Ultra-broadband acoustic absorption with inhomogeneous high-order Fabry–Pérot resonances

We present an ultra-broadband acoustic metamaterial with inhomogeneous high-order Fabry–Pérot (FP) resonances that achieves near-perfect continuous absorption in the range of 400–10000 Hz. The unit is composed of multiple FP channels with inhomogeneous cross-sectional areas, allowing for flexible adjustment of the impedance characteristics of each channel. This approach prevents the high-order peaks’ resistances from increasing rapidly and ensures a smooth resistance characteristic across a broad frequency range. As a result, the absorption performance in the high-frequency range above 3000 Hz can be significantly enhanced, eliminating the need for conventional porous materials. The inhomogeneous high-order absorption mechanism is investigated thoroughly by theoretical calculations and finite element simulations. By critically coupling the FP channels, a 27-cell broadband metamaterial is obtained with an average absorption coefficient above 90% over 400–10000 Hz, which is verified by experiments in a square impedance tube and an alpha-cabin reverberation room, respectively. Characterized by its extraordinary performance and easy-fabricated structure, this metamaterial has great potential in noise control engineering applications, especially in environments with low temperature, high humidity, or significant dustiness.