Microporous and Flexible Framework Acoustic Metamaterials for Sound Attenuation and Contrast Agent Applications.

The low-frequency (100-1250 Hz) acoustic properties of metal-organic framework (MOF) materials were examined in impedance tube experiments. The anomalously-high sound transmission loss of HKUST-1, FeBTC, and MIL-53(Al) quantitatively demonstrated that these prototypical MOFs are absorptive acoustic metamaterials. To the best of our knowledge this is the first example of MOFs that have been demonstrated to be acoustic metamaterials. The low-frequency acoustic dampening by subwavelength MOF metamate-rials is likely due to sound dissipation and absorption facilitated by multiple internal reflections within the microporous framework structure. Modification of MIL-53(Al) with flexible organic linkers clarified that acoustic signatures of the metal-organic frameworks may be tailored to add or alter certain diagnostic acoustic signatures. These results may be applied to the rational design of lightweight sound-insulating construction materials and acoustic contrast agents for subsurface mapping and monitoring applications at low fre-quency (100-1250 Hz).