Sound absorption enabled by critical coupling and mutual coupling in virtual boundary waveguide

Virtual boundary waveguides (VBWs) possess particular abilities for guiding sound transmission in a continuous and unbounded field. However, achieving acoustic mode manipulation in VBWs with non-invasive structures presents ongoing challenges. We propose and investigate a VBW incorporating coupling arrays for acoustic absorption through theoretical, numerical, and experimental methods. The efficiently sequencing Helmholtz resonators (HRs) form the waveguide edge, absorbing arrays, and scattering arrays. All HRs are embedded in an isolated acoustic metasurface layer without any structural intrusion into the acoustic propagation layer. Within the VBW, the lossy absorbing array achieves high acoustic energy dissipation under the critical coupling condition, while the lossless scattering array functions as an acoustic soft boundary. Furthermore, a serpentine VBW design is developed, and finite element simulations verify that sound absorption can be achieved at arbitrary locations along the flexible waveguide path within a confined space. Additionally, sound transmission and absorption are immune to excitation caused by mutual radiation between segmented waveguides. Our research enriches the function of boundaryless waveguide systems and provides a practical and efficient approach to acoustic wave regulation in fields such as biomedicine and pharmaceutics.