Phase plug

In a loudspeaker, a phase plug, phasing plug or acoustical transformer is a mechanical interface between a speaker driver and the audience. The phase plug extends high frequency response because it guides waves outward toward the listener rather than allowing them to interact destructively near the driver.

Phase plugs are commonly found in high-powered horn loudspeakers used in professional audio, in the mid- and high-frequency bandpasses, positioned between the compression driver diaphragm and the acoustic horn. They may also be present in front of woofer cones in some loudspeaker designs. In each case they serve to equalize sound wave path lengths from the driver to the listener, to prevent cancellations and frequency response problems. The phase plug can be considered a further narrowing of the horn throat, becoming an extension of the horn to the surface of the diaphragm.