Low-frequency absorption band in a thin acoustic metamaterial using acoustic black hole termination

The proposed metamaterial is a multi-pancake absorber, composed of periodically arranged thin annular cavities, with an alternative main pore profile that combines a constant section and an acoustic black hole termination to achieve an effective absorption band at low frequencies. The constant section at the opening enables low-frequency sound absorption, while the short acoustic black hole termination allows absorption band without a high-frequency shift. An equivalent mass-spring model using a single equivalent stiffness to represent the whole constant main pore section is proposed. This approach simplifies modeling and reduces computation time while capturing the metamaterial's multiple resonances and visualizing mass velocities at resonance frequencies, providing insight into its acoustic behavior. Validation is carried out using thermo-visco-acoustic finite element simulations and impedance tube measurements. First, profiles with an acoustic black hole termination of one to three main pores are studied to gain insight into the formation of combined modes and their impact on absorption. Next, optimizations are performed to achieve broadband absorption, resulting in the two best profiles. The first profile exhibits a 300 Hz absorption band starting at 550 Hz, while the second achieves a broader 500 Hz band at slightly higher frequencies, for a 3-cm thick material.