lots of know it alls in this thread for not one person to actually answer his question. Just saying it's a phase plug probably doesn't help him much. A phase plug is used to improve the off axis response of a speaker. When a speaker is listened to with the cone not facing you (off axis) it begins to do something known as beam, or beaming. Basically the top end of the frequency spectrum will begin to taper off. This is due to the inherent cancellation of a speaker with itself at higher frequencies. Normally we consider a single speaker to act as a single point of sound, which it will when the wavelengths of sound are much bigger than the speaker. However, we can also think of it this way. A single 6inch speaker can be considered 10000x very tiny pistons all producing the same wave, or 100000x even smaller ones, etc, every point of the cone is moving and interacting with itself if the wavelengths are shorter than the cone itself. Because of this, bigger speakers have a smaller off axis frequency response as they begin to beam earlier. That's also part of why tweets are so small, to keep frequency the speaker beams very, very high up. Anyway a phase plug in the center of a speaker to keep the speaker from interacting with itself. Some of the soundwaves that would normally interact from one side or the other will hit the phase plug, bounce the other way and then not cancel out since they didnt' interact with each other. This somewhat random dispersion is still better than full on cancellation.
Phase plugs can also help with power handling depending on the material as they can help draw heat from the coil. Their effect on cone area is pretty minimal in most cases.
