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We propose an acoustic metasurface for perfect absorption at dual frequencies within a compact space. Meta-molecules of the metasurface contain four subwavelength meta-atoms whose cavities are coiled by three foldings. The meta-atoms comprising a meta-molecule have slightly different neck sizes to obtain hybrid resonances at desired frequencies. In order to consider an effect of coiled spaces in the meta-atoms, we use equivalent straight cavities with an effective length in an analytical model. By using the analytical model, metasurfaces are optimally designed for perfect absorption at desired frequencies. The experimental results show that a λ/23-metasurface exhibits over 99% energy absorption at 294 Hz and 406 Hz. Furthermore, we define another optimization problem to minimize the thickness of the metasurface for perfect absorption at two desired frequencies. The experimental results show that a λ/32-metasurface exhibits over 91% energy absorption at 281 Hz and 99% sound energy absorption at 403 Hz.

This study starts with a simple question: can the vibration of plates or beams be efficiently reduced using a lightweight structure that occupies a small space? As an efficient technique to damp vibration, the concept of an acoustic black hole (ABH) is adopted with a simple modification of the geometry. The original shape of an ABH is a straight wedge-type profile with power-law thickness, with the reduction of vibration in beams or plates increasing as the length of the ABH increases. However, in real-world applications, there exists an upper bound of the length of an ABH due to space limitations. Therefore, in this study, the authors propose a curvilinear shaped ABH using the simple mathematical geometry of an Archimedean spiral, which allows a uniform gap distance between adjacent baselines of the spiral. In numerical simulations, the damping performance increases as the arc length of the Archimedean spiral increases, regardless of the curvature of the spiral in the mid- and high-frequency ranges. Adding damping material to an ABH can also strongly enhance the damping performance while not significantly increasing the weight. In addition, the radiated sound power of a spiral ABH is similar to that of a standard ABH.

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