Phase plug

Sometimes, cone drivers are fitted with a phase plug instead of a dust cap. While, the dust cap is permanently attached to the diaphragm, the phase plug is fitted in the same place but is attached to the pole piece of the loudspeaker and so does not move. The acoustic effect is that the sound propagated by the cone does not reflect from the pole piece. Phase displacement can, in this case, result in cancellations of the sound waves, which become audible as dropouts in the frequency response.