Fig. 6From: Acoustic wave science realized by metamaterials a–c are adapted from [41] a Membrane-type pipe. Membranes are inserted in each lattice constant d which is 7 cm, resulting in the negative mass density. b Helmholtz-type pipe. Side holes are inserted in series with lattice constant d, resulting in the negative bulk modulus. c Both membranes and side holes are inserted together, resulting in the negative phase velocity.Back to article page