The quantum spin Hall (QSH) phase is a paradigmatic time-reversal-symmetric topological insulator, carrying a ${\mathbb{Z}}_2$ bulk invariant and counterpropagating helical edge channels protected by spin-momentum locking. Since bosonic systems lack the microscopic Kramers degeneracy of electrons, the QSH phase in such systems is engineered from crystalline symmetry to form a synthetic spin-1/2 doublet. This analog has been realized in photonic crystals and in macroscopic kHz-MHz acoustic and elastic structures, but not yet on a chip-scale phononic platform in the microwave regime. The gigahertz (GHz) band is relevant for wireless communications and quantum acoustics, where on-chip work has so far been limited to the valley Hall effect. Here, we report the direct observation of pseudospin-momentum-locked helical phonon edge states at approximately 1 GHz in a nanoelectromechanical aluminum nitride (AlN) topological insulator. An enlarged supercell folds the $K$-point Dirac cones to the $\mathrm{\Gamma }$ point, and tuning the inner-cluster geometry drives a $p$-$d$ band inversion with nonzero spin Chern number, in direct correspondence with the Bernevig–Hughes–Zhang Hamiltonian. The helical edge channel along the domain wall is resolved by transmission-mode microwave impedance microscopy. A single-$K$-point Fourier filter of the complex displacement field reveals the opposite handedness of the two counterpropagating pseudospin branches. The channel propagates robustly through 120° corners of a Z-shaped waveguide and partitions selectively at a four-domain junction. Together with the valley Hall and Klein-tunneling phenomena already demonstrated on the same platform, our work establishes the acoustic analog of the QSH phase as a building block of an integrated gigahertz phononic toolbox on a CMOS-compatible piezoelectric substrate.

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• Phononic crystals
• Quantum spin Hall effect
• Topological edge modes
• Topological effects in acoustic systems
• Nano-electromechanical systems
• T-symmetry