Compact broadband acoustic absorber with coherently coupled weak resonances

Deep-subwavelength acoustic absorbers have received great attention due to their scientific and application values. Porous materials and micro-perforated absorbers, as the most conventional solutions, generally possess a structural thickness comparable to working wavelength, which hinders their potentials in the low frequency region. The recent advance in acoustic metamaterial/metasurface brings forward perfect absorbers (resonators) that possess ultra-thin thickness but relatively narrow working frequency bandwidth. Then these perfect resonators with quasi-perfect absorption are combined to piece together a broad absorbing band. However, the requirement of quasi-perfect absorption substantially places a very strict restriction on the impedance and thickness of the resonators.

Recently, the research teams from Tongji University and The Hong Kong Polytechnic University go a counter-intuitive way -- coupling imperfect components that possess low absorption peaks to achieve a compact broadband acoustic absorber. Their work opens a new pathway for this challenging goal by exerting the coherent coupling effect among the imperfect components to an unprecedentedly dominant role. This lifts the major restrictions on each component and frees the manipulation of coherent coupling effect. Results show that although each of the components exhibits rather low absorption peak individually, by suitably modulating the coherent coupling effect among the imperfect components, they work collectively to provide a broadband (870 - 3224 Hz) quasi-perfect absorption (average coefficient reaches 0.957). This work explores compact broadband acoustic absorbers with coherently coupled weak resonances and provides deep insight into the fundamental characteristics of these coupling systems, which may pave the way for developing novel acoustic devices against low frequency noise, and pave a way to modulate the surface acoustic impedance arbitrarily in broadband.