Additive manufacturing of functionally graded foams for acoustic insulation and absorption

Acoustic foams and foam-filled metamaterials excel at sound absorption but typically exhibit a low sound transmission loss (STL). Foams that precisely integrate tunable shapes, density gradients, and transitions between open-cell and closed-cell regions have the potential to simultaneously enhance absorption and STL as compared to uniform foams. However, fabrication of these materials is challenging even for small samples that consist of a few thousand unit cells. Here we show additive manufacturing of functionally graded foams via direct bubble writing, a method for generating and stacking bubbles into three-dimensional solid foam constructs with a throughput up to 100 ml/min. The density, pore morphology, flow resistivity, and dynamic mechanical behavior of homogeneous and graded foams are characterized. As a reference case, the STL and absorption of homogeneous samples were tested in an impedance tube for frequencies between 200 Hz and 2600 Hz. Graded samples were subsequently evaluated, revealing strongly enhanced peaks in STL (up to ∼ 68 dB) for closed-cell foams with a low-density core sandwiched between two high-density layers. A high-density core sandwiched between two low-density layers especially broadens the frequency range with high sound absorption and still enhances the STL. These results show that functionally graded closed-cell foams are a promising route towards structure-induced dissipation as required for materials that exhibit a high absorption and a high STL.