Speaker
Description
Helmholtz resonators (HRs) are widely used in the context of room acoustics to reduce the sound pressure peaks induced by room modes. A shortcoming of HRs is that they are very narrowband and can become ineffective as soon as target room eigenfrequencies change, e.g., due to occupancy changes. To address this, a controllable HR-based acoustic metamaterial (HRAM) with resonance frequencies that are reconfigurable over a wide frequency range is proposed here. Due to the large number of degrees of freedom required to model the thermoviscous dissipation in the HRAM design, finite element method (FEM) modelling of its interaction with an enclosure is highly computationally intensive. The use of an adequate boundary layer mesh to accurately capture the thermoviscous dissipation in the boundary layer of the HRAM further increases the complexity of the FE model. Instead of modelling the three-dimensional geometry of the HRAM explicitly, an alternative method is employed. In this contribution, the complexity of the FE model is reduced by employing a surface impedance method in which the HRAM is modelled using the specific acoustic impedance value evaluated at its opening (using a separate simulation model of only the HRAM). Simulation results of the pressure field inside an enclosure are presented, comparing the explicit HRAM modelling approach with the surface impedance method. The results show that the computational effort can be significantly reduced, with the surface impedance method achieving almost the same accuracy as the explicit HRAM modelling approach up to a limiting frequency at which the acoustic wavelength becomes comparable to the HRAM unit cells.