sentences:  Porous absorbers are most commonly made from fiberglass, mineral wool or open cell foam that resists the passage of air molecules through the interstitial space  
  .Porous absorbers often incorporate a foil or paper facing to reflect frequencies above 500 Hz.Facing also improves low bass absorption by translating the physical compression of air at the facing into physical compression of the fibers that are in contact with the facing while also maintaining the resistive loss of air as it is driven through the bulk of the fiber by the facing.