  We propose a class of linear elastic three-dimensional metamaterials for
which the effective parameters bulk modulus and mass density can be adjusted
independently over a large range|which is not possible for ordinary materials.
First, we systematically evaluate the static mechanical properties and the
phonon dispersion relations. We show that the two are quantitatively consistent
in the long-wavelength limit. To demonstrate the feasibility, corresponding
fabricated polymer microstructures are presented. Finally, we discuss
calculations for laminates composed of alternating layers of two different
metamaterials with equal bulk modulus yet different mass density. This leads to
metamaterials with effectively anisotropic uniaxial dynamic mass density
tensors.
