Reading DuMax Report: Symmetrical Field?

[kelticwizard]

This is the DuMax report for a Parts Express Dual Voice Coil 15" woofer. It has a very large 1-way excursion, 15mm.

Here is the link.
http://www.partsexpress.com/pdf/295-190dx.pdf

This woofer is being used in a Passive Radiator box. Such a box, along with a Vented box, should have a driver with a symmetrical magnetic field to prevent "suck-in" at resonance.

I am not really able to read a DuMax report. I understand they reveal a lot.

Is it possible to tell, from reading this DuMax report, whether this woofer has a symmetrical magnetic field?

Greatly appreciate any help on this.

[Lovan]

I have no idea what is meant as a symmetrical magnetic field but here´s my speculation.

I would guess that you read at for example -10mm displacement there is about 12 N/A and you should look at the Bl value at (+)10mm displacement which is also about 12 N/A.

But at -20 and (+)20 mm there is a slight difference in the Bl-value so...

I guess that the perfect "arch" would be the homogeneus one.

I hope you understand what I mean.

What is Kms?

[kelticwizard]

Lovan:

Don't know what Kms is-that is why I posted the DuMax report to see if anyone knew.

An abscence of a symmetrical field causes a woofer in a reflex box to move from the center position to the extreme of it's travel and stay there. It does this as the signal nears Fb.

Here is a thread explaining this.
forward diaphragm displacement--have you heard of this?

[Ron E]

If you mirror the Bl curve at x=0, the two curves should overlay, likewise for the Cms (=1/Kms) curve. If the curves were symmetrical about the rest position, the values for mag center and sus center would be 0.

You can see that the Kms curve is not symmetrical, but the Bl curve is closer. This should cause minor 2nd harmonic distortion for the driver that was measured, but we cannot assume that all of the DVC drivers will measure this way.

The original curves are in color, so it is difficult to say which curve is the actual one, but by the curve fit equation, the curve fit is a parabola, so the measured Kms curve must be the one with the flat bottom.

I would say that Dynamic offset, or oil canning, is a bit more complex than just magnetic assymetry - it has to do with the reduction in magnetic force with excursion, this reduces back emf and increases current flow through the coil, which increases excursion - and becomes a vicious cycle... I think to avoid dynamic offset you need both a symmetrical field and a spider that stiffens enough with excursion to balance the loss of Bl...

That said, I don't have enough experience reading these charts to be able to tell if the driver will have oilcanning trouble - or even if the chart has enough information to do so.