Metamaterials provide an effective approach to flexibly control acoustic and elastic waves. Our work started from the early milestone works of achieving tunable shear modulus of materials and hybrid elastic solids with exceptional polarization controllability that belongs to neither solid nor liquid (Read more). Such polarization control was experimentally realized by a meta-beam with indefinite mass density and negative moment of inertia (Read more). Recently, we unveiled a way to realize a meta-cage with the ability of sound insulation under the condition of ventilation (Read more).