Aerogel as a Soft Acoustic Metamaterial for Airborne Sound

To realize useful, exotic properties, acoustic metamaterials for airborne sound traditionally have relied on materials that are much harder than the surrounding fluid. Soft acoustic metamaterials utilizing mesoporous silica aerogel structures offer an alternative means to tune the effective properties of the resulting metamaterial, while simultaneously providing better coupling to the acoustic environment. This study presents systems demonstrating negative density, density near zero, and nonresonant, broadband ``slow sound'' propagation.