Thin metamaterial using acoustic black hole profiles for broadband sound absorption

The multi-pancake absorber is a thin acoustic metamaterial composed of periodically arranged thin annular cavities separated by the plates and connected by a main pore, enabling low frequency sound absorption. However, with a constant main pore radius, this structure exhibits tonal absorption behavior characterized by narrow peaks with varying amplitudes due to an imbalance in losses between the modes. This work investigates a geometric variation of the main pore, focusing on the designs with a gradually decreasing radius known as acoustic black hole profiles, to achieve broadband absorption. An analytical model based on the transfer matrix approach and lumped elements is proposed to predict the acoustic properties. Validation is conducted through thermo-visco-acoustic finite element simulations and impedance tube measurements. Multiple main pore profiles are investigated and the complex frequency plane representation is used to improve the balance of losses. An understanding of the influence of the main pore radius at the entrance and backing of the sample, allows design of a decreasing main pore radius profile that results in a high absorption coefficient value across a broad frequency range. Non-linear decreasing main pore profiles can be used, if necessary, to accentuate losses and achieve absorption peaks of similar amplitude.