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