Several of the checks need some explanation... Clearance between port intake and driver should equal to or larger than the port diameter Clearance between port exit and top cover (when fitted) should equal to or larger than the port diameter. The gap between the driver and the bottom plate should be such that the air leaving the driver doesn't encounter a cross-sectional area that is any smaller than the cone area. This check is simplified somewhat by basing it on the nominal diameter of the driver.