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Old 21st February 2004, 09:08 PM   #1
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Default adding magnets

I've got a couple of old 8" drivers whose cones have suffered from too much wine spilt on them, kid's fingers etc i.e. the cones are stuffed, but they've got big magnets which can be unscrewed.

Rather than ditch them, I was wonder what the effect would be of simply sticking these magnets on the back of some good drivers?

Increased sensitivity???

or is this just a silly idea?

Pete McK
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Old 21st February 2004, 09:32 PM   #2
sreten is offline sreten  United Kingdom
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glued on the way they dont want to go you will get a minor
increase in sensitivity and a major decrease in stray magnetic
field, assuming the magnets are removed from the pole pieces.

sreten.
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Old 22nd February 2004, 10:01 AM   #3
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I do know what happens when you do this kind of thing to a DC electric motor - it runs slower for the same applied voltage because it always tries to rotate at whatever rpm that generates the same as the supply voltage.

Translate that into loudspeaker-speak it means that if you double the magnet strength the sensitivity to drive voltage drops by 3db (perhaps 6dB?) This is because the voice coil impedance doubles. You have double the flux on the same length of wire. It is the same as the same flux on double the length of wire. If you also reduce the length of voice coil wire by the amount you increased the magnet strength, then...
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Old 22nd February 2004, 10:14 AM   #4
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Circlotron
I do know what happens when you do this kind of thing to a DC electric motor - it runs slower for the same applied voltage because it always tries to rotate at whatever rpm that generates the same as the supply voltage.

Translate that into loudspeaker-speak it means that if you double the magnet strength the sensitivity to drive voltage drops by 3db (perhaps 6dB?) This is because the voice coil impedance doubles. You have double the flux on the same length of wire. It is the same as the same flux on double the length of wire. If you also reduce the length of voice coil wire by the amount you increased the magnet strength, then...
But the glued on magnet is not part of the magnetic circuit ?

(Presume that the pole pieces are removed)

Glued on in opposition to the stray magnetic field in will cancel
a lot of it, if its too strong, most of its field will be cancelled ?

By making the stray field magnetic path "harder" you will get
a minor increase in the flux of the magnetic circuit ?

This is how I understand bucking magnets to work, and that
they are required in fully shielded drivers to prevent the steel
screening can magnetically saturating due to the stray field.

sreten.
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Old 22nd February 2004, 10:36 AM   #5
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Yeah, I was more talking about if you actually increased the total magnet strength, which you would not do just by putting another magnet on the outside. You would have to remove the back pole plate and put the new magnet directly in contact with the original one then put the pole plate back on, as you implied.

I was raving on about sensitivity because a stronger magnet seems to do the opposite of what you would expect.
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Old 22nd February 2004, 11:11 AM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Circlotron
Yeah, I was more talking about if you actually increased the total magnet strength, which you would not do just by putting another magnet on the outside. You would have to remove the back pole plate and put the new magnet directly in contact with the original one then put the pole plate back on, as you implied.

I was raving on about sensitivity because a stronger magnet seems to do the opposite of what you would expect.
You would also need to extend the centre
polepiece by the thickness of the magnet.

However this relatively simple engineering expedient is used.

But the magnetic circuit and the magnets cannot support the
doubling of flux, the whole thing is not optimally designed.

Some point of the magnetic circuit is forced deeper into saturation.
(Ceramic magnets are flat shaped, to give more area for the flux)

You can make the gap slightly longer, but not a lot.


Nevertheless Vifa for one use double magnets in some of their
drivers, presumably because engineeering wise its simple to
do, and there are some advantages.
But the magnet circuit design is sub-optimal.

Also your DC motor analogy must be flawed. Practical experience
shows that the more flux you have, the higher the sensitivity.

Taken to its logical limit - you need tiny magnets for high sensitivity -
and we all know that this simply isn't true, you need big magnets.

sreten.
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Old 22nd February 2004, 11:49 AM   #7
AGGEMAM is offline AGGEMAM  Denmark
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There are several thing that happen when you apply an extra magnet to an existing speaker driver.

If you mount it the way it has least resistance, ie in phase, you actually in magnetic flux loss, because the voice coil gap where the flux is intended to be concentrated is usually above the the existing magnet mounting another magnet in phase will pull magnetic flux away from the voice coil gap thereby decreasing the flux strength in the voice coil gap. Mounted this way will also result in increased stray magnetic radiation.

If you mount it the way it has most resistance the opposite will happen. The magnetic flux centre point will be pushed nearer towards the voice coil gap thereby increasing the magnetic flux strength in the voice coil gap. Mounted this way will also reduce stray magnetic radiation.

Almost all parameters of a driver is interdepended this means if the magnetic flux strength is changed almost all parameters of the driver is changed as well. And since it is almost impossible to predict how much the flux will change, having access to measurement equipment would be a nice thing.

As a general rule when the flux is increased though sensistivity will go up along with fs. Vas and Qt will go down however.

This most often has the result that bass and midrange will sound more controlled and with much improved definition. I wholeheartedly recommend it for basically every speaker driver. I know it's one of the first thing I do when I get a new driver, it almost always improve the driver over stock ones.

Please note that you should never mount a magnet that is bigger than the original magnet as this can have very unpredictable results.
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Old 22nd February 2004, 12:19 PM   #8
sreten is offline sreten  United Kingdom
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Sorry, but I can't see how Fs or Vas could possibly change.

sreten.
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Old 22nd February 2004, 12:36 PM   #9
AGGEMAM is offline AGGEMAM  Denmark
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Quote:
Originally posted by sreten
Sorry, but I can't see how Fs or Vas could possibly change.

Yes, I know that theoretically they shouldn't as neither parameter is directly controlled by the flux density. But it is my experience that it does. Only slightly though.

I believe it may be because the voice coil contains a slight amount of magnetic material and thereby decreasing Cs (and all parameter connected with that), but this is only my best guess.

NOTE EDITED
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Old 22nd February 2004, 02:14 PM   #10
tuneman is offline tuneman  Australia
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just thought i'd add something to this topic'
i had spare sub lying on my floor a wile back and one day i decided to move it cos it was in the way' i put it on top of one of my surround speakers(two 4 inch mids and a tweet) witch is on a stand,
i didnt really take much notice for about two weeks then one day i put on a dvd and i thought the other surround speaker wasn't working but infact the sub on the other one was making that one about double the volume

so its my conclusion that it increases sensitivity a lot in small mids and such but in larger speakers that already have large motors the effect would be minimal especially to the ear
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