They may appear to be solid but they are not, and thus can absorb sound. The other approach is a surface that is patterned with 3D triangles or pyramids, as found in acoustic foam panels.

The lightweight panels offer ease of application for face materials (laminate, veneer or coatings), with multiple edge finishing options available. Thickness options range from 3/4, 1, 1-1/4, 1-1/ ...

Cellular cast aluminum foam, sound absorbing properties, strong yet lightweight panels, versatile material, contains up to 100% recycled content, corrosion resistant. Alusion™ Small Cell ...

the same foam that is used as stucco backer board and is expanded into large blocks of a pound and a half density. The company creates 8” thick panels, allowing it to achieve an R-31 rating ...

Cymat Technologies Ltd is an innovative materials technology company that has the exclusive worldwide rights for producing Alusion™ Stabilized Aluminum Foam ... it has sound-absorbing properties ...