Figure 2.

Dispersion relations ω=ω(k) for the longitudinal waves of the relaxed micromorphic model with gradient micro-inertia ${\overline{\eta }}_3=(0,3\times {10}^{-3},\hspace{0.17em}3\times {10}^{-2})$ kg m⁻¹ and η=0. Black dots indicate the dispersion relations for a first gradient model with Lamé parameters μ_macro and λ_macro and the same inertiae ρ and ${\overline{\eta }}_3$ (a). The same picture obtained imposing λ_micro=0 (b): a very slight variation with respect to the first gradient case can be detected. (Online version in colour.)