Underwater metagratings for sub-kilohertz low frequency and broadband sound absorption

Investigation on Low-Frequency and Broadband Sound Absorption of the Compact Anechoic Coating Considering Hydrostatic Pressure

“…Viscoelastic material such as rubber was generally adopted as the host medium for the impedance matching with the water domain and the resultful shear losses [19,20]. The cross-section diagram of the multilayer coating is shown in Figure 1b, in which the different diameters of tandem perforative voids and heights of rubber layers determine the surface impedance precisely [21]. Water density and sound speed are ρ w = 1000 kg/m 3 , c w = 1500 m/s, while those of air are ρ a = 1.2 kg/m 3 , c a = 340 m/s.…”

“…The equivalent velocity 𝑐 𝑒 in the effective homogeneous layer (EHL) could then be calculated logically upon the above homogenization. Following the transfer matrix method [21,[28][29][30], the transitive relation between complex pressure and normal velocity satisfies (𝑃 𝐴 ; 𝑉 𝐴 ) = 𝑇(𝑃 𝐵 ; 𝑉 𝐵 ), and the subscripts A and B indicate the incident (water) and transmitted (air) domain, respectively. T is the transfer matrix, represented as ).…”

“…By analogy with the analysis for single-layer coating, the analytical and numerical absorption coefficients from the tandem coating are illustrated in Figure 5c. Benefiting from the increment of resonant modes and the coupling effect between multilayer voids [21,33,34], quasi-perfect absorption from 420 Hz to 1720 Hz could be approached via this compact broadband absorber under ordinary pressures. The three resonance peaks would skew toward the higher frequencies, accompanied by the reduced amplitude and broader bandwidth, as shown in Figure 5c,d, which enhances the absorption efficiency in the middle-frequency band (2.5~5 kHz) at 1 MPa.…”

“…In this section, we conduct the experiment in an impedance tube filled with water to demonstrate the absorption dependence around hydrostatic pressures. Note that the tandem coating adopted in the investigation here was constructed with different void periods in each layer (Table 4) to improve the coupling effect between themselves [21]. Because a viscoelastic medium with arbitrary mechanical parameters is difficult to prepare in engineering, and for the sake of the manufacturing reliability for raw material, polyurethane (PU) was chosen for the host medium of a specimen.…”

Machine Learning-Driven Design Optimization of Buckling-Induced Quasi-Zero Stiffness Metastructures for Low-Frequency Vibration Isolation

“…Metastructures, which are typically referred to as metamaterials, comprise artificially designed arrays of micro/macrostructures that exhibit unique characteristics. − These characteristics can vary depending on factors such as the size, shape, arrangement, and orientation of the micro/macrostructures. Metastructures possess diverse properties, including negative density, zero thermal expansion, and zero Poisson’s ratio, which are typically absent in natural materials. − Therefore, metastructures are currently receiving considerable attention across diverse fields, including the aerospace industry, stealth technology, photonic devices, and semiconductor design. − …”