wave
• Tight-binding model