A compact broadband absorber based on helical metasurfaces

Owing to the pronounced wave-matter interaction at resonance, meta-absorbers show impressive potentials for eliminating low-frequency sound even at thicknesses far below the corresponding wavelength. For a meta -absorber, a traditional way of achieving broadband low-frequency absorption is to piece together a graded bank of tuned resonators side by side, while this eventually leads to a growing size as more and more elements are combined. Here, we present a broadband absorber built up by detuned resonators in a compact manner. By leveraging the space-coiling concept, the absorber features a remarkable reduction in thickness, while, owing to the way that elements twisting around each other, the absorber further shows compactness in surface area. Simulations and experiments on a proof-of-concept prototype reveal an effective absorption band ranging from 299 to 623 Hz with a thickness of just 52 mm (around 1/22 of the lowest working wavelength). Given that the effective sound path in a meta-unit can be continuously adjustable by the helical pitch, our investigations offer a compact way for broadband yet customizable noise absorption. More essentially, given that wave-matter interaction is heavily dependent on the surface's impedance while the perfect absorption suggests an extreme impedance modulation (matched-impedance modulation), our work suggests a compact approach towards on -demand acoustic impedance engineering in broadband.