Rainbow labyrinthine metamaterials for sound absorption applications

This work presents a proof-of-concept experiment demonstrating the efficient noise absorption of a 3-D printed panel, designed with arranged space-coiling labyrinthine elementary cells of various sizes to induce a rainbow effect. Numerical simulations are used to simulate absorption characteristics of the single labyrinthine units, and optimization methods are employed to further investigate the performance dependence on the geometry and thermoviscous phenomena. The dependence of absorption characteristics on cell thickness and lateral size is then experimentally verified. The resonance frequency is found to scale close to linearly with respect to both thickness and lateral size, allowing for tunability of the working frequency. Using these data, a panel is designed by arranging cells of different sizes in quasi-periodic lattice to exploit the acoustic ”rainbow” effect, resulting in a wider absorption spectrum covering the frequency range between 800 and 1200 Hz, which can be of particular interest in aeronautic applications. The performance of the panel is experimentally validated in a small-scale reverberation room, showing close to ideal values of absorption at the desired frequency of operation. This work suggests a design procedure for noise-mitigation panel solutions and provides experimental evidence of the versatility and effectiveness of labyrinthine metamaterials for tunable frequency sound attenuation.