Table 1
Characteristics and performance of the devices investigated in literature when experiments have been presented in the paper. The devices are sorted as a function of the kh product, and the λ/h and natural frequencies are added for reference.
| Ref. | kh | λ/h | Target freq. or freq. range | Description | Performance | Exp. values (when available) |
|---|---|---|---|---|---|---|
| [58] | 0.15 | 43 | 700 Hz | Local resonance slab with steel plate backing. | Absorption coefficients (α) of the sample with the steel plate of different thickness. | α≈0.98 at 700 Hz. |
| [36] | 0.17 | 37.5 | 1 kHz | Viscoelastic rubber matrix, conical cavity, cylindrical oscillator and steel backing. | Acoustic absorption. | α≈0.67 around 800 Hz. |
| [45] | 0.2 | 31 | 800 Hz | Array of multi-resonant steel scatterers in a silicone rubber cylinder, inside a viscoelastic polymer matrix with steel backing. | Significant acoustic absorption on a large bandwidth (300–2000 Hz). | α≈0.9 under low hydrostatic pressure. |
| [13] | 0.21 | 30 | 5 kHz | Viscoelastic materials with micro-spheres (fluid-filled or elastic cavities). | Transmission loss (TL) increases with frequency in a wide band (up to 100 kHz). | TL around 3 and 13 dB at 5 and 100 kHz, respectively. |
| [59] | 0.23 | 27.3 | 1 kHz | Two types of local resonance units in the metamaterial slab. Steel spheres embedded in a viscoelastic polymer matrix. | Design deduced from optimization. Acoustic absorption. | α≈1 at 1 kHz. |
| [23] | 0.29 | 21.4 | 1.75 kHz | Periodic array of air cylindrical cavities in silicone. | Transmission (T) measurement. | T≈−31 dB at 2 kHz. |
| [62] | 0.32–1.58 | 20–4 | 4–20 kHz | Composite rods placed on a stepped stainless-steel base. | Measured transmittance, reflectance and absorption. Broad band performance. | α around 0.9 in the band. |
| [37] | 0.6–2.51 | 10.4–2.5 | 2.4–10 kHz | Gradient cavity shaped polyurethane composite array supported by carbon fiber honeycomb with steel backing. | Acoustic absorption on a large bandwidth under hydrostatic pressure (3 MPa). | α>0.9 in the band. |
| [19] | 0.71 | 8.8 | 170 kHz | Macro-porous silicone rubber microbeads embedded in a water-based gel matrix. | Layer with negative index (n). | n≈−0.8. |
| [63] | 1–2.51 | 6.2–2.5 | 12–30 kHz | Phononic glass: interpenetrating network structure is introduced into a locally resonant phononic crystal. | Impedance matching with water. Acoustic absorption. | α>0.9 in the band. |
| [26] | 1.35–2.79 | 4.7–2.2 | 1.4–2.9 MHz | Air inclusions in PDMS. Several layers. | Acoustic absorption. | α>0.95 in the band. |
| [64] | 2.28–6.45 | 2.8–1 | 14.3–40.5 kHz | Square holes drilled into a steel plate. | Scattering cross section reduction. | Below −10 dB in the band. |
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