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).

I don't notice any difference except (this may be my imagination) it seems to sound a little louder.

Should attaching another magnet to the back of the speaker magnet make a difference? What should attaching another magnet to the back of the speaker magnet do?

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.

• Wouldn't the extra magnet help the speaker to move in one direction and hinder it in the other, causing some frequency distortion? – Garrett Fogerlie Dec 24 '12 at 16:33
• Not as long as it is correctly centred. If its offcentre then there could be a little radially asymmetric flux in the gap, pushing the voice coil sideways. (But the gluing fixture guaranteed concentricity!) – Brian Drummond Dec 24 '12 at 16:38
• I understand that, however my question was with regards to the voice coil moving back towards the magnet and then forwards (away from the magnet). – Garrett Fogerlie Dec 24 '12 at 16:52
• A speaker may be quieter with a stronger magnetic field, but only at its resonant frequency, due to increased damping. Away from the resonant frequency (mids, highs), of course a speaker will be louder with a stronger magnet: per mA of current, there is more displacing force. The increased damping around the resonant frequency could be combated by raising the output impedance of the amp (e.g. by adding some current feedback to the voltage feedback). – Kaz Dec 24 '12 at 17:27
• @Kaz : you're correct that the increased damping is mainly visible at resonance, and the way to "combat" it. That's why low flux is a problem; you can't normally decrease the amplifier output impedance below zero! – Brian Drummond Dec 24 '12 at 18:26

It will cause the voice coil to not move how it was designed too, since the induced magnetic field will be different. It will most likely effect different frequencies differently, for example higher frequencies will probably not notice a difference, but lower ones may not be as sinusoidal as they originally were (because the cone may have less resistance going backwards, and more going forwards.) However the overall effect will probably be minor.

One thing to not is that it could cause premature damage to the speaker as the voice coil may make contact with the side at some points.

• If it does make contact with the side, it'll be easy to hear! – Brian Drummond Dec 24 '12 at 16:50

The back of the magnet is usually flat iron. Any magnet stuck to the back will have very little effect. The magnetic flux lines from an external magnet will not pass through the air gap that the voice coil moves in so they will have no effect.

If the speaker should be of a budget type that has a magnet exposed at the rear and no outer iron pole then there might be some reinforcement if the external magnet reinforces the field due to the existing magnet and the lines of magnetic flux pass into the metal gore and then radiate out through the voice coil.

Below is a diagram that shows how the typical speaker is constructed and how the add on magnet at the back will have very little effect at the voice coil. Illustrated are a more exotic annular pole magnet and a common top bottom pole magnet.