Perhaps you should try some spelling variations:
acoustical materials » acoustic materials, optical materials, critical materials
acoustic material » acoustic materials, acoustic metamaterial, acoustic metamaterials
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 (((acoustical materials) OR (acoustic material))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((optic materials) or (((acoustical materials) OR (acoustic material))))%22".
• Adding a wildcard symbol may retrieve word variants: %((optic materials) or (((acoustical materials) OR (acoustic material))))%22*.