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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?
 

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Quote:
Supposedly bored out pole pieces are better for cooling.
http://www.klippel.de/typo3temp/pics/902cf5e5eb.jpg

" Increase of voice coil temperature The figure above shows the heat transfer of a woofer in a loudspeaker system with and without vent in the pole piece. The air below the dust cap will be ventilated through the open vent, and the convection cooling of the coil is low giving a low bypass factor (pink curve in right diagram). After sealing the vent (shown in the left sectional view), the air is pressed through the air gap, and the high velocity of the air particles increases the bypass factor to 50 %. "

http://www.klippel.de/uploads/media/Nonlinear_Modeling_of_Heat_Transfer_01.pdf

http://jblpro.com/ProductAttachments/tn_v1n18.pdf

http://www.mountain-environment.com/AES_paper_1998_4667.pdf
 
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