Perhaps you should try some spelling variations:
acoustic materials » acoustic metamaterials, plastic materials, elastic materials
acoustic material » acoustic metamaterial, acoustic metamaterials, plastic material
optic materials » optical materials, basic materials, ionic materials

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %((optic materials) OR (((acoustic materials) OR (acoustic material))))%22.
• Adding a wildcard symbol may retrieve word variants: %((optic materials) or (((acoustic materials) or (acoustic material))))%22*.