Acoustic higher-order topological insulators protected by multipole chiral numbers

Higher-order topological insulators, which go beyond the conventional bulk-boundary correspondence, have been attracting extensive interest in past years. Very recently, it was pointed out that chiral-symmetric higherorder topological insulators can be characterized by a Z topological invariant (dubbed the multipole chiral number), which dictates the number of degenerate zero-energy states per corner. Here, we report an acoustic realization of the higher-order topological insulators protected by multipole chiral numbers. To do this, strong long-range couplings, which are not available in solid systems, are carefully designed in our acoustic system. Our acoustic measurements demonstrate unique spatial characteristics for such higher-order topological insulators protected by multipole chiral numbers. This study may enable possibilities for controlling sound, such as acoustic sensing and energy trapping.