Soft Materials with Broadband and Near-Total Absorption of Sound

Low- and broadband-frequency sound absorption promotes myriad practical applications and scientific endeavors. Yet emerging metamaterial concepts enable such sound absorption only by the use of a complex assembly of attenuator constituents or by the addition of heavy undesirable mass. In this study, multiple low-frequency hybrid resonances are achieved in a soft-lightweight-material-based Helmholtz resonator. By replacing the rigid walls of a traditional Helmholtz resonator with compliant walls and by tailoring the relative structural and acoustic compliances, a unique multiphysics coupling among the material, structure, and sound is realized. These coupling mechanisms yield hybrid resonances that can be used for subwavelength, broadband, and near-total absorption of sound. As a result, the compliant-material resonator exploits a lightweight-material monolithic design that exerts dramatic and tunable control over low-frequency and broadband acoustic-energy transfer. This paper details analytical and experimental investigations to test the concept and reveals strategies for near-total absorption at arbitrary subwavelength frequencies.