We report a lightweight tunable acoustic metamaterial with deep subwavelength thickness (e.g., λ/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.

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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 (α>0.999) at 158 Hz across a deep subwavelength thickness of λ/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.

A novel underwater composite anechoic layer is proposed by inserting periodically placed longitudinal parallel steel plates into a viscoelastic rubber matrix. Built upon the complex viscosity model of viscoelastic materials, a theoretical model is established to evaluate the sound absorption performance of the proposed anechoic layer. For validation, finite element simulations are carried out, and good agreements are achieved between theory and simulation. Compared with the reference structure purely made of rubber, the new anechoic layer exhibits greatly improved sound absorption performance. It is demonstrated that the steel plate insertions significantly enlarge the shear deformation of rubber at plate–rubber interfaces, causing greatly improved viscous dissipation of acoustic energy. Systematic variations of material properties and geometrical parameters reveal the dominant roles of rubber viscosity and plate spacing. Further, a theoretical model is developed to study the effect of non-tight connection at rubber–plate interfaces. This study broadens the application scope of the complex viscosity model and provides useful guidance for designing novel anechoic layers with tailored underwater acoustic performances.

The acoustic metamaterial in the form of a petal-shaped channel embedded Helmholtz resonator (P-CEHR) is proposed for perfect sound absorption. According to theoretical predictions, numerical simulations, and experiments, the P-CEHR achieves perfect low-frequency (e.g., 200 Hz) sound absorption across a deep subwavelength thickness (e.g., 1/34 of the corresponding acoustic wavelength). Compared with the circular-shaped channel embedded Helmholtz resonator, the sound absorption peak and bandwidth of P-CEHR are significantly improved (e.g., increased by 20.9% and 60.0%, respectively) under fixed overall dimensions. Physically, the introduction of the petal shape changes the fluid dynamic characteristics of the channel, resulting in the periodic distribution of particle velocity along the circumferential direction and the expansion of the area of the viscous boundary layer. By adjusting the morphology of the embedded channel, the tortuosity ratio and the relative static flow resistance of the channel can be regulated appropriately, so that the resonator can meet the acoustic impedance matching condition and achieve excellent sound absorption performance. This work provides a method for improving the performance of acoustic absorption metamaterials with built-in air channels and has guiding significance for the control of low-frequency noise.