Tunable low-frequency and broadband acoustic metamaterial absorber

Current sound-absorbing materials have fixed absorption spectra due to unalterable local resonance properties, which limit their application potential in many noise control scenarios. Clear motivation exists, therefore, to design an acoustic absorber to fit the actual noise spectrum with reconfigurable geometry and subwavelength thickness. Here, we analytically present and experimentally verify a tunable low-frequency acoustic absorber composed of multi-layered ring-shaped microslit tubes with a deep subwavelength thickness. This decreases the working frequency and significantly increases the acoustic absorption efficiency simultaneously. A physical model of the proposed metastructure is established on the basis of an acoustic equivalent circuit using microslit absorber theory. Superior impedance manipulation capability is achieved by rotating the middle microslit tube from 0° to 180°. This enables continuous tunability of the metamaterial absorber over a wide working frequency band. In both the simulated and measured results, highly efficient acoustic absorption (at least 0.9) is achieved in the range of 280–572 Hz. Simulations under oblique incidence are conducted to validate the wide-angle performance of the absorber. Based on the proposed tunable absorption mechanism, a hybrid metamaterial absorber is designed to produce adjustable broadband absorption with high efficiency. Our work helps pave the way to absorbing metamaterials being used in practical engineering applications such as noise control due to the advantages of tunable functionality, compactness, high efficiency, wide-angle absorption, and easy fabrication.