  The ability of extreme sound energy confinement with high-quality factor
(Q-factor) resonance is of vital importance for acoustic devices requiring high
intensity and hypersensitivity in biological ultrasonics, enhanced collimated
sound emission (i.e. sound laser) and high-resolution sensing. However,
structures reported so far demonstrated a limited quality factor (Q-factor) of
acoustic resonances, up to several tens in an open resonator. The emergence of
bound states in the continuum (BIC) makes it possible to realize high-Q factor
acoustic modes. Here, we report the theoretical design and experimental
demonstration of acoustic BICs supported by a single open resonator. We
predicted that such an open acoustic resonator could simultaneously support
three types of BICs, including symmetry protected BIC, Friedrich-Wintgen BIC
induced by mode interference, as well as a new kind of BIC: mirror-symmetry
induced BIC. We also experimentally demonstrated the existence of all three
types of BIC with Q-factor up to one order of magnitude greater than the
highest Q-factor reported in an open resonator.
