A hybrid acoustic metamaterial is proposed as a new class of sound absorber, which exhibits superior broadband low-frequency sound absorption as well as excellent mechanical stiffness/strength. Based on the honeycomb-corrugation hybrid core (H-C hybrid core), we introduce perforations on both top facesheet and corrugation, forming perforated honeycomb-corrugation hybrid (PHCH) to gain super broadband low-frequency sound absorption. Applying the theory of micro-perforated panel (MPP), we establish a theoretical method to calculate the sound absorption coefficient of this new kind of metamaterial. Perfect sound absorption is found at just a few hundreds hertz with two-octave 0.5 absorption bandwidth. To verify this model, a finite element model is developed to calculate the absorption coefficient and analyze the viscous-thermal energy dissipation. It is found that viscous energy dissipation at perforation regions dominates the total energy consumed. This new kind of acoustic metamaterials show promising engineering applications, which can serve as multiple functional materials with extraordinary low-frequency sound absorption, excellent stiffness/strength and impact energy absorption.

We report a lightweight tunable acoustic metamaterial with deep subwavelength thickness (e.g., $\lambda /300$ ) and strong load-bearing capability for underwater low-frequency and ultra-broadband acoustic perfect absorption. The metamaterial is constructed by introducing a rubber coating and an embedded metallic neck into a metallic hexagonal honeycomb Helmholtz resonator. Physically, the quasi-Helmholtz resonance triggered by the rubber coating together with the anti-phase cancellation caused by the embedded neck leads to superior sound absorption. Theoretical predictions of the metamaterial performance agree well with finite element simulation results. With fixed external morphology (e.g., honeycomb-cored sandwich panel) and fixed overall thickness (e.g., 50 mm), key internal geometrical parameters of the proposed metamaterial can be tailored to achieve tunable perfect absorption from, e.g., 100 Hz to 300 Hz. Further, combining such tunable quasi-Helmholtz resonance leads to ultra-broadband quasi-perfect absorption from, e.g., 306 Hz to 921 Hz. This work contributes to designing underwater acoustic metamaterials and controlling underwater acoustic waves.

Acoustic impedance regulation of a neck embedded Helmholtz resonator is realized by introducing surface roughness to the neck so as to convert the initially non-perfect sound absorber to a perfect sound absorber. The proposed roughened-neck embedded Helmholtz resonator (R-NEHR) achieves perfect sound absorption ($\alpha >0.999$ ) at 158 Hz across a deep subwavelength thickness of $\lambda /42$ . Theoretical predictions of the R-NEHR's performance are validated against experimental measurements. Physically, surface roughness triggers the periodic concentration effect of fluid vibration in the neck, thereby improving its acoustic mass and acoustic resistance and altering the resonant damping state of the absorber. As a result, the absorption peak position of the R-NEHR shifts by 16.0% to lower frequency, together with a peak value increase of 19.6%. This work provides an approach for perfect sound absorber design and impedance regulation of acoustic metamaterials.