Unklar. Seite 188, also kurz davor: Often, porous absorbers are covered in cloth or plastic. For example, a thin impervious membrane might be used to wrap a fibrous absorbent when it is used as a duct lining, to prevent fibres being lost at the ventilation system. Another example is a thin layer of light reflecting material, which, when placed in front of mineral wool, makes a sound absorbing mirror.34 Such impervious membranes will reduce the high-frequency absorption. At low frequencies, the membrane’s acoustic mass is small and the sound passes through largely unaltered—although a small increase in absorption may occur. At high frequencies, the membrane’s acoustic mass is high and it will prevent some or all of the sound wave entering the porous material. Und oben (S.190) steht membrane-wrapped, also in Membran eingewickelt, das klingt stark nach Folie. "impervious" heißt undurchlässig.