Acoustic bound states in the continuum in coupled Helmholtz resonators

Resonant states underpin a variety of metastructures that exhibit remarkable capabilities for the effective control of acoustic waves at subwavelength scales. The development of metamaterials relies on rigorous mode engineering, providing the implementation of the desired properties. At the same time, the application of metamaterials is still limited, as their building blocks are frequently characterized by complicated geometry and cannot be tuned easily. In this work, we consider a simple system of coupled Helmholtz resonators and study their properties associated with the tuning of coupling strength and symmetry breaking. We numerically and experimentally demonstrate the excitation of the quasibound state in the continuum in the resonators placed in free space and in a rectangular cavity. It is also shown that tuning the intrinsic losses via introducing porous inserts can lead to spectral splitting or merging of quasibound states in the continuum and the occurrence of exceptional points. The obtained results open opportunities for the development of simple and easily tunable metastructures based on Helmholtz resonances.