This paper investigates the permeability of several multi-layered, woven filter media using air, water and bubble point methods. Estimated filter efficiencies from the bubble point measurements were then compared to a new sonic challenge test method where precision microspheres are fluidised through the pores. Inconsistencies in the bubble point filter efficiencies were found to be dependent on the openness or permeability of the weave. The filter efficiency of a non woven metal filter medium was then related to the pore size distribution measured by microscopy.