  We describe here a class of acoustic metamaterials with fractal Hilbert
space-filling and coiled geometry with equal tortuosity for noise mitigation.
Experiments are performed using a four-microphone impedance tube and
benchmarked against non-viscous and viscothermal Finite Element models related
to configurations spanning up to five fractal/geometry orders. We show that the
acoustic absorption can be predicted by the resonance of the cavities
associated to the tortuous paths. For a given fractal/geometry order, the
acoustic absorption at specific frequencies is also enhanced by maximising the
difference between the minimum and maximum fluid particle velocity of the air
inside the patterns. These principles can be used to design high-performance
acoustic metamaterials for sound absorption over broad frequency ranges.
