Low frequency sound attenuation in a flow duct using a thin slow sound   material
  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. 
Yves Auregan and Maaz Farooqui and Jean-Philippe Groby
physics.class-ph physics.flu-dyn