Acoustic Bound States in the Continuum in Coupled Helmholtz Resonators

Abstract: Resonant states underlie a variety of metastructures that exhibit remarkable capabilities for effective control of acoustic waves at subwavelength scales. The development of metamaterials relies on the 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 can't 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 quasi-bound state in the continuum in the resonators placed in a 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 quasi-\textit{bound states in the continuum} and occurrence of \textit{exceptional points}. The obtained results will open new opportunities for the development of simple and easy-tunable metastructures based on Helmholtz resonances.

• The paper demonstrates that symmetry breaking and adjustable coupling in Helmholtz resonators enable the creation of quasi-bound states and exceptional points, validated experimentally and numerically.
• It employs both experimental setups and numerical simulations to analyze how changes in geometric parameters and porous inserts affect eigenfrequencies and quality factors.
• The study’s findings offer practical insights for designing tunable acoustic metastructures for noise control, sound imaging, and sensing applications.

Summary of "Acoustic Bound States in the Continuum in Coupled Helmholtz Resonators" (2405.07383)

This paper explores the creation and tuning of quasi-bound states in the continuum (quasi-BIC) and exceptional points (EP) within a system of coupled Helmholtz resonators. The authors investigate the effects of symmetry breaking and coupling strength adjustments on acoustic resonances, with implications for metamaterial design.

Acoustic Resonances and BIC

The paper begins with a detailed discussion of acoustic resonances, crucial in areas such as acoustics, photonics, and industrial applications. The focus is on metamaterials and using resonant states to control wave propagation at subwavelength scales. A highlight is the exploration of bound states in the continuum (BIC), which are non-radiative states that exist within the continuum spectrum due to interference effects.

Helmholtz Resonators System

The authors propose a simple system of coupled 2D Helmholtz resonators that can be adjusted mechanically or through intrinsic loss manipulation via porous inserts. Such a system is shown to support quasi-BICs and EPs. They provide experimental and numerical evidence that tuning the distance between resonators or their intrinsic losses can effectively manipulate these states.

Coupling and Symmetry

Tuning the coupling between resonators by modifying geometric parameters and the introduction of intrinsic losses are examined in detail. The study demonstrates how these changes influence the eigenfrequencies and quality factors, leading to phenomena such as spectral splitting or merging of quasi-BICs. The role of symmetry breaking is emphasized as crucial for exciting quasi-BICs, which in real-world applications have finite quality factors due to weak coupling with far-field modes.

Experimental and Numerical Analysis

Two primary experimental setups are used: one simulating free space and another in a transmission tube. Numerical simulations supplement these, providing detailed insights into the modes' real and imaginary parts. The study aligns closely with experimental data, showing good concordance in observed frequencies and quality factors, despite slight discrepancies due to manufacturing imperfections or material parameter variations.

Porous Inserts Impact

The research explores the impact of porous inserts as a means to tune resonator characteristics. Different flow resistivity levels modify coupling regimes significantly, potentially leading to EPs where acoustic eigenmodes coalesce. This provides a method for precise acoustic state control, useful for designing efficient metastructures.

Practical Implications

The findings have practical implications for developing tunable acoustic metastructures with applications in noise control, sound imaging, and acoustic sensing. The study demonstrates the feasibility of controlling acoustic resonances using straightforward and adjustable designs, reducing reliance on complex geometries and potentially extending operational frequency ranges.

Conclusion

The paper provides a thorough technical examination of resonance engineering using coupled Helmholtz resonators, offering new insights into designing metamaterials with tunable acoustic properties. The ability to create and adjust quasi-BICs and EPs in a controlled manner opens up possibilities for new acoustic devices and advanced materials, pushing forward the boundaries of acoustic metamaterials research.