Low-frequency broadband acoustic absorption of a high-order metamaterial with hybrid-damping coupling

The causality of passive linear materials dictates both sound absorption performance and structure thickness, making it challenging in the design of thin structures that can approach the causality limits. In contrast to the traditional structure with single damping, a hybrid-damping coupling method is introduced here based on a high-order metamaterial, allowing for superior absorption performance in a thinner structure. In the hybrid-damping high-order cell, the first-order peak is applied with an over-damping impedance to shift the peak toward a lower frequency with the structure thickness unchanged, while the high-order peaks are optimized with matched-damping impedance, avoiding their rapid attenuations in absorption ability. By introducing opposite phases for two first-order peaks, 100% absorption can also be achieved at lower frequencies upon strict coupling. Owing to the near-perfect absorption, the high-order peaks allows for a more sparse arrangement and a lower peak density when coupled, which results in fewer restrictions on peak frequency and greater design flexibility. Therefore, a wider absorption band with superior performance can be achieved in the high-order metamaterial with a fixed number of peaks. By critically coupling 15 high-order cells, a broadband Helmholtz metamaterial is obtained with 95% mean absorption within 210–1860 Hz under a thickness of 135 mm, which is verified by experiments in a square impedance tube. This metamaterial can provide a new way for the low-frequency broadband absorption design and show great potential in noise control engineering applications.