Abstract: The invention consists of a method and apparatus for very rapid, assembly-line inspection of fluid filters for manufacturing flaws; which, when the filter is in service, would pass particles harmful to the mechanism or device which the filter is designed to protect. A harmless and low-cost particle test agent, with a wide range of particle sizes, is blown into the filters. If particles of a size which would be potentionally harmful in service, because of their relatively large size, pass through the filter along with the harmless small particles, they are concentrated in the air stream by a cyclone. The particles and the small remaining stream of air, then enter an acoustic sensor which signals rejection of the filter, because it will signal only the larger, potentially harmful particles.