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Old 18th December 2013, 10:07 AM   #1
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Default Axial or Diametrical neo magnets for homemade speaker?

Hi guys,

I would like to build a homemade speaker from scratch using a 1'' by 1'' neodymium magnet. The voice coil will be wound on a former outside the magnet. Assuming that there will be no pole pieces attached to the magnet, what type of magnet would be suitable to achieve the strongest surface magnetic field? Should it be axially or diametrically magnetized?

It is my understanding that in order to increase magnetic flux/motor strength, the voice coil should be moving perpendicularly to the magnetic field of the permanent magnet. Doesn't this suggest a diametrical magnet?
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Old 19th December 2013, 10:38 AM   #2
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Any ideas?
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Old 19th December 2013, 11:00 AM   #3
AndrewT is offline AndrewT  Scotland
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Originally Posted by chlorofille View Post
....... 1'' by 1'' neodymium magnet. ............
cube, or square prism (cuboid), or sphere, or?
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Old 19th December 2013, 12:59 PM   #4
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It's a cylinder.
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Old 19th December 2013, 01:04 PM   #5
AndrewT is offline AndrewT  Scotland
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?
1" diameter by 1" long?
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Old 19th December 2013, 03:56 PM   #6
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Here's the exact magnet I had in mind.

Cheers,
Dinesh
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File Type: png Neo magnet.png (167.1 KB, 141 views)
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Old 22nd December 2013, 11:11 AM   #7
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Sketch of the set-up :
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File Type: png Diametrical vs Axial neo magnet.png (519.9 KB, 140 views)
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Old 22nd December 2013, 06:10 PM   #8
JMFahey is offline JMFahey  Argentina
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A) definitely won't work, you'll have the right half of the voice coil pulling "down" when other half pulls "up" and vice versa and there's no way to correct that.

B) is the same, top of VC pulls opposite to what bottom does, but it can be corrected by moving magnet up or down so most or all of it sits outside the VC.

The useful field is that which passes through wire turns perpendicular to VC axis.

Efficiency will be very poor, that's why we use magnetic circuits to drive lines of force where we need them, not all over the place.
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Old 22nd December 2013, 06:14 PM   #9
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JMFahey,

Thank you very much. Looks like the pole pieces are mandatory for optimal performance.

Rgds,
Dinesh
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Old 22nd December 2013, 06:41 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

You need axial, to start off with, and pole pieces for a concentrated gap,
which will contain far more flux than any open field arrangements.

Ceramics cannot support the flux levels of pole pieces hence they
they are wide and flat, but Neo's are much more like AlNiCo's
with similar flux capability so you need a similar arrangement.

Look at centre slug AlNiCo speakers, not outside ring.

Its not hard to optimise the gap with basic modelling.

rgds, sreten.
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