A sample is clamped over an opening and air is drawn through it at a set pressure drop, such as 100, 125 or 200 Pa, and flow is reported in liters per square meter per second or in cubic feet per minute per square foot. It is a core spec for filter media, breathable fabrics and protective garments. Higher basis weight, finer fibers and denser bonding all lower it.
Air permeability is 350 liters per square meter per second, right on the filter spec, so leave the needling alone.
Always report the test pressure with the result; values at different pressure drops cannot be compared directly.
Also heard as
- air perm
- air permeance
Related terms
Electret
Filter fabric, usually meltblown, carrying a long-lasting electrostatic charge that pulls particles out of the air.
Needlepunch
Mechanical bonding in which barbed needles punch repeatedly through a batt, dragging fibers through and tangling it.
Basis weight
Mass of a nonwoven per unit area, almost always given in grams per square meter.
BFE
Bacterial filtration efficiency: the percentage of a bacteria-laden aerosol that a mask or filter fabric stops.