Shorting rings are used to make motor movement more symmetrical, as you know. I have only seen evidence that this reduces even harmonics, i.e. 2nd, 4th, etc. Speaking only of harmonic distortion, are you saying that you believe odd harmonics are reduced too?

Other non-linearities are present in the system, to be sure. At high power levels in particular, there are many other matters than just push-pull flux symmetry. But push-pull symmetry is what the shorting ring is used for. Can you think of other benefits it may bring?

Wayne

I think that Our discussion comes down to what a shorting ring does.

In my understanding, and I have some considerable experience with large loudspeakers companies in this, a shorting ring does two things.

The first is basically what you said, except that it does not just make the flux symmetrical, but stabalizes it, symmetrical or not. A shorting ring, if it had zero resistance, would not allow for any flux that pasess through it to change. Any change in the flux would result in a ring current which would produce exactly the counter flux required to make this change zero. This change is MOSTLY symmetrical, but since the entire flux circuit is nonlinear it also has a non-symmetrical part. All changes - symmetrical or not - are canceled. But shorting rings are not zero resistance, so they cannot do this perfectly. The lower the resistance the better. JBL once used silver. Today we know that a lot of copper works just as well at a much lower cost.

The other effect of a shorting ring is to linearize the inductance change with position. This is closely coupled with the above effect in many respects in that the inductance change with current is closely associated with flux modulation. It is different than the inductance change with position. A shorting ring will tend to negate the inductance increase due to the steel core of the motor structure, thus decreasing the change with position. This lowering of the inductance is a primary effect in compression drivers particularly at high frequencies. The previous effect (flux modulation) is more dominate in woofers, but both effects exist in both components. The change of inductance with position is again dominately non-symmetrical, but it has signifcant symmetrical aspects to it.

I hope that this discussion sets well with your understanding.

Earl Geddes

When I speak of symmetry and flux stabilization, I'm talking about the fact that the voice coil adds to the fixed magnetic flux in one direction and subtracts from it going the other way. You're right that the shorting ring has to be carefully sized and positioned so that its effect balances these forces and reduces asymmetry from flux modulation. But when properly implemented, it does a very good job of making the motor movement more symmetrical.

One thing I've realized about shorting rings is that, like you said, the conductor isn't perfect so there is a minimum speed they work at. This translates to a lower frequency limit where they become ineffective. If the ring were superconductive, I suppose it could be made very small and it probably could work at very low frequencies. But since it isn't, a large conductor is required and that displaces magnetic material. There comes a lower limit where increases in ring size become prohibitive.

The geometry of the gap and the position and size of the voice coil are important features too. They sort of set a baseline, and if static flux is asymmetrical, there is little point in AC flux stabilization. Using a shorting ring on a motor with asymmetrical static flux in the gap is a waste. The really cheap drivers sound bad even at low volume levels because the static flux is asymmetrical. The diaphragm moves further in one direction than the other even at very low drive levels. But better drivers do a pretty good job at low levels, and only start becomming asymmetrical at high drive levels. That's where flux stabilization can help them achieve that extra level of symmetry.

We seem to see things differently and I don't want to be argumentative, but here are the palces that we differ on.

A shorting ring is purely resistive, no reactance at all. Hence ther can not be any time constants associated with it.

I don't agree that the flux has to be symmetrical for a shorting ring to work. I don't even agree that a symmetrical flux is even necessary at all. Sure if it is symmetrical the second harmonic will be reduced, but the third will go up. I no of no data to show that this is a subjective improvement.

Honestly, from the data that I have virtually all long held beliefe about nonlinear distortion are incorrect. Time will tell.

I don't find you to be argumentative at all. Your posts seem very well mannered to me, and I appreciate your comments. What sort of gets my goat is when people pop up on forums like these with patronizing or arrogant comments, or being downright rude and making personal attacks. But none of that is going on here. We're just kicking some ideas around, and I think it is great.

Have you seen John Eargle's document about magnet structures? It illustrates motors with static flux symmetry and those without, and it compares alnico, ferrite and flux stabilized ferrite magnets.

Wayne

John is a very old friend from my days of working with JBL. He is highly complimentary of my book.

I would really like to read his comments, but the link that you had was not readable on my browser. Do you have any other formats for it? Or a link to a PDF or something like that?

• Symmetrical Field Geometry, Page 1
  
• Symmetrical Field Geometry, Page 2
  
• Symmetrical Field Geometry, Page 3

Thanks

Yes this is old stuff. John's paper is also misleading or wrong as proved by JBL's own recent product moves.

Its not just the gap geometry that lowers the distortion in an ALNICO structure. ALNICO is a good electrical conductor, unlike ceramic, an so it is its own a shorting ring! So his conclusion that the geometry accounts for the changes is incorrect. JBL recently started to use ALNICO again for variuos reasons - lower flux modulation and near zero flux changes with themerature, both of which are very bad in ceramic.

Then there is the fact that second and third harmonics are not really the important ones.

John's paper was the prevailing philosophy at the time, but now we know more than we did then.

Its hard to beat ALNICO for performance, but its price and weight are real drawbacks. The better drivers today are using laminar motor structures with each lamina being copper plated.

But static symmetry is important too. I'll look around and try to find some comparitive data somewhere. In the meantime, a simple thought experiment should suffice. If gap geometry provides more force in one direction than the other, then motor movement will be asymmetrical, even without considering flux modulation. The shorting ring resists flux modulation, but statically symmetrical flux in the gap is important too.

The point of this paper is to "sell" JBL's change to ceramic magnets as "OK". John talks about both the flux ring and the symmetrical geometry. I think that I am missing the rest of your points. I don't think that I ever disputed that the excursion was not symetric in an unsymmetric field or that flux modulation does not modify the excursion in an unsymmetrical way.