Fixing a sub woofer

[VK3ZYZ]

I really do not know why these devices are so popular. Maybe my ears are different but to me, the sub woofer BOOM BOOM is quite annoying. Anyway, I'm in the process of fixing one for a friend.
The speaker is dead, grinding on the frame when moved.
Can anyone spot something odd with this speaker?

 

[VK3YNV]

From this angle the magnet looks to be off center?
Also something looks a bit funny with the left hand side spade, or is it just smaller to maintain phasing?

 

[VK2RK]

left hand side spade, or is it just smaller to maintain phasing?

That is normal as you say to maintain correct phasing.
The extra magnet stuck on the back plate is the odd part to me. Could be done to increase the flux

 

[VK3ZYZ]

My thought is the extra magnet is outside the magnetic circuit and has been added to just make it look bigger. Along the lines of extra flashing lights and chrome knobs???

 

[VK2RK]

The backing plate is part of the magnet and it has the centre core attached to it that the voice coil operates on, the extra magnet will definitely increase the flux at the core.

Quote of Opinion from other:
"Depends which way round it is. It's almost completely out of the main magnetic circuit, but its fringes will either add, or subtract, a little - maybe 10-15% - to the flux in the field where the voice coil operates.

If it subtracts from the flux the speaker will be slightly - paradoxically - louder (the same way an electric motor goes faster when you reduce the field strength - more motion is required to generate the same back EMF). Probably about 1dB louder which is quite difficult to hear. (Some preamps have digitally generated 1dB steps so you can test what 1dB sounds like, and yes it is just audible).

If it adds to the flux, the speaker will be quieter - but its bass will be better controlled.

I have seen some poor guy flown from Japan to Europe to spend a few weeks supergluing magnets onto the back of 5000 drive units, to increase the flux where the original magnets were weaker than the manufacturer had promised."

What should attaching another magnet to the back of the speaker magnet do?

The dupe speaker I have here 8 \$\Omega\$ 0.5W had it's magnet split from the coil/cone. So I did something thoughtless - attached the speaker magnet to the back of the other speaker (same specs)....

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