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