The air permeability of a material can be described as the resistance air encounters as it attempts to travel through that material. Materials with tight pore structures usually create more resistance to air than materials with larger pores. Therefore, materials with a 20µ or 40µ pore structure will have lower air permeability than materials with 100µ or 300µ pore structures. In the case of sintered porous plastics, air permeability is typically expressed in terms of air flow rate, differential pressure, and time.
Densometers are the accepted standard for measuring the porosity, air-permeability or air-resistance of sintered porous plastics and membranes. Densometers tend to measure the time required for a given volume of air (typically 25cc to 400cc) to flow through a standard area of material being tested under light, uniform pressure. Beyond this, certain models and procedures can also be used to measure surface smoothness and material softness.