Acoustic perfect absorbers via spiral metasurfaces with embedded apertures

In this work, we analytically and experimentally present perfect acoustic absorbers via spiral metasurfaces composed of coiled channels and embedded apertures. Perfect absorption (reaching 0.999 in experiments) is realized with an ultra-thin thickness down to ∼1/100th of the operating wavelength. Owing to the superior impedance manipulation provided by the embedded apertures, perfect absorption with tunable frequencies is demonstrated. Our results would contribute to paving a way towards designing thin and light absorbers for the low frequency absorption challenge.