Figure 1. (left) Photographs of soft porous silicone rubbers with various porosities ϕ ranging from 0% to 35% and SEM micrographs of the porous structure for two samples with ϕ = 1% and 35%.

The photograph of the 19%-sample is not shown here because it has the same appearance as the 12%-sample. (right) Phase velocities c versus porosity ϕ for both longitudinal and transverse elastic waves. Solid lines refer to the Kuster-Toksöz model (Eqs 2(a) and 2(b)) with K₀ = 1.2 GPa, G₀ = 0.3 MPa, ρ₀ = 1040 kg/m³ for the soft silicone rubber matrix, and K₁ = 0.14 MPa, G₁ = 0, ρ₁ = 1 kg/m³ for the air cavities. Dash lines refer to the Waterman-Truell model (equations not shown here) with the same material input parameters.