  In this work, we demonstrate in a proof of concept experiment the efficient
noise absorption of a 3-D printed panel designed with appropriately arranged
space-coiling labyrinthine acoustic elementary cells of various sizes. The
labyrinthine unit cells are analytically and numerically analysed to determine
their absorption characteristics and then fabricated and experimentally tested
in an impedance tube to verify the dependence of absorption characteristics on
cell thickness and lateral size. The resonance frequency of the unit cell is
seen to scale approximately linearly with respect to both thickness and lateral
size in the considered range, enabling easy tunability of the working
frequency. Using these data, a flat panel is designed and fabricated by
arranging cells of different dimensions in a quasi-periodic lattice, exploiting
the acoustic "rainbow" effect, i.e. superimposing the frequency response of the
different cells to generate a wider absorption spectrum, covering the target
frequency range, chosen between 800 and 1400 Hz. The panel is thinner and more
lightweight compared to traditional sound absorbing solutions and designed in
modular form, so as to be applicable to different geometries. The performance
of the panel is experimentally validated in a small-scale reverberation room,
and an absorption close to ideal values is demonstrated at the desired
frequencies of operation. Thus, this work suggests a design procedure for
noise-mitigation panel solutions and provides experimental proof of the
versatility and effectiveness of labyrinthine metamaterials for tunable mid- to
low-frequency sound attenuation.
