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