  Recently, artificial moire superlattices of classical waves have aroused
tremendous interest, inspired by the newly emergent twistronics that focuses on
the peculiar electronic properties induced by flat bands. However, so far, the
moire flat bands have not been observed directly. Here, we report the first
experimental observation of ultraflat bands in moire sonic crystals with
frequency-momentum spectra, together with a real-space visualization of the
hallmark localized states. Strikingly different from the established magic
angle mechanism for twisted bilayer graphene, the flat bands are formed by
weakly coupled moire potential-well states inside a wide band gap, and can be
realized over a broad range of twist angles. The average group velocity of the
moire localized states, as a faithful reflection of the band flatness, decays
exponentially with the moire period of the acoustic structure. Our findings,
applicable to all artificial crystals, enable new possibilities for
manipulating classical waves with moire structures.
