A horn-loaded compression driver or a loudspeaker has a phasing plug with multiple slots and a common annular chamber. The slots extend from an inlet side to the common annular chamber, which extends to an outlet side. Each slot has a path length extending to a common focal point in the common annular chamber. The common focal point has a common path length extending to the outlet side. The phasing plug provides an approximately flat acoustic wave front from the compression driver to the horn.