Perhaps you should try some spelling variations:
acoustical materials » acoustic materials, optical materials, critical materials
acoustic material » acoustic materials, acoustic metamaterial, acoustic metamaterials
optical material » optical materials, optimal maternal, optic 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: %((optical material) OR (((acoustical materials) OR (acoustic material))))%22.
• Adding a wildcard symbol may retrieve word variants: %((optical material) or (((acoustical materials) or (acoustic material))))%22*.