  We reveal that finite-size solid acoustic resonators can support genuine
bound states in the continuum (BICs) completely localized inside the resonator.
The developed theory provides the multipole classification of such BICs in the
resonators of various shapes. It is shown how breaking of the resonator's
symmetry turns BICs into quasi-BICs manifesting themselves in the scattering
spectra as high-Q Fano resonances. We believe that the revealed novel states
will push the performance limits of acoustic devices and will serve as high-Q
building blocks for acoustic sensors, antennas, and topological acoustic
structures.
