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