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Old 27th June 2009, 02:07 PM   #21
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Hi Wachara,

The distance should be the same as the width of the probe contact area, so they make up a square (and the dimensions are taken out of the equation). Hence Ohms/square.

If the probe width and distance are not equal you have to scale the measurement by the ratio of the width and the distance between the probes. My probe consists of two copper strips of 100mm wide and spaced 2.5mm apart, hence giving a amplification factor of about 40x.

Then there is the matter of contact resistance and professional measurement equipment uses a standardised four point probe to deal with this.

see also the wikipedia on sheet resistance
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Old 27th June 2009, 03:23 PM   #22
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Great work Arend-Jan, and thanks for sharing it. Did you use the same coating, but with different thicknesses, to achieve the different resistivities, or did you use two different types of coating?
Few
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Old 27th June 2009, 03:27 PM   #23
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Thanks Few,

They are different coatings. The low resistance is a conductive polymer and the high resistance is nylon.
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Old 27th June 2009, 03:55 PM   #24
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Ah, very interesting. Thanks for the response. Can you share any details about the nylon coating, or is it proprietary?
Few
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Old 28th June 2009, 07:30 AM   #25
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It's Elvamide, a soluble nylon from Dupont. Wachara reported similar values of resistance for graphite doped PVA glue.
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Old 28th June 2009, 08:20 AM   #26
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Hi Arend-Jan,

Please don't count on my surface resistance value. I just found out from you that my measurement reading could very well be wrong. I didn't know anything about how far apart the 2 measuring probes should be at. What I did was measuring a few places and finding an average value.

Sorry for the confusion.

Wachara C.
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Old 28th June 2009, 03:22 PM   #27
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Arend-Jan:
Elvamide...thanks very much! I hadn't heard of that material before. I see Dupont makes more than one type. Which one have you had success with? I'll have to see if I can find a source of small quantities in the US.

Also, nylon tends to absorb water to some degree. Have you seen any changes in resistivity when the humidity changes? Given your posted results regarding distortion and resistivity I now have yet another reason to be sure to use a coating that isn't sensitivity to humidity.

Thanks again for sharing.
Few
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Old 17th July 2009, 02:42 PM   #28
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Default Need for Higher Resistance Coatings

Quote:
Originally posted by arend-jan
As we can see, no increase in second harmonic this time! We have actually improved the linearity of the device by raising the surface resistance of the coating.

That this is so, is confirmed by a significant drop in second harmonic compared to the measurement from post #1, especially in the mid range (0.03% vs 0.1%)

Wachara has commented that he could hear the difference between low and high resistance coating in his headphones. These measurements support that claim.

Unfortunately many commercially available coatings for ESLs are in the 1E7 to 1E8 range. I'd say this makes a strong case for higher S/R coatings!

arend-jan,

You have clearly demonstrated the advantage of using higher resistance coatings to lower 2nd harmonic distortion for ESLs with large geometric asymmetry in their construction.

The question that comes to my mind is:
For the average ESL where asymmetry is typically <5% would the higher resistance coating provide any reduction in 2nd harmonic distortion? or is the lower resistance of the commercially available coatings more then adequate.

Is this something you could easily test with your setup?
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Old 17th July 2009, 02:56 PM   #29
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Hi,

I now realise that it was not completely obvious at first sight, but if you look at the graphs carefully you will see that the distortion is always lower with high SR coating, even without the added asymmetry. Here is the measurement for the normal constructed (e.g. NOT asymmetric) headphone panel:
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File Type: jpg distortion_vs_sr.jpg (76.6 KB, 206 views)
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Old 17th July 2009, 03:22 PM   #30
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Quote:
Originally posted by arend-jan
Hi,

I now realise that it was not completely obvious at first sight, but if you look at the graphs carefully you will see that the distortion is always lower with high SR coating, even without the added asymmetry. Here is the measurement for the normal constructed (e.g. NOT asymmetric) headphone panel:

Thanks!
You had mentioned running some tests at different SPL levels, so
I wasn't sure if the distortion data from the two separate plots could be compared directly.

I am still a bit confused as to why the higher resistance coating did not reduce the odd harmonic distortion as I know I had measured it in the past. Actually, know that I think about it, the measurements were for low frequency distortion only where the diaphragm was moving a large percentage of the gap.
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