Angle-dependent Absorption Property of 2D Infinite Periodic Arrangements of Helmholtz Resonators

In recent years, the potential of sound absorbing metamaterials as an alternative to traditional acoustic absorbers has been demonstrated in several publications. A common way to design these absorbing metamaterials, is through periodic arrangements of resonators. By combining acoustic resonators tuned to different resonance frequencies in each unit cell, the frequency range for which the material achieves high absorption can be extended. Nevertheless, there is limited research on their behavior when they are exposed to waves incident at oblique directions, and this information is highly relevant for a large amount of practical applications; particularly in room acoustics. In this work, the finite element method (FEM) is used, to study the angle-dependent absorption properties of infinitely large 2D surfaces consisting of a periodic arrangement of Helmholtz resonators. It is found that the incidence angle influences not only the magnitude of the (maximum) absorption coefficient, but also the frequencies at which the maximum absorption coefficient is observed, as well as the absorption bandwidth.