**Bonus #1** Be cautious with some loudspeaker manufacturers that use a quasi phase plug which looks like a phase plug for marketing purposes, but does not physically separate from the cone of the driver. These "so called" phase plugs can sometimes do more harm than good as they can provide an inconsistent surface area and can also increase mass of the cone yielding poorer frequency response and linearity.
A real phase plug essentially eliminates the need for a dust cap, which in turn, reduces cone mass, permitting extended frequency response. It allows more airflow through the motor structure, which improves cooling and thus increases power handling. A Phase plug also helps to reduce on-axis beaming by dispersing higher frequencies that the cone is producing.
It actually looks like they put condoms over drivers with phase plugs... 
Ridiculous! 
