So the top plate focuses the magnetic field of the permanent magnet on the voice coil. I have read that the pole piece is used to shape the magnetic field as well. How is the pole piece used in controlling the magnetic field? Is it simply carrying a magnetic charge from being in proximity to the permanent magnet? I have seen some pole pieces are bored out and some are solid. Supposedly bored out pole pieces are better for cooling. If the pole pieces carry a magnetic charge, wouldn't it be better as a solid piece for more magnetic charge? Which is more preferable with respect to performance?