We present a thin subwavelength material that can be flush mounted to a ductand which gives a large wide band attenuation at remarkably low frequencies inair flow channels. To decrease the material thickness, the sound is slowed inthe material using folded side branch tubes. The impedance of the material iscompared to the optimal value, which differs greatly from the characteristicimpedance. In particular, the viscous and thermal effects have to be very smallto have high transmission losses. Grazing flow on this material increases thelosses at the interface between the flow and the material.
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