Accidental bound states in the continuum in acoustic resonators with rotating obstacles

Bound states in the continuum (BICs) are embedded states in the radiation spectrum, which have garnered significant interest across various areas, including photonics and acoustics. In this work, by introducing rotational obstacles into acoustic resonators, we report a series of accidental BICs in coupled waveguide-resonator systems. We demonstrate that a general type of accidental BICs would emerge at specific rotating angles, supported by the mode symmetries at the boundary interface between resonators and attached waveguides. We further demonstrate that the presence or absence of accidental BICs is closely related to the geometric parameters of resonators and obstacles, and can be predicted by the mode evolution within closed resonators. Additionally, we show that two BICs of an individual mode could converge, merge, and vanish in a single resonator by changing the geometry parameters. We also explore the topological origins of these phenomena. Our study provides an efficient way to manipulate and engineer BICs through rotating obstacles in acoustic resonator systems.