Electric field and stator hole alignment

[arend-jan]

I tried to find out the effect on the uniformity of the electric field between stators as a function of hole alignment. There were two hole patterns discussed in the Wachara headphone thread, the regular straight alignment and the 'interleaving' alignment where each other row is shifted by 1/2. I tried to model the electric field with this simulator that I found on the web. Here are the results.

First the straight alignment.



and the shifted alignment:

[Few]

Thanks for posting these Arend-Jan. The results look about like what I would have guessed, for whatever that's worth. I guess the question now is how the diaphragm behaves in the two cases. I wonder, for example, whether the diaphragm will be affected by local differences in the acoustic resistances, or whether the stator just looks like a uniform resistance in both cases. The smaller the holes, hole spacing, and diaphragm-stator spacing, the more I would expect the stator to act like a uniformly distributed acoustic resistance. Is that resistance enough to prevent the nonuniform electric field from causing the diaphragm to rattle? I've long assumed that rattling is the "credit card sound" some people have complained about in some designs.

By the way, did you use FEMM or something else to do the simulations?

Few

[chinsettawong]

There is only one way to find out. Let's test the real thing. Arend-Jan, I'll send you a new set of stators. Please test and post your actual test results.

Wachara C.

[arend-jan]

Hi Few,

from the looks of it the electric field is more even (the density of the lines is more constant) in the shifted row configuration. With the straight alignment we can clearly see the lines widening and narrowing. The wiggling would be a very small factor, it's the density that counts.

The acoustic resistance will no doubt depend on the frequency. But let's assume that for a given frequency range there is a fluctuation in the acoustic resistance, then my guess would be that it would again be more even in the second case, because the diaphragm is non compressible so we can lump the front and back resistance together. This will give an even distribution in the second case, and a uneven distribution in the first case.

The sim is a Java applet

[Few]

Arend-Jan:
I agree the contour density is more important than the wiggling--good point.

With respect to the frequency dependence of the acoustic resistance, most of the dimensions of interest (hole diameter, stator spacing...) are small compared to virtually all the wavelengths being produced by the speaker. That is why I'm inclined to think the stator looks like a fairly pure acoustic resistance--for both the aligned and offset hole configurations. If it's a pure resistance over the frequency range of interest, then there shouldn't be any frequency dependence. I suppose that in some large-hole or large-gap designs there are some stator features that approach one wavelength at 20 KHz or above, but with my aging ears I'm not too concerned about those frequencies.

As a gentle nudge: which software did you use for the simulations?

Thanks,
Few

[arend-jan]

Hi,

The software is the Java applet I linked in my previous post. I did not write it.

[Few]

I completely missed the link in your previous message, Sorry about that.
Few

[Andersonix]

Originally posted by Few
Is that resistance enough to prevent the nonuniform electric field from causing the diaphragm to rattle? I've long assumed that rattling is the "credit card sound" some people have complained about in some designs.

Few: Do you have any more details about this complaint?

[chinsettawong]

Hi Arend-Jan,

I'll mail a set of new stators to you tomorrow. When you put two stators together, all the holes of one stator will be covered by the no hole area of the other stator. If you flip one of the stators up side down, you will get all the holes of the two stators 100% align with each other. Therefore, you can test the differences between 100% align and 100% non-align.



Please keep us posted on your findings.

Wachara C.

[Few]

Hi Tosh,
I'm afraid I don't have many details to offer. The coloration description is something I've read in reviewers' comments over the years but it's been so many years it would be almost impossible to track it down by memory.

Then again, there's always Google... I found this example at the InnerSound site. Apparently it comes from the Absolute Sound. I'm pretty sure there are other references to the coloration as well, but this is typical.

"More surprising, at least for me, were the most extended high frequencies, which don't roll-off so sharply and
dramatically as some 'stats (try Martin-Logan's Request or the Quad, old or new). It just keeps on going up, letting air
and a kind of sweet breeziness into the top octave that translates down into bloom and that kind of cream-like sound on
massed violins. Even a small section of violins, if indeed that section has poised intonation. It will surprise no one to learn
that the transients, from the speaker's crossover point of 450 Hz upward, are as good as they come and without that
peculiar, somehow inherent electrostatic credit-card coloration. Instruments being reproduced in the soundspace do not
have that disembodied quality, that lack of palpability that sometimes affects very fast ribbons and 'stats."

By the way, it's not my intention to argue that I've been troubled by the coloration Pearson describes. I'm just suggesting that rattling of the diaphragm because of a non-uniform driving force or insufficient damping might explain such a perception.

Few