We report a direct determination of the dynamic behavior of confined acoustic phonons in nanocavities by picosecond acoustics. We provide the broadband, high resolution transmission amplitude curve in the sub‐terahertz range and we give evidence of resonant transmission peaks in three successive stop bands, in quantitative agreement with transfer matrix simulations. We demonstrate transit times in the nanosecond range at the cavity peaks reflecting the strong phonon confinement within the cavity layer and picosecond times in the stop bands, shorter than in any of the constituting materials, a tunneling effect known in photonic systems as superluminal propagation.

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