Sound insulation properties in low-density, mechanically strong and ductile nanoporous polyurea aerogels

Aerogels are quasi-stable, nanoporous, low-density, three-dimensional assemblies of nanoparticles. In this paper, an extremely high sound transmission loss for a family of ductile polyurea aerogels (e.g., over 30 dB within 1 to 4 kHz at bulk density 0.25 g/cm3 and 5 mm thickness) is reported. The fundamental mechanisms behind the aerogel acoustic attenuations are investigated. Sharing striking similarities with acoustic metamaterials, initially, aerogels are studied via a one-dimensional multi degree-of-freedom mass-spring system. Different effects such as spring constant disparity are investigated in regards to the structural vibration wave transmission loss. Results are given for different configurations consistent with the aerogel nano/microstructures. A significant wave attenuation is observed by considering a random spring distribution. In the next step towards modeling such a complex hierarchical and random structural material, the continuum Biot's dynamic theory of poroelasticity is implemented to analyze the experimental sound transmission loss results. In this framework, a two-dimensional plane strain analysis is considered for the interaction of a steady state time-harmonic plane acoustic wave with an infinite aerogel layer immersed in and saturated with air. The effects of bulk density and thickness on the aerogel sound transmission loss are elucidated. By comparing the theoretical results with the experimental observations, this study develops a qualitative/quantitative basis for the dynamics of the aerogel nanoparticle network as well as the air flow and solid vibroacoustic interactions. This basis provides a better understanding on the overall acoustic properties of the aerogels that might also be helpful in the design of the future hierarchical materials.