A metamaterial having a frequency dependent elasticity tensor and a zero   effective mass density
  Within the context of linear elasticity we show that a two-terminal network of springs and masses, can respond exactly the same as a normal spring, but with a frequency dependent spring constant. As a consequence a network of such springs can have a frequency dependent effective elasticity tensor but zero effective mass density. The internal masses influence the elasticity tensor, but do not contribute to the effective mass density at any frequency. 
Graeme Milton and Pierre Seppecher
physics.class-ph cond-mat.mtrl-sci math-ph math.AP math.MP